Microscopical Researches/On Cells as the Basis of all Tissues of the Animal Body

by Theodor Schwann

SECTION II

ON CELLS AS THE BASIS OF ALL TISSUES OF THE ANIMAL BODY.

The young cells contained within the cartilage-cells (see plate I, fig. 8, ff) may be regarded as the elementary form of the tissues previously considered, and may be described as round cells having a characteristic nucleus, firmly attached to the internal surface of the wall. As the above were proved to correspond with the vegetable cells, it follows, that it is only necessary to trace back the elementary structure of the rest of the tissues to the same formation, in order to show their analogy also with the cells of plants. In some tissues this proof is easy, and immediately afforded ; in others, however, it is obtained with much difficulty, and it would frequently be altogether impossible to demonstrate the cellular nature of some, if the connexion between the different steps in this investigation were lost sight of. The difficulty arises from the following circumstances: Ist. The minuteness of the cells; in consequence of which it is not only necessary to use a power magnifying from 400 to 500 diameters, but it is also frequently, indeed generally found impossible to press out their contents. 2dly. The delicate nature of the cell-membrane. When this has a certain density, its external as well as internal outline may be recognized, and the distinction between it and the cell-contents may thus be placed beyond a doubt. But if the cell-membrane be very delicate, the two outlines meet together in one line, and this may readily be regarded as the boundary line of a globule, not enclosed by a special enveloping membrane. 3dly. The similar power of refraction possessed by the cell-wall and cell-contents, in consequence of which the internal outline of the former cannot be observed. 4thly. The granulous nature of the cell-membrane, which when the contents are also granulous, cannot be distinguished from them. Lastly, the variety of form presented by the cells, for they may be flattened even to the total disappearance of the cavity, or elongated into cylinders and fibres. From these circumstances, many of the cells which now come before us for consideration, have been described as mere globules, or granules, terms which do not express their true signification, and even when they were spoken of as cells, or cells furnished with a nucleus, the description rested only upon a slight analogy, since but very few of them (for example, the pigment-cells), were proved to be actually hollow cells. But—as the precise signification of the nucleus is unknown, and as the cell-membrane is not proved to be anything essential to those cells (and this follows from their accordance with vegetable cells), upon the analogy with which the proof of the cellular nature of the rest of the globules provided with a nucleus will be based,— there is no contradiction involved in the supposition that a nucleus may be contained in a solid globule as well as in a cell.

From the difficulties of this investigation above detailed, it will be seen that a given object may really be a cell, when even the common characteristics of that structure, namely, the perceptibility of the cell-membrane, and the flowing out of the cell-contents, cannot be brought under observation. The possibility that an object may be a cell, does not, however, advance us much; the presence of positive characteristics 1s necessary in order to enable us to regard it as such. In many instances these difficulties do not present themselves, and the cellular nature of the object is immediately recognized; in others, the impediments are not so great but that the distinction between cell-membrane and cell-contents is at least indicated, and in such cases other circumstances may advance that supposition to a certainty. The most important and abundant proof as to the existence of a cell is the presence or absence of the nucleus. Its sharp outline and dark colour render it in most instances easily perceptible; its characteristic figure, especially when it encloses nucleoli, and remarkable position in the globule under examination, (being within it, but eccentrical, and separated from the surface only by the thickness of the assumed cell-wail,) all combine to prove it the cell-nucleus, and render its analogy with the nucleus of the young cells contained in cartilage, and with those of vegetables, as also the analogy between the globules under examination, in which it lies, and those cells, consequently the existence of a spherical cell-membrane in the globules, extremely probable. More than nine tenths of the globules in question present such a nucleus; in many the special cell-membrane is indubitable, in most it is more or less distinct. Under such circumstances, we may be permitted to conclude that all those globules which present a nucleus of the characteristic form and position, have also a cell-membrane, although, from the causes before specified, it may not be perceptible. The different tissues will also afford us many instances of other circumstances which tend to prove the existence of an actual cell-membrane. An example of what is referred to would be afforded by an instance, in which a certain corpuscle (furnished with a nucleus), about the cellular nature of which a doubt existed, could be proved to be only a stage of development, or modification in form, of an indubitable cell. The cell-nuclei and their distance from each other when scattered in a tissue, also serve as indications, when the outlines of the cells have to be sought for. They likewise serve to guide conjecture as to the earlier existence of separate cells, in instances where they have coalesced in the progress of develop- ment. When a globule does not exhibit a nucleus during any one of the stages of its development, it is either not a cell, or may at least be preliminarily rejected, if there be no other circumstances to prove it such.’ Fortunately, these cells devoid of nuclei are rare.

In addition, however, to the cellular nature of the elementary structures of animal tissues, there are yet other points of accordance between them and the cells of plants, which may generally be shown in the progress of their development, and which give increased weight to the evidence tending to prove that these elementary structures are cells. The exceedingly frequent, if not absolutely universal presence of the nucleus, even in the latest formed cells, proves its great importance for their existence. We cannot, it is true, at present assert that, with regard to all cells furnished with a nucleus, the latter is universally the primary and the cell the secondary formation, that is to say, that in every instance the cell is formed around the previously existing nucleus. It is probable, however, that such is the case generally, for we not only meet with separate nuclei in most of the tissues, distinct from those which have cells around them, but we also find that the younger the cells are, the smaller they are in proportion to the nucleus. The ultimate destiny also of the nucleus is similar to that of the vegetable cells. As in the last named, so in most animal cells it is subsequently absorbed, and remains as a permanent structure in some few only. In plants, according to Schleiden, the young cells are always developed within parent cells, and we have also seen such a development of new cells within those already formed in the chorda dorsalis and cartilage. If, however, any doubt existed as to whether the primary cells of these tissues were formed within previously existing parent cells, none such can arise in reference to many of the tissues next to be considered. We shall indeed frequently meet with a formation of young cells within older ones, but it is not the rule, and does not occur at all with regard to many of them.

The following admits of universal application to the formation of cells; there is, in the first instance, a structureless ^[1] substance present, which is sometimes quite fluid, at others more or less gelatinous, This substance possesses within itself, in a greater or lesser measure according to its chemical qualities and the degree of its vitality, a capacity to occasion the production of cells. When this takes place the nucleus usually appears to be formed first, and then the cell around it. The formation of cells bears the same relation to organic nature that crystallization does to inorganic. The cell, when once formed, continues to grow by its own individual powers, but is at the same time directed by the influence of the entire organism in such manner, as the design of the whole requires. This is the fundamental phenomenon of all animal and vegetable vegetation. It is alike equally consistent with those instances in which young cells are formed within parent cells, as with those in which the formation goes on outside of them. The generation of the cells takes place in a fluid, or in a structureless substance in both cases. We will name this substance in which the cells are formed, cell-germinating material (Zellenkeimstoff), or cytoblastema. It may be figuratively, but only figuratively, compared to the motherlye from which crystals are deposited.

We shall refer to this point at greater length hereafter, and only anticipate our subject with this result of the investigation, in order to facilitate the comprehension of what follows.

In the previous section of this work we have discussed in detail the course of development of some of the animal cells, having taken the chorda dorsalis and cartilage for our examples. We are now required to prove, as far as is possible, that all the tissues either originate from, or consist of cells. We separate this investigation into two divisions. The first treats of the Ovum and Germinal membrane, in so far as they form the common basis of all the subsequent tissues. The second division embraces the permanent tissues of the animal body, with the omission of the two already described.

FIRST DIVISION.

On the Ovum and Germinal Membrane.

The ovum of Mammalia lies, as is known, within the Graafian vesicle. I have not made any investigation as to whether that vesicle may be considered to have the signification of a cell. It is ineed a cell in the general sense of the word, being a cavity in the substance of the ovary, it has even a special membrane ; but as we here only receive the word cell as signifying an elementary part of animals and plants, it becomes necessary to inquire whether this membrane may not be a secondary formation resulting from the junction of other structures which are elementary. The history of the development of the Graafian vesicle must show whether that be the case, or whether it originate by the mere growth of a cell furnished with a structureless cell-membrane, which cell may formerly, perhaps, have had a nucleus. ^[2] Within this vesicle lies the ovum or vesicle of Baer, embedded in a layer of granules. When these granules are examined with a magnifymg power of 450, they are readily recognized to be cells, that is, round vesicles containing a nucleus, which is situated upon the internal surface of the wall. The nucleus being granulous and darker than the rest of the object falls under observation first. It encloses one or two nucleoli. The cell surrounding it varies in size, being in the average about half as large again in diameter, but some are much larger. The cells are for the most part extremely delicate, and round, when separated from one another. When in connexion, they often flatten against one another, and assume a polyhedral form. In addition to these cells, isolated nuclei appear also to be present within the Graafian vesicle, perhaps as the germs of new cells. The pro- duction of these cells proceeds according to the fundamental law mentioned at page 39, within the fluid of the Graafian vesicle, that being their germinative material or cytoblastema. Whether this fluid is to be regarded as cell-contents, and the cells produced in it as being formed within a parent cell, must depend upon the solution of the question, as to whether the Graafian vesicle be an elementary cell or not; but the deci- sion of this point is not essential, for the rule that cells originate within others is not universal. When the inde- pendent vitality of cells is borne in mind, we can readily conceive how these, when they (after the bursting of the vesicle) arrive with the ovum im the uterus, may be further developed into other structures (the chorion according to Krause.) Within this granulous or rather cellular disc then the ovum or vesicle of Baer lies embedded, (see the representation, plate II, fig. 1, taken from Krause.) The first object which attracts observation is the dark spherical yelk, surrounded by a transparent space, (zona pellucida of Baer, chorion of Wagner.) Krause found (Miiller’s Archiv, 1837, p. 27) that the yelk is surrounded by a peculiar membrane, d (vitelline membrane),and that the transparent space is enclosed externally by a very delicate pellicle, the albumen-membrane, b, also that the transparent substance itself (albumen) is sufficiently fluid to permit of such a degree of displacement of the yelk as to allow of its coming into contact even with the albumen-membrane. Although I have never yet succeeded in observing this pellicle, and though in my researches the transparent membrane, on the bursting of the yelk, always tore with smooth edges like a solid substance, yet the observations of the respected discoverer are too precise to admit of a doubt upon it. It is also supported by the analogy of most of the ova of other classes of animals, in which chorion and vitelline membrane may generally be distinguished, notwithstanding that they sometimes lie close upon each other. The albumen-membrane has probably the signification of a cell-membrane, in which case the albumen will be the cell-contents, and the yelk a young cell. According to Wharton Jones, the transparent areola (zona pellucida) of the ovum, or the albuminous layer in the fecundated ovum of mammalia, becomes considerably expanded in the tubes, a fact which would be readily explained by the inherent energy of the albumen-membrane when regarded as a cell. In such case, however, the mode of formation of the albumen would be very different from the corresponding process in the bird’s egg, where, according to Purkinje, it is secreted by the oviduct, and a membrane (chorion) is formed around it subsequently, which cannot therefore have the signification of a cell-membrane, and is moreover not simple in structure, but composed of fibres. Meanwhile an investigation might be made, as to whether the albumen in the egg may not also be first surrounded and formed by an equally thin pellicle, around which a secondary external membrane may subsequently be produced. According to Purkinje, however, this is not the case, and I could not discover any such pellicle upon the inner surface of the shell-membrane of the excluded egg. I have not made any inquiry as to whether the chorion of fishes is a cell-membrane or not. It is covered internally with a very beautiful epithelium, which is made up of more or less flat hexagonal cells, each of which has its nucleus.

Within the transparent areola, or, according to Krause, the albuminous layer, lies the vesicle of Baer, or the yelk ; which, from Krause’s statement, is enclosed by a peculiar structureless

membrane, the double outline of which he recognised, (plate II, fig. 1, d.) It is thus highly probable that the yelk of the mammalian ovum is a cell. Even if, as Wagner intimates, the vitelline membrane in other animals should sometimes be formed only secondarily within the chorion, it would not materially interfere with our purpose, since in that case the chorion would be the cell-membrane. The ovum universally possesses an external closed membrane (whether it be chorion or vitelline membrane), which is structureless, and not gene- rated from other elementary structures, and therefore is the ovum always a cell. The yelk-cell encloses the vitelline substance as its cell-contents, and upon its internal surface lies the germinal vesicle, or vesicle of Purkinje, (fig. 1, f) This, as is known, is a very transparent thin-walled vesicle, containing a pellucid fluid, according to Wagner coagulable by spirits of wine. It encloses almost universally (Wagner cites but very few exceptions) upon the internal surface of its wall, a corpuscle, called by the discoverer, R. Wagner, germinal spot, or germinal disc, (fig. 1, g.) In mammalia it is generally flat. In many instances several of these spots are present, their number, however, is said by Wagner to bear proportion to the age of the ovum, they being fewer and much more firmly attached to the wall of the germinal vesicle in young ova, I have frequently observed in osseous fishes (where they are often pre- sent in such numbers as to prevent the fluid in the vesicle from being seen) that when one of these corpuscles, after the bursting of the germ-vesicle, passed through a narrow space, it first became considerably elongated, and then drawn out in the centre to a thin thread, which soon broke. The two ends afterwards retracted, and thus two round globules were produced from one corpuscle, in a similar manner to what we may observe in the drops of fat upon soup. They appear, therefore, to be composed of a tenacious substance which is not miscible with water. Purkinje states that the germinal vesicle in birds is firmly fixed to the vitelline membrane, but Baer and Wagner describe it as lying in the centre of the yelk at first, and rising to the surface at a subsequent period.

The decision of the question, as to the precise signification of the germinal vesicle, now becomes of great importance. Is it a young cell generated within the yelk-cell, or is it the nucleus of the yelk-cell? If the former, it is in all probability the most essential rudiment of the embryo; but if it be the nucleus of the yelk-cell its importance vanishes with the formation of the yelk-cell, and according to the analogy of most cell-nuclei, it must either become absorbed altogether at a subsequent period, or continue for a time simply rudimentary, without forming any important new structure. The following is the ordinary career of a simple cell: a nucleus is present in the first instance; around it a cell is formed; the nucleus at first often increases in size as the cell grows, but their growth is by no means proportionate, that of the cell being much more rapid ; the cell-contents are at first transparent; a firm precipitate or new formation next commences in the cell, and this occurs immediately around the nucleus, which is at first enclosed by it; the nucleus then either becomes entirely absorbed, or continues only rudimentary and (with the following exception) I have never observed it to give origin to any other essential formation. One or more oil-globules once appeared to me to be formed during the absorption of the nucleus in the adipose cells within the cranial cavity of a young carp. The importance of the decision of this question in reference to the germinal vesicle thus becomes very obvious. Unfortunately, however, neither the observations upon the subsequent relations of the germ-vesicle, nor those on the origination of the ovum, are sufficiently extensive or certain for the purpose.

We shall next proceed to analyse both views of the question more minutely, and afterwards compare them with the observations. If the germ-vesicle be a young cell, in the first place, it is absolutely necessary that the yelk-cell should first exist, and that the germ-vesicle should afterwards be developed within it; 2dly, the germ-vesicle must not be connected with the vitelline-membrane, but must be developed free at some chosen spot within the cavity of the yelk; 3dly, the germ-vesicle may be regarded either as a cell without a nucleus, and in that case the spots of Wagner belong to the cell-contents, or Wagner’s spot, when it is single, is the nucleus; when there are several present, the others either differ essentially from one particular spot, and pertai to the cell-contents, or they are nuclei of young cells afterwards to be developed within the germ-vesicle. Before the spot can be considered to be the nucleus, it is necessary that it should, in the first instance at least, be connected with the wall of the vesicle. If, however, the germinal vesicle be the nucleus of the yelk-cell, it is essential, in the first place, that it should, in all probability, be present before the yelk-cell; at all events, that in proportion as the ovum is younger, should the vesicle be larger in relation to the cell; 2dly, it must, at first, he upon the vitelline- membrane, and be more or less intimately connected with it; 3dly, the germinal-vesicle, when regarded as a nucleus, either has no nucleoli, or Wagner’s spots are to be considered to represent them; in the first case they form the contents of the nucleus. In the enumeration of these points, no regard is had to the relations of the germ-vesicle subsequent to impregnation, because it is desirable to determine its ultimate destiny, to a certain extent a priori, from its signification, and thus to be enabled at the least to afford a guide to the much moré difficult observation of the fecundated ovum. If the researches were complete, the distinctions above cited would be sufficient for the correct determination of the question at issue, the decision of the first point indeed would of itseif be ample evidence.

When we take into consideration the first point raised on either side, we should be compelled to decide in favour of the latter view, and regard the germ-vesicle as a nucleus, if it were proved to be first present, and also that the yelk-cell is formed around it as a simple cell, narrowly encompassing it in the first instance, and becoming gradually expanded. In the next place, it is certain that at an early period the germ-vesicle is much larger in proportion to the yelk-cell, and that it at first grows pari passu with the yelk-cell, but that subsequently the latter increases in size in a much greater ratio, whilst the vesicle remains stationary; and these are precisely the relations in which the vesicle should stand in order to be regarded as a nucleus. But these facts are not entirely irreconcilable with the first view. A young cell, the germ-vesicle, might be imagined to form within the yelk-cell at a very early period of its growth, which young cell might at first increase in size more rapidly than the original one, but cease to do so earlier, whilst the parent-cell might continue to be developed in size. Such a circumstance is, however, very rare, and the weight of evidence before us is much in favour of the second view; but in order to determine this point, it is necessary to inquire whether the vesicle exist before the cell. That such is the case is not yet proved, although Baer and Purkinje suppose it to be so, and an observation of Wagner’s favours the supposition. (Prodromus Physiologiæ Generationis, p. 9, fig. xviii, a.) He found the posterior extremity of the oviduct of Acheta campestris full of germinal vesicles, which became gradually expanded in their progress through the oviduct. The oviduct becomes dilated in its further course; globules are observed in it, which Wagner regards as yelk-globules, and between them lie the germ-vesicles; then “ each vesicle becomes surrounded by its yelk and chorion, and thus the individual ova become separated.” He does not state, however, in what manner the vitelline-membrane is produced. Is it formed as a cell, at first narrowly encompassing the germ-vesicle, and then gradually expanding ; or does it at the same time enclose a quantity of the surrounding yelk-globules? It is difficult to conceive the latter mode of formation ; but if the former be the correct one, the globules surrounding the germ-vesicles in the oviduct cannot be yelk-globules. Fresh researches are therefore necessary, which, if they should be confirmatory of the first view, will also be decisive for considering the germ- vesicle as a cell-nucleus.^[3]

With regard to the second point, — namely, as to whether the germ-vesicle be more or less intimately connected with the membrane of the yelk-cell at an early period, or lie free within it, — any evidence afforded by its solution would be comparatively inconclusive. According to Baer and Wagner, the vesicle in the first instance lies in the centre of the yelk-cell, and only rises to its wall at a later period. Baer quotes the ova of frogs as examples in which it lies for a long time in the centre of the yelk. The germ-vesicle is generally found on the wall of the cell; and in birds, according to Purkinje, it is frequently so intimately connected with it, that it tears in the attempt to separate them. Although the position of the vesicle in the middle of the yelk-cell affords evidence rather in fayour of its being regarded as a young cell, yet it is not altogether inconsistent with its character as a nucleus; for it is only during the earliest formation of the cell that the nucleus is required to be connected with it; it is frequently disconnected at a later period, and lies loose in the cell. At that stage of development, however, in which the vitelline-membrane closely encompasses the germ-vesicle, it is impossible to decide whether it lie in the middle or on the wall of the cell. This point, therefore, is of more ideal than practical importance for the prosecution of the investigation.

The third point relates to the signification which attaches to the individual parts of the germ-vesicle. It may be hollow consistently with both views. Although we are not as yet acquainted with any hollow nuclei in plants ^[4], we have nevertheless found nuclei in cartilages which were hollow, and decidedly to be regarded as cytoblasts. The question now arises, what are Wagner’s spots or spot? If the germ-vesicle be considered to be a young cell, one of them may be its nucleus, and the rest cell-contents, or nuclei of young cells, which will be developed afterwards; if it be regarded as nucleus, the spots may either be nucleoli, or merely its contents. It is a fact in favour of the former view, that only one spot is present in most instances, the others being usually produced at a later period. Wagner has sometimes observed one or more minute points in this single spot, and has delineated them from Alcedo hispida, Lepus cuniculus, Ovis aries, &c.; I have also sometimes met with small points of this kind which gave the spot, in some degree, the appearance of a nucleus adhering to the wall of the cell, and containing within it these little points as its nucleoli. Meanwhile, their presence is too inconstant, and they are generally too indefinite, to permit of our attributing any importance to them in the decision of the present question. The extra- ordinary number in which they frequently occur is opposed to their being regarded as nucleoli within the germ-vesicle, pre- suming it to be a cell-nucleus, for in fishes they sometimes fill the entire vesicle, at least, being closely crowded, they cover the internal surface of it. Three is the largest number of nucleoli which I have observed in other nuclei, and Schleiden has in some very rare instances seen four in plants. If, however, they are only the contents of the nucleus, and not nucleoli, it must be allowed that they differ very much from the contents of almost all other nuclei, which are generally yellowish, and made up of extremely minute granules. The only exception which I have met with was that already mentioned respecting the nucleus of the adipose cells in the cranial cavity of a young carp. This last point seems therefore rather in favour of the germ-vesicle being regarded as a young cell. ^[5]

When the whole of the above detailed evidence is reflected upon in connexion, it will be seen that it is as yet impossible to decide the question as to whether the germinal vesicle be cell or nucleus. The opinion that the vesicle is to be regarded as a cell-nucleus, seems for the present to have the ascendancy, inasmuch as the observations upon the first and most important point, viz. the prior existence of the germ-vesicle to that of the yelk-cell appear to be in favour of that view. ^[6] The subsequent relations of the vesicle seem also to afford evidence in its favour. The disc, for instance, is formed around it, and this perhaps corresponds to the granulous precipitate which

<ref follow="p74"> mals of several ova in one ovicapsule is difficult of explanation by Barry’s view. In the further investigation of this subject, attention must continue to be fixed upon the possible, and even probable, existence of a nucleus to the ovicapsule. Wagner saw certain follicles in the mole, in which he could not detect a trace of any enclosed body.

Wagner expresses himself in his new work (Lehrbuch der Physiologie, Leipzig, 1839, p. 34) as being doubtful whether the vesicles met with in his observations on the preformation of the germinal vesicle in the ova of insects, were actually vesicles or not. The observations of Barry on the ova of mammalia and birds, are, however, in favour of the explanation of the ovum of the insect originally given by the first-named highly respected investigator, and therefore also of that which represents the germ-vesicle as nucleus of the ovum-cell. It is true it might be said, that, regarding the germ-vesicle as a cell, a second one, the ovum-cell was formed around it; but as opposed to that view, it must be remembered that no example of a second cell being formed around the first is afforded amongst all the other cells which exhibit a nucleus of the decidedly characteristic form. The point in dispute, as to the interpretation to be placed upon the germ-yesicle, loses, however, somewhat of its importance if the theory which I shall propose (see the conclusion of the treatise) be received, inasmuch as I shall there endeavour to prove the formation of the cell around the nucleus to be merely a repetition of the process by which the nucleus is formed around the nucleolus, and that the whole process of development of the cell may be reduced to a single or many times repeated formation of strata. The germinal-vesicle accordingly is the first stratum, or a cell of the first order; the yelk-cell the second stratum, or a cell of the second order. As above stated at page 47, a minute point was observed in the germinal spot by Wagner, and subsequently by myself also; and my respected colleague Vanbeneden lately found germinal spots in the ova of certain polypes (Genus Zoanthus), and also in ova of Anodonta, which had not as yet left the ovary, that appeared granulous, but at the same time seemed to be hollow, and some of which distinctly contained a very small round corpuscle. This observation accords most completely with the theory which regards the cells as produced by a stratified formation. This small corpuscle, which may be called a secondary nucleolus, would here be the primordial formation; the germinal spot would be the first stratum around it, that having in this instance become developed into a vesicle, in a manner likewise to be explained hereafter by the Cell-Theory; the germinal vesicle would be the second, and the yelk-cell the third stratum. The formation of even a fourth stratum, the albumen membrane, around the yelk-cell, would involve nothing contradictory to the theory ; but in such case we certainly could not avoid regarding it as a second cell, which had become formed around a previously existing one: for the yelk-cell cannot well be considered to be a nucleus. The mode of formation of this albumen membrane must, however, in the first instance, be ascertained by investigation. usually takes place around the nucleus in other cells; and again, the germ-vesicle disappears, precisely as the nucleus of other cells is generally absorbed. There is then no evidence that the fluid of the germinal vesicle exercises a fructifying influence; but if it be the cell nucleus, it disappears, because it has completed its office,—the formation of the yelk-cell. The dise, which has formed around it, becomes developed into the germinal membrane, and it is uncertain whether the remains of the germ-vesicle also take part in that formation.

We shall next proceed to the consideration of the other contents which the yelk-cell includes in addition to the germ-vesicle, making use of the bird’s egg for the purpose. Setting aside some points of distinction of slighter importance, the globules, well known as present in the yelk of the hen’s egg when laid, may be divided into two principal classes: a, the globules of the yelk-cavity; and b, those of the true yelk-substance. The former (a) are not only present in the yelk-cavity, but occur also in the canal leading from it to the germinal membrane, and in the little prominence, called by Pander the nucleus of the tread (Kern des Hahnentritts). When many of them lie close together, they exhibit a white colour, whilst the true yelk-globules in such circumstances appear yellow. They may also be distinguished from the latter globules under the microscope, (see pl. II, fig. 2.) They are perfectly round globules, with quite smooth edges, each enclosing a smaller one, which is also perfectly spherical, and looks like an oil-globule, being rendered very distinct by its sharp outline.

The remaining space in the large globules is usually transparent, and not granulous. But some may be observed which have granulous contents, and they then completely resemble the true yelk-globules, except that the latter do not generally contain any smaller ones with such dark outlines. Sometimes also, the globules of the yelk-cavity contain two or more such smaller ones. The common yelk-globules (4), that is, those of the true yelk-substance, may be distinguished from the above-described by the following characteristics : they are upon the whole larger, they have all granulous contents, and, for the most part, do not enclose any smaller globules. They are very sensitive to the action of water, which causes them to fall to pieces, and then the granules enclosed within them becoming free, give a milk-white colour to the fluid. These granules, which are of various size, resemble milk-globules, and, as has been frequently remarked by others, exhibit also like them a brisk molecular motion. In consequence of the speedy action of water upon these globules, they must be examined in albumen or a weak solution of common salt, which preserves them better. These fluids also do not impart a white colour to the surface of a yelk which is opened in them, as water does. The globule, when crushed under the compressorium, tears somewhat suddenly on one side, the other margins remaining smooth, and then, without any increase of the pressure, a large quantity of the globules contained in it flow slowly forth. This fact indicates an external membrane belonging to the globules, but it must be a very soft and delicate one. Baer, who distinguishes four kinds of them, believes that he has also sometimes seen such a mem- brane in the yelk-globules of immature ovarian eggs. The yelk-globules when isolated are round, but, in their natural position in the yelk, they flatten against one another into angular shapes, in which manner the crystal-like bodies observed by Purkinje in the boiled yelk are produced. These bodies generally make up the whole of the true yelk-substance of a fresh egg, so that, with the exception of the contents of the yelk-globules, we do not usually meet with any free granulous substance in the yelk. The minutely granulous substance which is observed in addition to the yelk-globules, particularly after the action of water upon them, appears in most instances, and on the external layers of the yelk invariably, to be produced solely by the destruction of the yelk-globules. In the vicinity of the yelk-cavity of a boiled egg, however, we frequently find a coagulated substance composed of granules similar to those contained in the yelk-globules, and which appears to be actually free yelk substance not enclosed within globules.

It is necessary to examine the eggs while still contained in the ovary, if we wish to become acquainted with the process of formation of these two kinds of globules (those of the yelk- cavity and yelk-substance), and the mode of production of the yelk-cavity and its canal. The younger eggs, having a diameter of one or two lines, have a grayish-white colour, but are not yellow ; if such an one be cut through the centre, under water, it is found to contain a thick, semi-fluid, grayish-white mass, part of which flows slowly out. Around this mass lies a more consistent, cohering, membrane-like stratum, which lines the cavity of the little egg. When a portion of this mass is examined under the microscope, a great many round and very transparent vesicles or cells are observed in it, each of which encloses a dark corpuscle resembling an oil-globule. Many such globules float about free, and in addition to them there is also a good deal of minutely granulous substance present. In order, however, to examine this mass in a perfectly natural condition, the use of water must be avoided; one of the little eggs, of from half a line to a line in diameter, should be placed upon the dry object plate, and then pierced, a drop of its contents being allowed to flow out. This drop will be found to consist entirely of very pale cells, most variable in size, each one containing a round globule, the size of which is about proportionate to that of the cell. This globule or nucleus resembles an oil-globule, in consequence of its dark outline, (see pl. II, fig. 3.) Many of these cells with their nuclei are so small, that, when lying close together, they might be regarded as a merely granulous substance; the cells may, however, be recognised with a favorable light. Some of the larger ones occasionally contain two or three of the globules or nuclei before mentioned. The contents of the cells are usually quite transparent, but some isolated ones are seen, in which a minutely granulous precipitate has formed. These cells are enclosed within the egg, in a small quantity of transparent fluid. In order to explain the somewhat variable appearance which the contents of the egg assume after contact with water, a small one should be placed upon a glass with a drop of that fluid, and some of its contents pressed out whilst under the microscope. A quantity of these cells will then be seen to burst quite suddenly in the water, precisely like soap-bubbles in the air. In consequence of their paleness, the fact of the bursting is rendered manifest, in the first instance, only by the sudden motion of the nucleus, which, together with some minutely granulous substance, remains behind. If these cells were solid, although ever so soft, this sudden bursting would not be possible. They are therefore true cells. I cannot say whether the globule enclosed in them is to be regarded as the nucleus. Although it resembles an oil-globule, it does not appear to be fat; for if acetic acid be applied to a drop of the contents of the egg, it does not appear to act materially upon the cells, and the contained corpuscle becomes paler and somewhat swollen, which could not well take place if it were fat. These cells, then, are the earlier stage of development of the subsequent globules of the yelk-cavity. The larger ones already resemble them perfectly. These globules of the yelk-cavity are therefore likewise cells. The nucleus-globule (Kernkugel) is acted on by acetic acid precisely in the same way as it was in the earlier condition. It does not lie centrally in the cell, but on the internal surface of the wall, as is seen when the cells are caused to roll under the microscope. When at rest, however, they are generally so placed that the nucleus-globule occupies the most depending point (because probably it is the heaviest portion of the cell), and, on that account, it then appears to lie in the centre of the cell. The yelk in the first instance contains only the yelk-cavity, with its cells; the proper yelk-substance with its globules not being as yet formed. The colour of these young eggs is there- fore also white, like the contents of the yelk-cavity.

The membrane-like layer which surrounds the above-described contents of the egg, may be completely separated from the parts which surround it externally with facility, after the egg has been divided through the centre. It is not connected with them, and appears, to the unaided eye at least, to be pretty smooth on its external surface; it is not possible to trace it towards the interior. Its structure is peculiar. Purkinje, who discovered it, describes it as consisting of globules, which re- semble in form and size, but are more transparent than the blood-corpuscles. When spread out upon a plate of glass, and examined with the microscope, it is seen to consist of two parts, an internal minutely granulous stratum, and an external layer of cells. Numerous little granules are observed in the internal stratum, which resemble the nuclei of the above-described cells of the yelk-cavity in their earliest stage, and I conjecture that the cells of the yelk-cavity are formed from this stratum, so that in fact it still pertains to the yelk-cavity. The external layer consists of small round granulous cells, each of which contains a nucleus, which again in many instances encloses one or two nucleoli. Two or three such layers of cells lie one above another, These layers of cells are surrounded externally by a very transparent, perfectly structureless membrane, which represents a closed cell-membrane, having as little connexion with the ovary as with the layers of cells, and which is denominated vitellne membrane, It is as readily separated from the ovary as from the layer of cells, the latter, therefore, cannot be merely its epithelium.

If we now proceed to examine larger eggs from the ovary, such, for instance, as have attained a diameter of half an inch or more, and are already yellow-coloured, on their being divided across the centre under water, a white substance, the yelk-cavity, will be found in their interior. This cavity contains those cells, now in a higher stage of development, which in the first instance alone formed the contents of the egg. Around these a stratum of yellow substance, the proper yelk-substance, appears, and round this again lies the layer of cells. Globules may be recognised in the proper yelk-substance with the aid of the microscope, as in the same substance of the mature yelk. These globules, then, have been formed between the yelk-cavity and the layer of cells. The question, however, arises how this has been effected? The following may be supposed to be the mode of their production: — the innermost portion of the yelk, the yelk-cavity, is the part which is first formed, the innermost yelk-globules are therefore also the oldest, and the formation of the new yelk-globules takes place externally upon the internal surface of the layer of cells. If a small portion of the layer of cells be so placed under the microscope that the inner surface becomes turned towards the eye, and a spot be sought for at which a thin layer of yelk-substance is attached to it, it will be seen that the yelk-globules do actually become smaller in the proximity of the layer of cells, whilst in other respects they retain their general appearance. The smallest of them, which lie immediately upon the inner surface of the layer of cells, are even smaller than the cells of the layer itself. It is therefore extremely probable, that the formation of new yelk-globules takes place on the inner surface of the layer of cells, and that the globules then expand to their normal size somewhat quickly, for the stratum of small ones is but thin. Meanwhile new ones continue to form externally, until the yelk has reached its normal size. The formation of the canal leading from the yelk-cavity to the germinal vesicle may also be explained in the same manner; for instance, no formation of yelk-globules can go on at that point at which the germ-vesicle and the stratum for the germinal membrane are in connexion with the layer of cells, but at that spot there must be a gap in each stratum of yelk-globules, which by the increasing thickness of the yelk-substance becomes a canal, necessarily conducting from the yelk-cavity towards the germinal membrane, and into which cells from the yelk-cavity become crowded. Now are these globules of the proper yelk-substance cells? I cannot prove decisively that they are so; the following arguments, however, render it probable: 1st, because Baer believes that he observed. an external membrane in some of them; 2dly, because, when ruptured at a particular spot by the compressorium, they at once pour out a large portion of their contents without the pressure being increased ; 3dly, because, notwithstanding that they lie close together in the yelk and flatten against one another, they do not run together; 4thly, because they so closely resemble some of the cells of the yelk-cavity which are furnished with granulous contents; 5thly, because they, like cells, appear to have an independent growth. These reasons are sufficiently strong to render it probable that the yelk-globules have a cellular structure, though they cannot be received as decisive of the point. However, inasmuch as they all form the contents of a larger cell, it is not absolutely necessary for our purpose that they should be distinctly proved to be cells. Both the indubitable cells of the yelk-cavity, and those problematical ones of the proper yelk-substance, have an independent growth in a fluid, and within another cell. They are cells within cells. For although the formation of new cells takes place only at the outside, yet they are still separated from the organized substance, not only by the cell-membrane of the entire ovum, but also by the layer of cells which is situated immediately beneath it. We here, then, meet with an instance of just such a formation and independent growth of cells within a fluid as was expressed by the fundamental phenomenon previously laid down. It is a point open to investigation, whether the cleaving of the yelk described by Baer, Rusconi, and others, in the development of the lower animals, the ova of frogs for example, may not also depend upon a process of cell-formation, two cells being developed within the yelk in the first instance, and in each of these again two new ones, and so on.

We next proceed to consider the changes undergone by the external layer of cells furnished with nuclei. In eggs which have a diameter of a line, this entire membrane, if it may be so called, appears to be made up merely of cells. In such as have reached a higher stage of development, such as have a diameter of upwards of half an inch, for instance, it consists of two strata, the external of which is granulous, and no longer exhibits cells ; the internal, however, is composed of cells, which are flat, hexagonal, but also granulous, and bear the relation of a covering of epithelium to the outer one. The external stratum passes away over the germinal vesicle and the foundation of the germinal membrane, so that these structures may easily be removed from its inner surface without injury to it. The internal cellular stratum, on the contrary, is interrupted at the spot where the germinal vesicle lies. I have not traced the mode of formation of this external granulous stratum through all its details; I suppose it to be produced by a blending of the outer cells, which composed the original membrane when it was made up entirely of cells. As the period approaches at which the egg leaves the ovary, the epithelium-like stratum of cells gradually disappears, and the granulous membrane alone remains. It does not exhibit any disposition to unite with the structureless external membrane of the egg, even in eggs which are almost sufficiently mature for extrusion. If such an egg be cut open under water, and the investment derived from the ovary be drawn off, this granulous membrane frequently remains lying upon the yelk, whilst the structureless membrane follows the above-mentioned investment, and may readily be proved to be connected with it, when they are folded so that the inner surface forms a sharp edge. By the aid of the compressorium this structureless membrane may then be seen, projecting out from the border of the preparation. It often separates in large pieces during this manipulation, so that it has likewise no connexion with the parts pertaining to the ovary. If the signification of vitelline membrane is to be assigned to this structure, a blending between it and the granulous stratum must take place in the oviduct, in order to form the subsequent vitelline membrane of the extruded egg.

We now pass on to that portion of the egg from which the embryo is first formed, the germinal membrane. It represents, as is known, a round, white, little disc, somewhat above a line in breadth, which lies between the vitelline membrane and the yelk-substance. This little disc, in a fresh-laid hen’s egg, consists of globules, which are of unequal size in different parts of the germinal membrane. When examined with the microscope, they appear much darker than the yelk-globules, (see plate II, fig. 4.) They lie in close contact, so that they flatten against one another to an hexagonal form. The boundaries of the dis- tinct globules may be clearly distinguished, even when in con- nexion. They may also be readily isolated from one another, and are then round. They contain many smaller round gra- nules of various size, with very dark outlines, which float about singly when the globules are burst by pressure. Although these granules, in most instances, completely fill the globules, yet some globules may be observed where that is not the case, and where a portion of the globule is transparent, and free from granules, (a b, of the above figure.) I thought that I distinctly saw a double external outline on one of these globules (a), which would be evidence of the presence of a cell-membrane. In most instances, however, this is not distinct, and my principal reason for concluding that they are cells, is, that it is so extremely probable that they are developed to form the indubitable cells of the incubated germinal membrane. I have not, however, fully investigated this process, and only communicate my observations on the point, incomplete as they are. If the unincubated germinal membrane be folded in such a manner that its external surface form a sharp margin, that surface is found to be tolerably even, dark, and composed immediately of the globules of the germinal membrane already described ; the surface of the germinal membrane of an egg which has been exposed to brooding heat for four hours, presents a precisely similar appearance. The same membrane, when examined also upon its general surface, differs but very slightly in appearance from one which has not undergone incubation. The globules of which it consists merely appear to have more minutely granulous contents. But if a germinal membrane after eight ^[7] hours’ incubation be folded in the same manner, its margin at many points is found to be no longer dark and even, but to be composed of extremely pale transparent cells. These cells present every variety of size, some being as large and even larger than the primitive globules of the germinal membrane. They either project forward in the form of half-spheres, or the greater portion of their spherical surface juts out in some instances, and they may be completely separated by pressure. They contain a pellucid fluid, but no nucleus. The following fact shows them to be cells; some of them contain very minute, isolated, black granules, which resemble the molecules described by Brown, and exhibit molecular motion within the cell. This fact proves that the contents of the cell must be fluid. A fluid which is miscible with water cannot, however, preserve any definite form, unless it be encompassed by a membrane. Such a structure must, therefore, exist in this instance. It is not altogether easy to convince one’s self that these granules, exhibiting molecular motion, do actually he within the cells; but it may be concluded from the fact, that they do not flow away when the surrounding fluid is allowed to escape, and that they are not moved beyond the limits of the cell, but only to its wall and back again. Beneath this stratum of cells le the globules of the unincubated germinal membrane, which, however, appear to have become still more clear and minutely granulous than those of the membrane examined after four hours’ incubation. In addition to these, separate cell-nuclei may be observed, such as occur in the cells of the serous layer at a subsequent period, and may be seen in plate II, fig. 6. Still more internally than this layer, we meet with perfectly dark globules. The serous and mucous layers of the germinal membrane are perfectly formed in the egg after sixteen hours’ incubation. If the membrane at that period be folded so that its external surface may be seen, it will be found

to be composed of cells, which project forwards in the form of half-spheres, (plate II, fig. 5). A nucleus of the characteristic form may be recognised in some of them. It lies upon the internal surface of the cell-wall, is round, and contains one or two nucleoli. In most instances, however, no nucleus can be seen, either because none is present, or because it lies upon the posterior side of the cell, in which position it cannot be perceived, in consequence of the dark substance lying beneath it. The cells also contain a transparent fluid, and some minute granules with molecular motion, which is evidence sufficient for the existence of a peculiar cell-membrane. If, after the germinal membrane has lain for a time in water, the mucous layer be washed off, the general surface of these cells may be observed. They are then seen to lie close together, and to flatten against one another to hexagonal forms, (see plate II, fig. 6). They contain a beautiful nucleus, which encloses one or two nucleoli. They also present many minute granules, which exhibit molecular motion. The cells may also be observed in the recent germinal membrane, especially on its margin, at which part it is more transparent, and there they project forward in the form of large segments of a sphere. These cells then re- present the serous layer of the germinal membrane,—which, therefore, consists of round cells (their polyedrical form being referrible solely to their lying so closely together), furnished on the inner surface of their wall with the characteristic nucleus, and containing a clear fluid, and some isolated smaller granules. They might be conceived to be a mere covering of epithelium to the serous layer. But if the serous layer be separated after the blood has formed, for example, in an egg which has undergone forty-eight hours’ incubation, the vascular layer remains lying immediately upon this stratum of cells. Valentin has already recognised these cell-nuclei, for he says, that each of these layers of the germinal membrane consists of a transparent vitreous jelly, but that they are to be distinguished by the corpuscles which they contain. (Entwicklungsgeschichte, page 287.) These corpuscles are the cell-nuclei, the transparent substance in which they he is composed of the cells, and is gelatinous only in appearance. The cells have only a minimum of intercellular substance between them.

When, in the next place, we proceed to examine the mucous layer of the germinal membrane of an egg after sixteen hours’ incubation, we find it to be composed of globules, which vary greatly both in size and appearance, (see plate II, fig. 7.) The large globules, which form the greater proportion, may be proved to be cells, and Baer has already named them vesicles. The molecular motion, which is frequently visible in isolated globules within them, although much slighter in these instances than in the cells of the serous layer, affords sufficient evidence of their cellular character. They contain a transparent fluid and granules of various kinds. One particular globule, having very dark outlines, resembling those remarked in the cells of the yelk-cavity, may be observed in almost every cell. Several of the globules, and of all gradations of size, are frequently seen in a cell. In addition to the above, a minutely granulated substance is present in many of them. These cells lie somewhat loosely together in a structureless, tenacious, intercellular substance, which is their cytoblastema, so that at this stage they are but slightly flattened against one another. This intercellular substance contains, in addition, perfectly dark globules and smaller granules, but I do not know what relation they bear to the cells. A portion of them may, perhaps, be nuclei of new cells. Yet I could not decide whether the one dark globule, which is generally so very prominent in the cells of the mucous layer, had actually the signification of a cell-nucleus. It differs in form from the usual cell-nucleus very materially. During the progressive development of the germinal membrane, the quantity of intercellular substance, and of those globules the cellular nature of which is not demonstrable, diminishes very much, so that at a subsequent period the cells lie close together, and present the appearance of vegetable cellular tissue. The description here given applies only to the mucous layer on the outside of the area pellucida. Within that the cells have quite a different appearance. They are very much smaller, of pretty equal size, very transparent, and contain no coarse granules, but only very small globules. They do not appear to have any nucleus, and this fact distinguishes them from the cells of the serous layer, which possess a nucleus even within the area pellucida.

The first rudiments of the embryo appear to be formed from the cells of the serous and mucous layers of the germinal membrane, that is, from such cells as are met with in the area pellucida, so that the embryo is composed, partly of small cells without nuclei, and partly of cells furnished with the characteristic nucleus. It presents, however, besides them, an extraordinary quantity of simple cell-nuclei with nucleoli, around which no cells have as yet formed.

I have made but few researches with respect to the structure of the vascular layer, and from them, I could not (with the exception of the vessels themselves and the blood) detect any such essential difference between it and the mucous layer, as was exhibited between the latter and the serous layer. As, however, the formation of the vessels themselves, although it ap- pears to depend upon a production of cells, is not a process peculiar to the germinal membrane, we shall defer it, to be resumed at a subsequent stage of our investigation.

I have not ascertained the relation which these cells of the layers of the germinal membrane have to the primitive globules of the membrane before incubation, or within eight hours after that process has commenced; but inasmuch as it is probable that at least one of those kinds of cells owes its origin to the development of the primitive globules, we may be permitted to suppose that those globules are likewise cells.

For the purpose of giving, in outline, a connected view of the changes which the egg undergoes, from its first formation up to the period at which the actual development of the embryo commences,—in so far as the foregoing, more or less complete, observations enable us to form a provisional conception of the process of development,—we will proceed on the understanding, that the germ-vesicle is the nucleus of the yelk-cell ; at the same time, however, we expressly refer the reader to the more detailed statement above furnished for the certainty both of this and of every other separate point which occurs in the following exposition. It is probable that the germ-vesicle is the first structure, and that the yelk-cell forms around it as its cell-nucleus. Both advance in growth, the latter, however, much more rapidly than the former. A precipitate, the commencement of the germinal membrane, next forms around the germ-vesicle. Young cells are simultaneously formed in the remaining space of the yelk-cell, these are the cells of the subsequent yelk-cavity. Then cells of another kind originate beneath the vitelline membrane, which are the subsequent cells of the proper yelk-substance. They are formed round about the neighbourhood of the vitelline membrane, with the exception of that spot where the germ-vesicle and the rudiments of the germinal membrane lie. ‘These cells expand very rapidly, while at the same time a new layer is formed on the outside of them, and so on successively. In this manner they surround the white cells of the yelk-cavity with a layer of yellow cells, which is constantly increasing in thickness ; as, however, a vacant space remains at the spot where the germinal vesicle and germinal membrane are situated, by the increasing thickness of the yelk-substance, the space becomes converted into a canal. The development of the vitelline membrane proceeds continuously with these changes, in pro- portion as the increasing contents require. When the yelk-cell has attained its due size and the egg leaves the ovary, the germ-vesicle, like most other cell-nuclei, disappears, and the now more fully developed germinal membrane remains. It is made up of globules, probably cells, having coarsely-granulated contents. It grows during the process of incubation by the con- tinual development of new cells. After sixteen hours’ incubation, a distinction may be observed in the cells composing the membrane. The more external ones form a layer, in which the cells exhibit a nucleus of the characteristic form, and contain a quantity of transparent fluid and minute isolated granules. These cells are therefore clear, and firmly united together, and have only a minimum of intercellular substance between them; they represent the serous layer of the germinal membrane. The under stratum of the germinal membrane or mucous layer contains cells of another kind; they have no nucleus of the characteristic form, but contain one or more dark globules, and frequently also some minutely granulous substance. These cells lie loosely together in a larger quantity of intercellular substance, which contains smaller granules of different kinds, in addition. When this division of the membrane into the two layers is completed, and its superficies has become considerably extended, and after a transparent spot, the area pellucida, has formed in its centre—(the cells of the mucous layer in this area being much smaller, but of pretty equal size, as compared with one another, and having transparent contents with very minute isolated granules),—the embryo is developed, as a portion of the germinal membrane separating from the whole by a constriction. Both layers contribute to its formation, and it therefore consists of small transparent cells, some of which (probably those pertaining to the mucous layer) contain no nucleus, whilst others (those derived from the serous layer) exhibit the characteristic cell-nucleus with its nucleoli. In addition to these cells it contains a great many nuclei, around which no cells have as yet formed. Between the two layers of the germinal membrane other cells arise, which may be regarded as representing a third layer, the vascular, although they do not really form a connected independent layer; of these we shall treat hereafter. These three layers then, and pre-eminently the first two, form the mediate basis of all the subsequent tissues.

The yelk is not a lifeless aliment for the embryo,—as it is when taken as food by the adult, to whose organism it is dead and must be chemically dissolved,—but the cells of the yelk take part in the vitality called forth by incubation. They effect an alteration in their contents, whereby the albumen which they contain loses its property of coagulating, and the granules become dissolved, in the same manner in which the granules of starch dissolve in the cells of the vegetable embryo. In short, the yelk bears the same relation to the embryo as regards its nutritive property, that the albumen bears to the vegetable embryo.

In accordance with the analogy between the cells we are treating of and those of vegetables, all the changes in the egg, the growth of the germinal membrane, and even the first formation of the embryo, proceed entirely without vessels. SECOND DIVISION.

Permanent Tissues of the Animal Body.

The foregoing investigation having taught us that the entire ovum, from its first origin up to that period at which, by the formation of the serous and mucous layers of the germinal membrane, the foundation of all the subsequent tissues is laid, exhibits simply a continual formation and more extended development of cells, and having found the primordial substance of the tissues itself to be composed of cells, we are now required to prove, that the tissues do not only originate from cells in this general manner, but that the special basis of each individual tissue is a matter composed of cells, and that all tissues either consist entirely of or are formed from cells which pass through a variety of transformations. These modifications, which some of the cells undergo in the progress of their development to the subsequent tissues, are very important, since thereby the cells not infrequently cease to exist as separate independent structures. We have already (in the Introduction) seen such changes in plants, forexample, in the coalescence of the cell-walls observed by Schleiden in the bark of the Cacti, and the blending of several cells to form a tube in the spiral and lactiferous vessels. This takes place to a much greater extent in animals, and, in general, the higher the importance of a tissue is, the more do the cells lose their individuality. We shall not, however, enumerate these modifications here; we shall become acquainted with them as the result of investigation of the separate tissues, and, at the conclusion of the work, we shall combine them into a connected representation of Cell-life. It is necessary, however, to mention the most important of them at least preliminarily in this place, in order to make a classification of the tissues.

Since all organic structure is primarily formed from cells, the most scientific classification of general anatomy would manifestly be one founded upon the more or less high degree of development at which the cells must arrive, in order to form a tissue. The complete retention, or relinquishment, to a greater or less extent, of their individuality by the cells, should serve as the scale for their degree of development. We give the name of independent cells to those in which the wall remains distinguishable from the neighbouring structures throughout the whole progress of its expansion. We apply the term coalesced cells to those in which the wall blends, either partially or entirely, with the neighbouring cells, or intercellular substance, so as to form an homogeneous substance. The cell-cavities, in such instances, are separated from one another only by a single wall, as we have already observed in cartilage. This is the first degree of coalescence; the cacti present an example of it in vegetables. The second is that in which the walls of several cells lying lengthwise together, coalesce with one another at their points of contact, and the partition walls of the cell-cavities become absorbed. In this way not only the walls but the cavities of the cells also become united, as in the spiral and lactiferous vessels in plants.

Upon these more or less important modifications of the Cell-life the following classification of the tissues is based: 1st. Isolated, independent cells, which either exist in fluids, or merely lie unconnected and moveable, beside each other. 2d. Independent cells applied firmly together, so as to form a coherent tissue. 3d. Tissues, in which the cell-walls (but not the cell-cavities) have coalesced together, or with the intercellular substance. Lastly, tissues in which both the walls and cavities of many cells blend together. In addition to these, however, there is yet another very natural section of the tissues, namely, the fibre-cells, in which independent cells are extended out on one or more sides into bundles of fibres. The naturalness of this group will form my excuse for sacrificing logical classification to it, and inserting it as the fourth class (4th), consequently, that last mentioned, consisting of tissues, in which the cell-walls and cell-cavities coalesce, becomes the fifth (5th).

All tissues of the animal body may be comprised under these five classes; the classification, however, gives rise to some difficulties. For instance, the fibres of cellular tissue and fat must be placed in very different classes, so also the enamel of the teeth and the proper dental substance. A second difficulty arises from the fact, that transitions take place, the isolated cells, for example, passing over into those with blended walls ; and again, a tissue which usually consists of isolated cells, occasionally exhibits in different situations coalesced cells. Such difficulties, however, present themselves in all classifications of natural objects. Nature is very unwilling to accommodate herself to our schemes. The object of her aim is quite opposed to that of our intellect. She accords and accommodates all contrarieties by gentle transitions: the intellect disjoins, and seeks everywhere for strongly-marked contrasts. If, however, regard be had to the most important structure only in each individual tissue,—for example, in the nervous system, to the nervous fibres and not to the ganglion-globules, in cellular tissue, to its fibres and not to the fat, and so forth,—and further, if we regard only that which is the general rule as to these structures, all tissues may then be readily brought under these five classes. With the desire of making this work as complete as possible, I have applied this arrangement to all the tissues in the way which has appeared to be most probably correct, according to the investigations I have hitherto made. Those researches are, however, far from complete, and continued observations may perhaps render it necessary, at some future time, to assign a different position to some of the tissues. This may serve as a preliminary sketch:

Class I. Isolated, independent cells. To this class the cells in fluids pre-eminently belong; Lymph-globules, Blood-corpuscles, Mucus- and Pus-corpuscles, &c.

Class II. Independent cells united into continuous tissues. Such as the Horny tissues and the Crystalline lens.

Class III. Cells, in which only the cell-walls have coalesced: Cartilage, Bone, and the substantia propria (ivory) of the Teeth.

Class IV. Fibre-cells: Cellular (areolar), Fibrous, and Elastic tissue.

Class V. Cells, im which both the cell-walls and cell-cavities have coalesced: Muscle, Nerve, Capillary vessels.

1. ' [Strukturlos. — I have ventured to translate this word as above, although I am aware it is open to objection. The idea intended to be conveyed by the author is that of a substance in which no definite structure can be detected. As the word will be frequently used in the following pages, the reader is requested to assign this signification to it invariably. —TRANS. ]
2. ' According to the researches of Martin Barry (Phil. Trans. Part II, 1838, p. 305, &c.), both cases appear to occur, so that a cell composed of a structureless membrane is first formed, (the ovisac of Barry,) and subsequently an external vascular covering of cellular tissue. On the relation of this follicle to the mode of development of the ovary itself, see Valentin in Müller’s Archiv, 1838, p. 526.
3. See the Supplement. The observations of Wagner upon the ova of insects which are there quoted, and the recent researches of Barry on those of mammalia and birds, (l. c. p. 308,) prove the germinal vesicle to be first formed, and then the vitelline membrane round it.
4. See Note, p. 33.
5. Since in vegetable cells the nucleolus is the primary formation, and the nucleus a secondary one around it, and as the same has been shown to be most probably the case in animal cells, (see page 20, on the production of the nucleus of cartilage-cells,) so also in this case the signification to be assigned to Wagner’s spot depends upon the history of the development of the germ-vesicle. The observations of Wagner, quoted in the Supplement, show, however, that the single germinal spot of the ova of insects is first formed, and the germinal vesicle afterwards around it. The former must then be considered as nucleolus to the vesicle, which correspondsto the nucleus. When several of Wagner’s spots occur, their signification is totally different from that of the first one, and they are to be regarded only as secondary formations in the interior of the germ-vesicle. In fact, the younger the ova of fishes and frogs, the fewer spots are observed in them.
6. The following is the probable course of formation of the ovum, according to the researches now before us; the ovisac (Eisach, ovisac of Barry, internal membrane of the Graafian vesicle) is first developed. In this (according to analogy with Wagner’s observations on the ova of insects) a germinal spot is generated, asnucleolus to the ovum. Around that spot the germinal vesicle is formed as nucleus to the ovum; and round this again the ovum-cell (Eizelle.) Martin Barry, indeed, (1. c. p. 308,) conjectures that the germ-vesicle is formed previously to the ovisac; but my respected friend expresses himself with great caution on the ques- tion; and it would in fact be difficult to determine whether a given vesicle were a germinal vesicle, around which no ovisac had as yet formed, or an ovisac within which no germ-vesicle had as yet formed. The occurrence also in the lower ani-
7. It is quite as impossible to define with any certainty a fixed time for a precise stage of development of the elementary cells of the germinal membrane, as it is to connect the formation of the area pellucida, the embryo, and its separate parts, with any degree of certainty to any precise hour of incubation. The periods cited should therefore only be taken as being near about the true determinations of the time. The cells in the germinal membrane, before incubation even, do not appear to be al- ways at the same stage of development; thus, plate II, fig. 4, c, and fig. 4, a, b, represents cells from two different membranes. A great portion of the germinal membrane from which c was taken consisted of such cells as that delineated, and I thought I perceived molecular motion in the granules contained in some of them, which, if correct, would clearly prove them to be cells.