Microscopical Researches/SECTION III.
SECTION III.
REVIEW OF THE PREVIOUS RESEARCHES—THE FORMATIVE PROCESS OF CELLS——THE CELL THEORY.
The two foregoing sections of this work have been devoted to a detailed imvestigation of the formation of the different tissues from cells, to the mode in which these cells are developed, and to a comparison of the different cells with one another. We must now lay aside detail, take a more extended view of these researches, and grasp the subject in its more intimate relations. The principal object of our investigation was to prove the accordance of the elementary parts of animals with the cells of plants. But the expression “plant-like life” (pflanzen-ahnliches Leben) is so ambiguous that it is received as almost synonymous with growth without vessels; and it was, therefore, explained at page 6 that in order to prove this accordance, the elementary particles of animals and plants must be shown to be products of the same formative powers, because the phenomena attending their development are similar; that all elementary particles of animals and plants are formed upon a common principle. Having traced the formation of the separate tissues, we can more readily comprehend the object to be attained by this comparison of the different elementary particles with one another, a subject on which we must dwell a little, not only because it is the fundamental idea of these researches, but because all physiological deductions depend upon a correct apprehension of this principle,
When organic nature, animals and plants, is regarded as a Whole, in contradistinction to the inorganic kingdom, we do not find that all organisms and all their separate organs are compact masses, but that they are composed of innumerable small particles of a definite form. These elementary particles, however, are subject to the most extraordinary diversity of figure, especially in animals ; in plants they are, for the most part or exclusively, cells. This variety in the elementary parts seemed to hold some relation to their more diversified physiological function in animals, so that it might be established as a principle, that every diversity in the physiological signification of an organ requires a difference in its elementary particles ; and, on the contrary, the similarity of two elementary particles seemed to justify the conclusion that they were physiologically similar. It was natural that among the very different forms presented by the elementary particles, there should be some more or less alike, and that they might be divided, according to their similarity of figure, into fibres, which compose the great mass of the bodies of animals, into cells, tubes, globules, &c. The division was, of course, only one of natural history, not expressive of any physiological idea, and just as a primitive muscular fibre, for example, might seem to differ from one of areolar tissue, or all fibres from cells, so would there be in like manner a difference, however gradually marked between the different kinds of cells. It seemed as if the organism arranged the molecules in the definite forms exhibited by its different elementary particles, in the way required by its physiological function. It might be expected that there would be a definite mode of development for each separate kind of elementary structure, and that it would be similar in those structures which were physiologically identical, and such a mode of development was, indeed, already more or less perfectly known with regard to muscular fibres, blood-corpuscles, the ovum (see the Supplement), and epithelium-cells. The only process common to all of them, however, seemed to be the expansion of their elementary particles after they had once assumed their proper form. The manner in which their different elementary particles were first formed appeared to vary very much. In muscular fibres they were globules, which were placed together im rows, and coalesced to form a fibre, whose growth proceeded in the direction of its length. In the blood-corpuscles it was a globule, around which a vesicle was formed, and continued to grow; in the case of the ovum, it was a globule, around which a vesicle was developed and continued to grow, and around his again a second vesicle was formed. The formative process of the cells of plants was clearly explained by the researches of Schleiden, and appeared to be the same in all vegetable cells. So that when plants were regarded as something special, as quite distinct from the animal kingdom, one universal principle of development was observed in all the elementary particles of the vegetable organism, and physiological deductions might be drawn from it with regard to the independent vitality of the individual cells of plants, &e. But when the elementary particles of animals and plants were considered from a common point, the vege- table cells seemed to be merely a separate species, co-ordinate with the different species of animal cells, just as the entire class of cells was cu-ordinate with the fibres, &c., and the uniform principle of development in vegetable cells might be explained by the slight physiological difference of their elementary particles.
The object, then, of the present investigation was to show, that the mode in which the molecules composing the elementary particles of organisms are combined does not vary according to the physiological signification of those particles, but that they are everywhere arranged according to the same laws; so that whether a muscular fibre, a nerve-tube, an ovum, or a blood-corpuscle is to be formed, a corpuscle of a certain form, subject only to some modifications, a cell-nucleus, is universally generated in the first instance; around this corpuscle a cell is developed, and it is the changes which one or more of these cells undergo that determine the subsequent forms of the elementary particles ; in short, that there is one common principle of development for all the elementary particles of organisms.
In order to establish this point it was necessary to trace the progress of development in two given elementary parts, physiologically dissimilar, and to compare them with one another. If these not only completely agreed in growth, but in their mode of generation also, the principle was established that elementary parts, quite distinct in a physiological sense, may be developed according to the same laws. This was the theme of the first section of this work. The course of development of the cells of cartilage and of the cells of the chorda dorsalis was compared with that of vegetable cells. Were the cells of plants developed merely as infinitely minute vesicles which progressively expand, were the circumstances of their development less characteristic than those pointed out by Schleiden, a comparison, in the sense here required, would scarcely have been possible. We endeavoured to prove in the first section that the complicated process of development in the cells of plants recurs in those of cartilage and of the chorda dorsalis. We remarked the similarity in the formation of the cell-nucleus, and of its nucleolus in all its modifications, with the nucleus of vegetable cells, the pre-existence of the cell-nucleus and the development of the cell around it, the similar situation of the nucleus in relation to the cell, the growth of the cells, and the thickening of their wall during growth, the formation of cells within cells, and the transformation of the cell-contents just as in the cells of plants. Here, then, was a complete accordance in every known stage in the progress of development of two elementary parts which are quite distinct, in a physiological sense, and it was established that the principle of development in two such parts may be the same, and so far as could be ascertained in the cases here compared, it is really the same.
But regarding the subject from this point of view we are compelled to prove the universality of this principle of development, and such was the object of the second section. For so long as we admit that there are elementary parts which originate according to entirely different laws, and between which and the cells which have just been compared as to the principle of their development there is no connexion, we must presume that there may still be some unknown difference in the laws of the formation of the parts just compared, even though they agree in many points. But, on the contrary, the greater the number of physiologically different elementary parts, which, so far as can be known, originate in a similar manner, and the greater the difference of these parts in form and physiological signification, while they agree in the perceptible phenomena of their mode of formation, the more safely may we assume that all elementary parts have one and the same fundamental principle of development. It was, in fact, shown that the elementary parts of most tissues, when traced backwards from their state of complete development to their primary condition are only developments of cells, which so far as our observations, still incomplete, extend, seemed to be formed in a similar manner to the cells compared in the first section. As might be expected, according to this principle the cells, in their earliest stage, were almost always furnished. with the characteristic nuclei, in some the pre-existence of this nucleus, and the formation of the cell around it was proved, and it was then that the cells began to undergo the various modifications, from which the diverse forms of the elementary parts of animals resulted. Thus the apparent difference in the mode of development of muscular fibres and blood-corpuscles, the former originating by the arrangement of globules in rows, the latter by the formation of a vesicle around a globule, was reconciled in the fact that muscular fibres are not elementary parts co-ordinate with blood-corpuscles, but that the globules composing muscular fibres at first correspond to the blood-corpuscles, and are like them, vesicles or cells, containing the characteristic cell-nucleus, which, like the nucleus of the blood-corpuscles, is probably formed before the cell. The elementary parts of all tissues are formed of cells in an analogous, though very diversified manner, so that it may be asserted, that there is one universal principle of development for the elementary parts of organisms, however different, and that this principle is the formation of cells. This is the chief result of the foregoing observations.
The same process of development and transformation of cells within a structureless substance is repeated in the formation of all the organs of an organism, as well as in the formation of new organisms; and the fundamental phenomenon attending the exertion of productive power in organic nature is accordingly as follows: a structureless substance is pre- sent in the first instance, which lies either around or in the interior of cells already existing; and cells are formed in it in accordance with certain laws, which cells become developed in various ways into the elementary parts of organisms.
The development of the proposition, that there exists one gene ral principle for the formation of all organic productions, and that this principle is the formation of cells, as well as the conclusions which may be drawn from this proposition, may be comprised under the term cell-theory, using it in its more extended signification, whilst in a more limited sense, by theory of the cells we understand whatever may be inferred from this proposition with respect to the powers from which these phenomena result.
But though this principle, regarded as the direct result of these more or less complete observations, may be stated to be generally correct, it must not be concealed that there are some exceptions, or at least differences, which as yet remain unexplained. Such, for instance, is the splitting into fibres of the walls of the cells in the interior of the chorda dorsalis of osseous fishes, which was alluded to at page 14. Several observers have also drawn attention to the fibrous structure of the firm substance of some cartilages. In the costal cartilages of old persons for example, these fibres are very distinct. They do not, however, seem to be uniformly diffused throughout the cartilage, but to be scattered merely here and there. I have not observed them at all in newborn children. It appears as if the previously structureless cytoblastema in this instance became split into fibres; I have not, however, investigated the point accurately. Our observations also fail to supply us with any explanation of the formation of the medullary canaliculi in bones, and an analogy between their mode of origin and that of capillary vessels, was merely suggested hypothetically. The formation of bony lamellae around these canaliculi, is also an instance of the cytoblastema assuming a distinct form. But we will return presently to an explanation of this phenomenon that is not altogether improbable. In many glands, as for instance, the kidneys of a young mammalian foetus, the stratum of cells surrounding the cavity of the duct, is enclosed by an exceedingly delicate membrane, which appears to be an elementary structure, and not to be composed of areolar tissue. The origin of this membrane is not at all clear, although we may imagine various ways of reconciling it with the formative process of cells. (These gland-cylinders seem at first to have no free cavity, but to be quite filled with cells. In the kidneys of the embryos of pigs, I found many cells in the cylinders, which were so large as to occupy almost the entire thickness of the canal. In other cylinders, the cellular layer, which was subsequently to line their walls, was formed, but the cavity was filled with very pale transparent cells, which could be pressed out from the free end of the tube.)
These and similar phenomena may remain for a time unexplained. Although they merit the greatest attention and require further investigations, we may be allowed to leave them for a moment, for history shows that in the laying down of every general principle, there are almost always anomalies at first, which are subsequently cleared up.
The elementary particles of organisms, then, no longer lie side by side unconnectedly, like productions which are merely capable of classification in natural history, according to similarity of form; they are united by a common bond, the similarity of their formative principle, and they may be compared together and physiologically arranged in accordance with the various modifications under which that principle is exhibited. In the foregoing part of this work, we have treated of the tissues in accordance with this physiological arrangement, and have compared the different tissues with one another, proving thereby, that although different, but similarly formed, elementary parts may be grouped together in a natural-history arrangement, yet such a classification does not necessarily admit of a conclusion with regard to their physiological position, as based upon the laws of development. Thus, for example, the natural-history division, "cells," would, in a general sense, become a physiological arrangement also, inasmuch as most of the elementary parts comprised under it have the same principle of development; but yet it was necessary to separate some from this division ; as, for instance, the germinal vesicle, all hollow cell-nuclei, and cells with walls composed of other elementary parts, although the germinal vesicle is a cell in the natural-history sense of the term. It does not correspond to an epithelium-cell, but to the nucleus of one. The difference in the two modes of classification was still more remarkable in respect to fibres. The mode of their origin is most varied, for, as we saw, a fibre of areolar tissue is essentially different from a muscular fibre; while, on the other hand, a whole primitive muscular fasciculus is identical in its mode of origin with a nervous fibre, and so on. The existence of a common principle of development for all the elementary parts of organic bodies lays the foundation of a new section of general anatomy, to which the term philosophical might be applied, having for its object—firstly, to prove the general laws by which the elementary parts of organisms are developed; and, secondly, to point out the different elementary parts in accordance with the general principle of development, and to compare them with one another.
SURVEY OF CELL-LIFE.
The foregoing investigation has conducted us to the principle upon which the elementary parts of organized bodies are developed, by tracing these elementary parts, from their perfected condition, back to the earlier stages of development. Starting now from the principle of development, we will reconstruct the elementary parts as they appear in the matured state, so that we may be enabled to take a comprehensive view of the laws which regulate the formation of the elementary particles. We have, therefore, to consider—l, the cytoblastema; 2, the laws by which new cells are generated in the cytoblastema ; 3, the formative process of the cells themselves; 4, the very various modes in which cells are developed into the elementary parts of organisms.
Cytoblastema. — The cytoblastema, or the amorphous substance in which new cells are to be formed, is found either contained within cells already existing, or else between them in the form of intercellular substance. The cytoblastema, which lies on the outside of existing cells, is the only form of which we have to treat at present, as the cell-contents form matter for subsequent consideration. Its quantity varies exceedingly, sometimes there is so little that it cannot be recognized with certainty between the fully-developed cells, and can only be observed between those most recently formed; for instance, in the second class of tissues; at other times there is so large a quantity present, that the cells contained in it do not come into contact, as is the case in most cartilages. The chemical and physical properties of the cytoblastema are not the same in all parts. In cartilages it is very consistent, and ranks among the most solid parts of the body; in areolar tissue it is gelatinous; in blood quite fluid. These physical distinctions imply also a chemical difference. The cytoblastema of cartilage becomes converted by boiling into gelatine, which is not the case with the blood; and the mucus in which the mucus-cells are formed differs from the cytoblastema of the cells of blood and cartilage. The cytoblastema, external to the existing cells, appears to be subject to the same changes as the cell-contents; in general it is a homogeneous substance; yet it may become minutely granulous as the result of a chemical transformation, for instance, in areolar tissue and the cells of the shaft of the feather, &c. As a general rule, it diminishes in quantity, relatively with the development of the cells, though it seems that in cartilages there may be even a relative increase of the cytoblastema proportionate to the growth of the tissue. The physiological relation which the cytoblastema holds to the cells may be twofold : first, it must contain the material for the nutrition of the cells; secondly, it must contain at least a part of what remains of this nutritive material after the cells have withdrawn from it what they required for their growth. In animals, the cyto- blastema receives the fresh nutritive material from the blood- vessels ; in plants it passes chiefly through the elongated cells and vascular fasciculi; there are, however, many plants which consist of simple cells, so that there must also be a transmission of nutrient fluid through the simple cells; blood-vessels and vascular fasciculi are, however, merely modifications of cells.
Laws of the generation of new cells in the cytoblastema.— In every tissue, composed of a definite kind of cells, new cells of the same kind are formed at those parts only where the fresh nutrient material immediately penetrates the tissue. On this depends the distinction between organized or vascular, and unorganized or non-vascular tissues. In the former, the nutritive fluid, the liquor sanguinis, permeates by means of the vessels the whole tissue, and therefore new cells originate throughout its entire thickness. Non-vascular tissues, on the contrary, such as the epidermis, receive the nutritive fluid only from the tissue beneath; and new cells therefore originate only on their under surface, that is, at the part where the tissue is in connexion with organized substance. So also in the earlier period of the growth of cartilage, while it is yet without vessels new cartilage-cells are formed around its surface only, or at least in the neighbourhood of it, because the cartilage is connected with the organized substance at that part, and the cytoblastema penetrates from without. We can readily conceive this to be the case, if we assume that a more concentrated cytoblastema is requisite for the formation of new cells than for the growth of those already formed. In the epidermis, for instance, the cytoblastema below must contain a more concentrated nutritive material. When young cells are formed in that situation, the cytoblastema, which penetrates into the upper layers, is less concentrated, and may therefore serve very well for the growth of cells already formed, but not be capable of generating new ones. This constitutes the distinction which was formerly made between a growth by apposition and one by intussusception; "growth by apposition" is a correct term, if it be applied to the generation of new cells, and not to the growth of those already existing, the new cells in the epidermis for example, are formed only on its under surface, and are pushed upwards when other new ones are formed beneath them; but the new cells are generated throughout the entire thickness of the organized tissues. The cells, however, grow individually by intussusception in both instances. The bones occupy, to a certain extent, a middle position between the organized and unorganized tissues. The cartilage in the first instance has no vessels, and the new cells are, therefore, formed in the neighbourhood of the external surface only; at a subsequent period it receives vessels, which traverse the medullary or Haver sian canals, the latter, however, are not sufficiently numerous to allow of the entire tissue becoming equably saturated with the fluid parts of the blood, a process which would be still further impeded by the greater firmness of cartilage and bone. According to the above law, then, the formation of new cytoblastema and new cells may take place partly upon the
| surface and partly around these medullary canals. Now, the structure of bone becomes most simple, if we assume that, in consequence of the firmness of the osseous substance, this process goes on in layers, which do not completely coalesce together. It must consist of a double system of layers, one being concentric to each of the medullary canals, and the other to the external surface of the bone. When the bone is hollow, the layers must also be concentric to the cavity; and when small medullary cavities exist in the place of canals, as in the spongy bones, the layers must also be concentric to them. The difference in the growth of animals and plants also rests upon the same law. In plants, the nutritive fluid is not so equably distributed throughout the entire tissues, as it is in the organized tissues of animals, but is conveyed in isolated fasciculi of vessels, widely separated from one another, more after the manner of bone. These fasciculi of vessels are also observed to be surrounded with small (most likely younger) cells, so that, in all probability, the formation of their new cells also takes place around these vessels, as it does in bones around the medullary canaliculi. In the stem of dicotyledonous plants the sap is conducted between the bark and the wood, and on that account the new cells are generated in strata concentric to the layers of the previous year. The variety in the mode of growth, as to whether the new cells are developed merely in separate situations in the tissue, or equally throughout its whole thickness, does not, therefore, constitute any primary distinction, but is the consequence of a difference in the mode in which their nutritive fluid is conveyed.
The generation of cells of a different character, such as fat- cells, in the interior of a non-vascular tissue (in cartilage which does not as yet contain vessels, for example), appears at first sight to form an exception to the law just laid down. But such is not really the case; the circumstance is capable of two explanations, either the cytoblastema for this kind of cells is furnished by the true cells of the tissue only when they have attained a certain stage of their development, or, the cytoblastema which penetrates into the depth of the tissue contains the nutritive material for the true cells of the tissue in a less concentrated state, whilst it is still sufficiently impregnated with the nutritive material for the other kind of cells.
According to Schleiden, new cells are never formed in the intercellular substance in plants; in animals, on the contrary, a generation of cells within cells is the less frequent mode, but this does occur, and in such a way, that a threefold or fourfold generation may take place in succession within one cell. Thus, according to R. Wagner’s observations (see the Supple- ment), the Graafian vesicle appears to be an elementary cell; the ovum is developed within it in like manner as an elementary cell; within this, again, according at least to observations made upon the bird’s egg, cells are generated, some of which contain young cells. It appears also, that a formation of true cartilage-cells can sometimes take place within those which already exist, and that young cells (fat-cells?) may be generated within them again. Several such examples might be brought forward; but by far the greater portion of the cells of cartilage are formed in the cytoblastema on the outside of the cells already present, and we never meet with a generation of cells within cells in the case of fibre, muscle, or nerve.
General phenomena of the formation of cells. Round corpuscles make their appearance after a certain time in the cytoblastema which, in the first instance, is structureless or minutely granulous. These bodies may either be cells in their earliest condition (and some may be recognized even at this stage), that is, hollow vesicles furnished with a peculiar structureless wall, cells without nuclei, or they may be cell-nuclei or the rudiments of cell-nuclei, round which cells will afterwards be formed.
The cells without nuclei, or, more correctly, the cells in which no nuclei have as yet been observed, occur only in the lower plants, and are also rare in animals. For the present, however, the following must be regarded as such, viz.: the young cells contained within others in the chorda dorsalis (see p. 13), the cells of the yelk-substance in the bird’s egg (p. 50), the cells in the mucous layer of the ger- minal membrane of the bird’s egg (p. 60), and some cells of the crystalline lens (p.88). Pl. I, fig. 10, c, represents one of these cells without nuclei. Thus the mode of growth, in this instance, is similar to that of the nucleated cells, after the formation of their cell-membrane.
By far the greater portion of the animal body, at least ninety-nine hundredths of all the elementary parts of the bodies of mammalia are developed from nucleated cells.
The cell-nucleus is a corpuscle, having a very characteristic form, by which it may in general be easily recognized. It is rather round or oval, spherical or flat. In the majority of fully-developed animal cells its average size would be about 0-0020-0:0030 Paris inch; but we meet with nuclei which are very much larger, and others, again, much smaller than this. The germinal vesicle of the bird’s egg may be regarded as the largest cell-nucleus; the nuclei of the blood-corpuscles of warm-blooded animals afford examples of very small cell- nuclei. If the latter were but a very little smaller they would escape observation altogether, and the blood-corpuscles would then appear to be cells without nuclei. No other structure can be detected in these very small nuclei, nor can their characteristic form be further demonstrated. On the other hand, that of the larger blood-corpuscles may be distinctly recognized as a cell-nucleus.
The cell-nucleus is generally dark, granulous, often somewhat yellowish; but some occur which are quite pellucid and smooth. It is either solid, and composed of a more or less minutely granulated mass, or hollow. Most nuclei of animal cells exhibit more or less distinct trace of a cavity, at least, their external contour is generally somewhat darker, and the substance of the nucleus seems to be somewhat more com- pact at the circumference. The nucleus may often be traced through its progressive stages of development from a solid body to a perfect vesicle ; this may be observed in the nuclei of the cartilage-cells in the branchial cartilages of tadpoles. The membrane of the cell-nucleus and its contents may be distinguished in those which are hollow. The membrane is smooth, structureless, and never of any remarkable thickness, that of the germinal vesicle beimg the thickest. The con- tents are either very minutely granulous, especially in the small hollow cell-nuclei, or pellucid, as in the germinal vesicle, and the larger nuclei in the cells of the branchial cartilages of the tadpole, or larger corpuscles may be subsequently formed in the interior of hollow nuclei, for instance, the innumerable corpuscles in the germinal vesicle of the fish, and fat-globules in the nucleus of the fat-cells in the cranial cavity of fishes.
The nucleus, in most instances, contains one or two, more rarely three or four small dark corpuscles, the nucleoli. Their size varies from that of a spot which is scarcely discernible to that of Wagner’s spot (macula germinativa) in the germinal vesicle. Nucleoli cannot be distinctly recognized in all cell-nuclei. They may be distinguished from the larger corpuscles, which are sometimes developed in certain hollow nuclei, from the circumstance of their being formed at a much earlier period; they exist, indeed, before the cell-nucleus. They are placed eccentrically in the round nuclei, and in the hollow ones are distinctly seen to lie upon the internal surface of the wall. It is very difficult to ascertain their nature; it may also vary very much in different cells, They sometimes appear to be capable of considerable enlargement, as in the nuclei of the fat-cells in the cranial cavity of the fish, and in such instances often have the appearance of fat. According to Schleiden, hollow nucleoli also frequently occur in plants.
Most cell-nuclei agree in the peculiarity of not being dissolved, or rendered transparent by acetic acid, at least not rapidly so, whilst the cell-membrane of animal cells is in most cases very sensitive to its action. Some cells, (such as those of the yelk-cavity of the egg, plate II, fig. 3,) which have no perceptible nucleus of the ordinary form, exhibit a globule having the appearance of a fat-globule, which grows as the cell expands, though not in the same proportion, and was probably formed previous to the cell. Whether such a globule have the signification of a nucleus or not, must re- main an undecided question.
The formation of the cell-nucleus. In plants, according to Schleiden, the nucleolus is first formed, and the nucleus around it. The same appears to be the case in animals. According to the observations of R. Wagner on the development of ovain the ovary of Agrion virgo, [1] the germinal spot is first SURVEY OF CELL-LIFE. 175
formed, and around that the germinal vesicle, which is the nucleus of the ovum-cell, Eizelle. [2] The youngest germinal vesicle there represented by Wagner, appears to be hollow. This is not generally the case, however, in the formation of cell-nuclei. Plate III, fig. 1, e, appears to be a cell-nucleus of a cartilage-cell in the act of forming. A small round corpuscle is there seen, surrounded by some minutely granulous substance, whilst the rest of the cytoblastema is homogeneous. This granulous substance is gradually lost around the object; at a subsequent period it begins to be sharply defined, and then exhibits the form of a cell- nucleus, which continues to grow for a certain period. (See pl. III, fig. 1, a, b.) Such a nucleus usually appears solid in the first instance, and many nuclei remain in this condition; in others, on the contrary, the portion of the substance situated nearest to the external surface continually becomes darker, and not unfrequently at last forms a distinctly perceptible membrane, so that the nucleus is hollow in such instances. The formative process of the nucleus may, accordingly, be conceived to be as follows: A nucleolus is first formed; around this a stratum of substance is deposited, which is usually minutely granulous, but not as yet sharply defined on the outside. As new molecules are constantly being deposited in this stratum between those already present, and as this takes place within a precise distance of the nucleolus only, the stratum becomes defined externally, and a cell-nucleus having a more or less sharp con- tour is formed. The nucleus grows by a continuous deposition of new molecules between those already existing, that is, by intussusception. If this go on equably throughout the entire thickness of the stratum, the nucleus may remain solid ; but if it go on more vigorously in the external part, the latter will become more dense, and may become hardened into a membrane, and such are the hollow nuclei. The circumstance of the layer generally becoming more dense on its exterior, may be explained by the fact that the nutritive fluid is conveyed to it from the outside, and is therefore more concentrated in that situation. Now if the deposition of the new molecules between the particles of this membrane takes place in such a manner that more molecules are deposited between those particles which he side by side upon its surface than there are between those which lie one beneath another in its thickness, the expansion of the membrane must proceed more vigorously than its increase in thickness, and therefore a constantly increasing space must be formed between it and the nucleolus, whereby the latter remains adherent to one side of its internal surface.
I have made no observations on the formation of nuclei with more than one nucleolus. But it is easy to comprehend how it may occur, if we conceive that two nucleoli may lie so close together that the layers which form around them become united before they are defined externally, and that by the progressive deposition of new molecules, the external limitation is so effected that two corpuscles are enclosed by it at the same time, and then the development proceeds as though only one nucleolus were present.
When the nucleus has reached a certain stage of development, the cell is formed around it. The following appears to be the process by which this takes place. A stratum of substance, which differs from the cytoblastema, is deposited upon the exterior of the nucleus. (See pl. III, fig. 1, d.) In the first instance this stratum is not sharply defined externally, but becomes so in consequence of the progressive deposition of new molecules. The stratum is more or less thick, some- times homogeneous, sometimes granulous; the latter is most frequently the case in the thick strata which occur in the formation of the majority of animal cells. We cannot at this period distinguish a cell-cavity and cell-wall. The deposition of new molecules between those already existing proceeds, however, and is so effected that when the stratum is thin, the entire layer—and when it is thick, only the external portion—he- comes gradually consolidated into a membrane. The external portion of the layer may begin to become consolidated soon after it is defined on the outside; but, generally, the membrane does not become perceptible until a later period, when it is thicker and more defined internally ; many cells, however, do not exhibit any appearance of the formation of a cell-membrane, but they seem to be solid, and all that can be remarked SURVEY OF CELL-LIFE. 177
is that the external portion of the layer is somewhat more compact. |
Immediately that the cell-membrane has become consolidated, its expansion proceeds as the result of the progressive reception of new molecules between the existing ones, that is to say, by virtue of a growth by intussusception, while at the same time it becomes separated from the cell-nucleus. We may therefore conclude that the deposition of the new molecules takes place more vigorously between those which lie side by side upon the surface of the membrane, than it does between those which lie one upon another in its thickness. The interspace between the cell-membrane and cell-nucleus is at the same time filled with fluid, and this constitutes the cell-contents. During this expansion the nucleus remains attached to a spot on the internal surface of the cell-membrane. If the entire stratum, in which the formation of the cell commenced, have become consolidated into a cell-membrane, the nucleus must lie free upon the cell-wall; but if only the external portion of the stratum have become consolidated, the nucleus must remain surrounded by the internal part, and adherent to a spot upon the internal surface of the cell-membrane. It would seem that the portion of the stratum which remains may be disposed of in two ways: either it is dissolved and forms a part of the cell-contents, in which case the nucleus will lie free upon the cell-wall as before; or it gradually becomes condensed into a substance similar to the cell-membrane, and then the nucleus appears to lie in the thickness of the cell-wall. This explains the variety in the position of the nucleus with respect to the cell-membrane. According to Schleiden, it sometimes lies in the thickness of the membrane in plants, so that its internal surface, which is directed towards the cell-cavity, is covered by a lamella of the cell-wall. In animals it also sometimes appears to be slightly sunk in the cell-membrane; but I have never observed a lamella passing over its inner surface ; on the contrary, in almost all instances it hes quite free, adherent only to the internal surface of the cell-membrane.
The particular stage of development of the nucleus at which the cell commences to be formed around it varies very much. In some instances the nucleus has already become a distinct vesicle ere it occurs; the germinal vesicle, for example; in others, and this is the most common, the nucleus is still solid, and its development into a vesicle does not take place until a later period, or perhaps the change never occurs at all. When the cell is developed, the nucleus either remains stationary at its previous stage of development, or its growth proceeds, but not in proportion to the expansion of the cell, so that the intermediate space between it and the cell-membrane, the cell- cavity, is also constantly becoming relatively larger. If the growth of a cell is impeded by the neighbouring cells, or if the new molecules added between the existing particles of the cell-membrane are applied to the thickening of the cell-wall instead of to its expansion, it may occur that the nucleus becomes more vigorously expanded than the cell, and gradually fills a larger portion of the cell-cavity. An example of this was brought forward at page 23, from the branchial cartilages of the tadpole; on the whole, however, such instances are very rare. As the nuclei, in the course of their development, and especially in such instances as that just mentioned, continually lose their granulous contents and become pellucid, and as in some cases, the germinal vesicle for example, other corpuscles, such as fat-globules, &c., may be developed in these contents of the nucleus (a circumstance which never occurs with respect to the cell-cavities) it is often difficult to distinguish such enlarged nuclei from young cells. The presence of two nucleoli is often sufficient to enable us to distinguish such an enlarged hollow nucleus. The observation of the stages of transition, between the characteristic form of the cell-nucleus and these nuclei which so much resemble cells, will also aid us in obtaining the information desired. As in the case of the germinal vesicle, however, a positive decision can only be obtained by demonstrating that such a nucleus has precisely the same relation to the cell that an ordinary cell-nucleus has; that is to say, that such a nucleus is formed before the cell, that the latter is formed as a stratum around it, and that the nucleus is afterwards surrounded by the cell. Whether the nucleus undergoes any further development, as the expansion of the cell proceeds, or not, the usual result is that it becomes absorbed. This does not take place, however, SURVEY OF CELL-LIFE. _ 179
in all cases, for, according to Schleiden, it remains persistent in most cells in the Euphorbiaceae, and the blood-corpuscles may be quoted as an example to the same effect in animals. The fact that many nuclei are developed into hollow vesicles, and the difficulty of distinguishing some of these hollow nuclei from cells, forms quite sufficient ground for the supposition that a nucleus does not differ essentially from a cell; that an ordinary nucleated cell is nothing more than a cell formed around the outside of another cell, the nucleus; and that the only difference between the two consists in the inner one being more slowly and less completely developed, after the external one has been formed around it. If this description were correct, we might express ourselves with more precision, and designate the nuclei as cells of the first order, and the ordinary nucleated cells as cells of the second order. Hitherto we have decidedly maintained a distinction between cell and nucleus; and it was convenient to do so as long as we were engaged in merely describing the observations. There can be no doubt that the nuclei correspond to one another in all cells; but the designation, “cells of the first order,” includes a theoretical view of the matter which has yet to be proved, namely, the identity of the formative process of the cell and the nucleus. This identity, however, is of the greatest importance for our theory, and we must therefore compare the two processes somewhat more closely. The formation of the cell commenced with the deposition of a precipitate around the nucleus; the same occurs in the formation of the nucleus around the nucleolus. The deposit becomes defined externally into a solid stratum: the same takes place in the formation of the nucleus. The development proceeds no farther in many nuclei, and we also meet with cells which remain stationary at the same point. The further development of the cells is manifested either by the entire stratum, or only the external part of it becoming consolidated into a membrane; this is precisely what occurs with the nuclei which undergo further development. The cell-membrane increases in its superficies, and often in thickness also, and separates from the nucleus, which remains lying on the wall; the membrane of the hollow cell-nuclei grows in the same manner, and the nucleolus remains adherent to a spot upon the wall. A transformation of the cell-contents frequently follows, giving rise to a formation of new products in the cell-cavity. In most of the hollow cell-nuclei, the contents become paler, less granulous, and in some of them fat-globules, &c., are formed. (See pages 173, 4.) We may therefore say that the formation of cells is but a repetition around the nucleus of the same process by which the nucleus was formed around the nucleolus, the only difference being that the process is more intense and complete in the formation of cells than in that of nuclei.
According to the foregoing, then, the whole process of the formation of a cell consists in this, that a small corpuscle (the nucleolus) is the earliest formation, that a stratum (the nucleus) is first deposited around it, and then subsequently a second stratum (substance of the cell) around this again. The separate strata grow by the reception of new molecules between the existing ones, by intussusception, and we have here an illustration of the law, in deference to which the deposition takes place more vigorously in the external part of each stratum than it does in the internal, and more vigorously in the entire external stratum than in the internal. In obedience to this law it often happens that only the external part of each stratum becomes condensed into a membrane (membrane of the nucleus and membrane of the cell), and the external stratum becomes more perfectly developed to form a cell, than the nucleus does. When the nucleoli are hollow, which, according to Schleiden, is the case in some instances in plants, perhaps a threefold process of the kind takes place, so that the cell-membrane forms the third, the nucleus the second, and the nucleolus the first stratum. Probably merely a single stratum is formed around an immeasurably small corpuscle in the case of those cells which have no nuclei.
Varieties in the development of the cells in different tissues. Although, as we have just seen, the formative process of the cells is essentially the same throughout, and dependent upon a formation of one or many strata, and upon a growth of those strata by intussusception, the changes, on the other hand, which the cells, when once formed, undergo in the different tissues, are, in their phenomena at least, much more varied. They may be arranged in two classes according as the individuality of the original cell is retained (independent cells), or as it is more or less completely lost (coalescing cells, and cells which undergo division).
The varieties which occur amongst the independent cells, are partly of a chemical nature, and partly have reference to a difference in the growth of the cell-membrane, by which means a change in the form of the cell may be produced.
The cell-membrane differs in respect to its chemical qualities in different kinds of cells. That of the blood-corpuscles, for instance, is dissolved by acetic acid, whilst that of the cartilage-cell is not. The chemical composition of the cell- membrane differs even in the same cell according to its age, so that a transformation of the substance of the membrane itself takes place in plants; for, according to Schleiden, the cell-membrane of the youngest cells dissolves in water, the fully-developed cells not being acted upon by that fluid. The simple cells are still more remarkable for their cell-contents. One cell forms fat, another pigment, a third etherial oil; and here also a transformation of the cell-contents takes place. A granulous precipitate is seen to form gradually in what was in the first instance a pellucid cell, and this usually takes place first around the cell-nucleus; or, vice versa, during incubation, the granulous (fatty) contents of the cells of the yelk-substance gradually undergo partial solution. According to Schleiden, this transformation of the substance of the cell-contents proceeds in accordance with a certain rule; I have not made any investigations upon the subject in animals.
We should also include under this head the formation of the secondary deposits upon the internal surface of the cell- membrane, so very frequently met with in plants. If a firm cohering substance be formed from the cell-contents, it may be deposited upon the internal surface of the cell-membrane. In plants this deposition generally takes place in layers, a stratum being formed in the first instance upon the internal surface of the cell-membrane, upon the internal surface of that one a second, and so on until at last the entire cavity may be almost filled by them. According to Valentin, these surrounding deposits always take place in spiral lines which are subject to great varieties in their arrangement, for there may be one or many of them, and they may either completely 182 SURVEY OF CELL-LIFE.
cover the internal surface of the cell-membrane, or not be in contact with each other at all. I have not observed any such secondary stratified depositions in animals.
The variations which may occur in the growth of the cell- membrane in simple cells, depend upon the circumstance as to whether or not the addition of new molecules takes place equably at all parts of the cell-membrane. In the first case the form of the cell remains unchanged, and the only other distinction possible would be grounded upon the fact as to whether the greater part of the new molecules were deposited between the particles which lay side by side upon the superficies of the cell-membrane, or between those which lay one behind another in its thickness. The first mode of growth produces an expansion of the cell-membrane, the effect of the second is more especially to thicken it. Both modes are gene- rally combined, but in such a manner that the expansion of the cell-membrane prevails in most instances.
A great variety of modifications in the form of the cells may be produced by the irregular distribution of the new molecules. The globular, which is their fundamental form, may be converted into a polyhedral figure, or the cells may become flattened into a round or oval or angular tablet, or the expansion of the cells may take place on one or on two opposite sides, so as to form a fibre, and these fibres again may either be flat, being at the same time in some instances serrated, or lastly, the expansion of the cells into fibres may take place on different sides so as to give them the stellate form. Some of these changes of form are, no doubt, due to mechanical causes. Thus, for example, the polyhedral form is produced by the close crowding of the spherical cells, and these, when separated from one another, sometimes assume their round figure again ; such is the case with the yelk-cells. Some of the other changes would seem to be capable of explanation by exosmosis. If, for example, the contents of a round cell be so changed, that a fluid is generated in it which is less dense than the surrounding fluid, the cell will lose some of its contents by exosmosis, and must, therefore, collapse, and may become flattened into a table as in the blood-corpuscles. Such explanations, however, are unsatisfactory in by far the greatest number of instances, and we are compelled to assume, that the growth does not necessarily proceed equably on all sides, but that the new molecules may be deposited in greater abundance in certain situations. Let us take the instance of a round _ cell, the cell-membrane of which is already developed, and suppose the deposition of new molecules to be confined to one particular part of the cell-membrane, that part would become expanded, and so a hollow fibre would grow forth from the cell, the cavity of which would communicate with the cell-cavity. The same result would take place, but more easily, if the new molecules were disposed unequally previous to the period when the external stratum of the precipitate, which is formed around the nucleus, had become condensed into a distinctly perceptible cell-membrane. The hollowing out of the fibre would then be less perfect, and the growth of the fibre must advance, particularly as regarded its thickness, before any manifest distinction between wall and cavity could be perceived.
The cause of this irregular disposition of the new molecules may, in some instances, be due to circumstances altogether external to the cell. If, for instance, a cell lay in such a position that one side of it was in contact with a concentrated nutritive material, one could conceive that side of the cell growing more vigorously, even though the force, which produces the growth of the cell, should operate equably throughout the entire cell. Such an explanation cannot, however, be received at all in most instances, but we must admit modifications in the principle of development of the cells, of such a nature, as that the force, which affects the general growth of the cells, is enabled to occasion an equable disposition of new molecules in one cell, and an unequal one in another.
Amongst the changes which more or less completely deprive the original cells of their individuality, are to be classed, in the first place, the coalescence of the cell-walls with one another, or with the intercellular substance; secondly, the division of one cell into several; and, thirdly, the coalescence of several primary cells to form a secondary one.
A coalescence of the cell-membrane with the intercellular substance, or with a neighbouring cell-wall, appears to take place in some cartilages for example. At first the cell-membrane has a sharply-defined external contour, by degrees the boundary line becomes paler, and at last is no longer perceptible with the microscope. We cannot, at present, lay down any general law respecting the circumstances under which such a coalescence occurs; it presupposes that the cell-membrane and intercellular substance are homogeneous structures, and may perhaps always take place when such a state exists.
As regards the subdivision of the cells, we have already seen how a jutting out of the cell-membrane may be produced by its more vigorous growth in certain situations. But a jutting inwards into the cavity of the cell may also result from the very same process. Now, if we imagine this jutting inwards to take place in a circular form around a cell, as the consequence of a partial increase in the force of its growth, it may proceed to such an extent, that one cell may be separated into two, connected together only by a short peduncle, which may afterwards be absorbed. This would illustrate the most simple form of subdivision in a cell. In the animal cells, however, which undergo subdivision, that is, the fibre-cells, the process is more complicated; firstly, because when an elongated cell subdivides, it splits into many fibres; and, secondly, because the cells are so very minute. The process, therefore, cannot for these reasons be accurately traced, and the following is all that we can detect : a cell becomes elongated on two opposite sides into several fibres ; from the angle, which the fibres on either side form with each other, a striated appearance gradually extends over the body of the cell ; this formation of striae becomes more and more distinct, until the body of the cell splits entirely into fibres.
The coalescence of several primary cells to form a secondary cell is, to a certain extent, the opposite process to the last. Several primary cells, of muscle for instance, are arranged close together in rows, and coalesce into a cylinder, in the thickness of which lie the nuclei of the primary cells. This cylinder is hollow and not interrupted by septa, and the nuclei le upon the internal surface of its wall. These are the facts of the process, so far as they have as yet been observed. One can form a conception of so much as is yet required to render them complete. If two perfectly-developed cells coalesce together, their walls must first unite at the point of contact, and then the partition-wall between the cavities must be absorbed. Nature, however, does not by any means require that these acts should occur at precisely defined periods. The coalescence may take place before the cell-wall and cell-cavity exist as distinct structures, somewhat in the following manner: the nuclei are formed first, around them a new stratum of substance is deposited, the external portion of which, in accordance with the course of formation of an ordinary simple cell, would become condensed into a cell-membrane. But in this instance the nuclei lie so close together, that the strata forming around them and corre- sponding to the cells, flow together, to form a cylinder, the ex-lternal portion of which becomes condensed into a membrane, just in the same manner as in simple cells, where merely the external portion of the stratum formed around the nucleus, becomes hardened on the outside into a membrane, in consequence of the reception of new molecules. There is, therefore, nothing in this which differs so very materially from the course of development of a simple cell; indeed, we seemed to be compelled to assume a similar process for the formation of the nuclei furnished with two or more nucleoli. (See page 176.) It is possible that there may be stages of transition between the ordinary simple cell and these secondary cells. It has been already mentioned at pages 117-118, that fat-cells occur in the cranial cavity of fishes, many of which contain two nuclei. It is possible that only one of them is the cytoblast of the cell, and that the second is a nucleus which has formed subsequently; but they resemble one another so completely in their characteristic position on the cell-membrane (see pl. III, fig. 10,) that perhaps they may both be cytoblasts of a cell which has been formed around both nuclei, in consequence of the external stratum of the precipitate having become condensed in such a manner that the membrane enclosed both nuclei. Meanwhile observation affords no demonstrative proof on the subject, and the similarity in the position of these two nuclei may be explained in another way. Fat thrusts all bodies which have imbibed water towards the outside of the cell, in order that it may assume its own globular form. If now a second nucleus should form in one of these fat-cells, it will be thrust towards the outside, and must gradually raise the cell-membrane into a prominence. It may also be observed, that opportunities of demonstrating the actual absorption of the fully-developed partition-wall between two cells do occur in the spiral vessels of plants.
THEORY OF THE CELLS.
The whole of the foregoing investigation has been conducted with the object of exhibiting from observation alone the mode in which the elementary parts of organized bodies are formed. Theoretical views have been either entirely excluded, or where they were required (as in the foregoing retrospect of the cell-life), for the purpose of rendering facts more clear, or preventing subsequent repetitions, they have been so presented that it can be easily seen how much is observation and how much argument. But a question inevitably arises as to the basis of all these phenomena; and an attempt to solve it will be more readily permitted us, since by making a marked separation between theory and observation the hypothetical may be clearly distinguished from that which is positive. An hypothesis is never prejudicial so long as we are conscious of the degree of reliance which may be placed upon it, and of the grounds on which it rests. Indeed it is advantageous, if not necessary for science, that when a certain series of phenomena is proved by observation, some provisional explanation should be conceived that will suit them as nearly as possible, even though it be in danger of being overthrown by subsequent observations; for it is only in this manner that we are rationally led to new discoveries, which either establish or refute the explanation. It is from this point of view I would beg that the following theory of organization may be regarded ; for the inquiry into the source of development of the elementary parts of organisms is, in fact, identical with the theory of organized bodies.
The various opinions entertained with respect to the fundamental powers of an organized body may be reduced to two, which are essentially different from one another. The first is, that every organism originates with an inherent power, which models it into conformity with a predominant idea, arranging the molecules in the relation necessary for accomplishing certain purposes held forth by this idea. Here, therefore, that which arranges and combines the molecules is a power acting with a definite purpose. A power of this kind would be essentially different from all the powers of inorganic nature, because action