Microscopical Researches/CLASS V. Tissues generated from cells, the walls and cavities of which coalesce together
CLASS V.
Tissues, generated from cells, the walls and cavities of which coalesce together.
The following is the type of formation in this class: independent cells, by which we mean such as have a special wall and cavity, are present in the first instance; these we shall call primary cells. They are either round or cylindrical, or of a stellate figure. When round or cylindrical, the primary cells are applied together in rows, the contiguous portions of the cell-walls then become blended, in such manner that merely simple septa remain, dividing each succeeding cell-cavity from its neighbour. These septa, however, become absorbed, so that the cavities of the different cells communicate. Instead of a number of primary cells, we then have one single long one, which we shall call a secondary cell. The cavity of such a one, therefore, consists of the united cavities of the original cells, and its cell-membrane of all their blended cell-mem- branes, except that the parts with which they were in contact are absorbed. ‘The growth of the secondary cell proceeds hike that of any simple independent cell. This appears to be the process of formation in muscle and nerve, so far, at least, as the observations, which will presently be communi- cated, extend. When the primary cells have a stellate figure, their bodies are not applied in rows, as in nerve and muscle, but are generated in larger interspaces filled with cytoblastema or with cells of another kind. ‘Their prolongations, however, come in contact, the walls coalesce at the points of junction, and the blended septa then become absorbed, so that the cell-cavities, which were at first separated, now com- municate. In this manner, when several prolongations of one cell come into contact with those of another, or of several others, we obtain, in the place of isolated, hollow, stellate cells, a network of canals, which are, in the first instance, somewhat thicker at the parts corresponding to the bodies of the cells, but become of pretty equal dimensions, in consequence of more vigorous expansion of the communicating prolongations. This appears to be the mode in which the capillary vessels are formed. The following detailed statement of observations upon the relation which muscles, nerves, and capillary vessels bear to elementary cells, will show how far the description just given, as the probable mode of formation, is to be regarded as proved by these, as yet, very incomplete researches, and will also indicate what deficiencies have yet to be supplied.
1. Muscle. To ascertain the relation which this tissue bears to the elementary cells, we must have recourse to the history of its development. I was, unfortunately, prevented from investigating the earliest formation of muscular fibre, in consequence of not being able to obtain any very young embryos; but the deficiency in my researches may be supplied from the description given by Valentin (Entwicklungs-Geschichte, p. 268), from which the following passage is extracted: “Long before separate muscular fibres can be discerned, the globules of the primitive mass are seen, arranged in parallel lines, particularly when they are lightly pressed between two pieces of glass. The granules then appear to be drawn somewhat nearer together, to become completely coalesced, in some situations, while at others the blending takes place only on the one or the other side, and to be combined into one transparent mass. In this way filaments are formed, which, in some situations, have an appearance like strings of pearls, at others, on the contrary, are less sharply indented; they often also continue slightly puckered on one side, whilst the margin of the other has already become more straight. At a subsequent period, all trace of granules or division in the filament vanishes, and its outline becomes symmetrically transparent and cylindrical. The muscular fibre usually undergoes no other change until the sixth month, except that its substance becomes somewhat darker and its cohesion closer. The first traces of transverse striæ are exhibited in the sixth month. These fibres are the primitive fasciculi of muscle and not the primitive fibrils, which latter are formed by a splitting of the fasciculus into smaller fibres. From the period at which the muscular filaments become transparent and uniform, masses of globules, of a more or less spherical form and somewhat larger than the blood-corpuscles, begin to accumulate between them. They diminish again afterwards, and, blending with the gelatiniform mass which connects them, become converted into the connecting areolar tissue.”
The youngest embryos in which I have investigated the generation of muscle were those of the pig, measuring three and a half inches in length. If a portion of one of the superficial dorsal muscles be removed from an embryo pig of that size, and examined under the microscope upon a black ground, a transparent gelatiniform mass is observed, in which parallel fibres (primitive fasciculi of muscle) run in close contact, having a whiter appearance than the surrounding gelatinous substance. As development proceeds, the transparent substance diminishes in quantity, the muscular fibres lie closer together, and have a more intensely white appearance upon the black ground. When some of this transparent substance, taken from a foetus of the size before mentioned (and in order to exclude as completely as possible the embryonal cellular tissue which surrounds the entire muscle, a portion should be cut out from the centre of the muscle), is examined with a magnifying power of 450, it exhibits various kinds of granules differing m size, and lying in a finely granulous mass. On examining these granules more accurately, they are found to vary, both in size and appearance, being round or oval, more or less opaque or transparent. A great many of them may be recognized as cell-nuclei by their form. In many instances, even when they are still connected together, the granulous substance around them is more or less distinctly seen to have a defined globular figure, within which the nucleus lies. This is, however, observed most distinctly when any of the granules become separated from the transparent substance, and float about in the fluid upon the object-glass. A quantity of globules are then seen floating about isolated, each one containing the characteristic cell-nucleus, which is placed eccentrical, varies much as to its size, and is often furnished with nucleoli. (See pl. III, fig. 18.) We are already familiar with this as the rudimentary form of most cells. The finely granulous portion of the transparent mass is formed, in part, of the bodies of the cells, which, when in close contact, are difficult to distin- guish, and in part, of the cytoblastema in which the cells have been generated. Some of these cells which float about are becoming elongated into fibres, which are manifestly those of areolar tissue. Such instances, however, are rare, and these cells seem to be something quite peculiar. They might be regarded as the primitive cells of new muscular fibres; but from the manner in which Valentin expresses himself, one should infer that they are formed at a later period, for he says, “masses of globules begin to accumulate between the muscular fasciculi from the period at which they become transparent;” it is clear that he here refers to the nuclei of these cells. This must, therefore, remain an undecided point.
We next examine the muscular fibres (primitive fasciculi) in the dorsal muscles of the same foetus. They do not all resemble one another in general character ; some are more irregular, more granulous, whilst others are relatively smooth. The smoother ones represent cylinders, which are generally more or less flattened (see pl. IV, fig. 3), in which they are delineated from the brachial muscles of a foetal pig seven inches in length, a representing the flat surface, b the marginal. The cylinder a presents a dark margin, and an internal clear portion, a distinction which is yet more manifest in c, where the dark margin is broader and sharply defined on its inner edge, so that it has quite the appearance of a hollow cylinder. I must, however, remark, that but very few fibres present this appearance sufficiently distinct to satisfy the mind of the ob- server. But in many instances it was so manifest, that no other explanation seemed left than to suppose the fibre a hollow tube. In the clear portion of the cylinder, which corresponds to the cavity, (in addition to some small granules,) larger oval corpuscles are seen, which are often very much extended in the longitudinal direction. Their form at once shows them to be nuclei, and they frequently contain one or two nucleoli. The distance at which they lie from one another is more or less regular in different instances. They do not lie in the axis of the fibre, but eccentrically, upon and within the thick- ness of the wall, as is seen when the fibre rests upon its margin. (See the fibre b.) That delineation exhibits a regu- larity in their position, since a nucleus lies upon the one side of the wall, the second on the opposite, and the third again upon the first side, and so on; such, however, does not appear to be the case in every instance. The nuclei are flat, for when viewed edgeways they have the appearance of mere stripes. The thickness of the wall of the cylinder seems to vary, as is shown by a comparison of a with c. The latter, c, the wall of which is the thicker, already presents an appearance of transverse striae. The nuclei, however, are also still visible in it, as well as small isolated globules which are contained in its cavity. Muscular fibre does not present any appearance of a cavity after the period of development before mentioned has passed, but the nuclei remain visible for a long time, lying in the thickness of the fibre, and often project upon the outside in the form of small prominences.
The other form of muscular fibre is delineated in pl. IV, fig. 1, from the dorsal muscles of a foetal pig of three inches and a half in length. They are in general somewhat thicker than those last described, more irregular, not so smooth, but more granulated. The existence of a special wall to the fibre and of a cavity in its interior, may be quite as distinctly, or even more clearly, recognised in many of these. (See the fibre c in fig. 1.) The wall is not so smooth as in the other form of muscular fibre. The contents are always very granulous. Distinct cell-nuclei, and not unfrequently nucleoli also, even in the natural state, may often be perceived in them. Com- monly, however, only the circular or oval outlines of the nuclei are distinctly perceptible, in consequence of the other granules which are contained in the cavity of the fibre lying above them, and the general granulous nature of the fibre renders an accurate discernment of the nucleus particularly difficult. But if a drop of acetic acid be added, the fibre becomes perfectly transparent, and swells; the nuclei, on the contrary, remain dark, shrivel up slightly, and may then be distinguished with perfect accuracy. This is exemplified by fig. 2, which represents the fibre c of fig. 1 after having been treated with acetic acid. The indubitable cell-nuclei, partially furnished with nucleoli, are there seen, with isolated small dark granules between them. The nuclei have indeed under- gone a slight change from the acetic acid, but they do not all present a regular aspect even in the recent state. The majority of them are flat. In the recent state, some appear to be placed on their edges, presenting an appearance as though the cavity of the fibre were divided into compartments by small thick transverse striae. The nuclei he much nearer together in this than in the form of muscular fibre previously described, so that the distance of the central points of two nuclei from one another, is generally equal to, or even less than, the thickness of the fibre.
This second form of muscular fibre appears to be an earlier condition of the first. The younger the embryo the more abundant is this form of fibre, and it gradually becomes less so as development proceeds. ‘The steps of this transition may readily be conceived. The fibre becomes extended in its entire length, is thereby rendered thinner, the cell-nuclei are removed farther from one another, and in some instances are also clongated in the direction of the fibre. Some of the nuclei, those for instance which appear to be placed on their edges, may possibly become absorbed at the same time, for they never present that position at a later period. The development of the whole cylinder proceeds simultaneously, its granulous aspect disappearing, and the small granules of the cavity also diminishing in quantity. All the stages of transition from the second form into that first described may be observed. The extension does not appear to take place quite regularly, but may be stronger at particular parts, so that, for a considerable extent, a fibre may be somewhat narrow, and present no nucleus, and then again an intumescence oc- curs in which a nucleus lies.
It now, however, becomes a question how the form of muscular fibre last described is generated, and what its elementary form may be. It presented a cylinder, which is most probably hollow, and may be presumed to be closed at both ends, since the muscular fibres terminate abruptly at the tendons, with a well-defined and bluntly-rounded extremity. Cell-nuclei lie within this cylinder at very small distances from one another. Is the cylinder an elongated cell, in which nuclei are formed as the rudiments of new cells, which, however, are not deve- loped; or are the nuclei the remains of cells, which, by coalescence with one another and absorption of the septa, form the entire fibre or cylinder? Or, in other words, is the fibre generated by a coalescence of cells?
I have not observed the stages of transition m which original cells arranged themselves in a linear series to form a fibre, the recent embryos at my command not being sufficiently young for the purpose. I have, indeed, met with an appearance in the form of muscular fibre last described, which might be regarded as an indication that those fibres are composed of small portions joined together. Their margins were incurvated at different spots, and a line, indicative of a division, ran transversely across the entire thickness of the fibre. I have endeavoured to delineate this appearance in pl. IV, fig. 1, b, but I have not succeeded in representing its true character, and it was not, in itself, conclusive. There are some other argu- ments in favour of the fibre of muscle being composed of separate particles. Many of the muscles of fishes or tadpoles, for 136 MUSCLE.
instance, when simply torn, separate into microscopic particles, which have an almost similar length. The same takes place, according to C. H. Schultz, during the digestion of muscle in the stomach, and, according to Purkinje, in muscle which is exposed to the action of an artificial digestive fluid. The observations of Valentin, already mentioned, admit, however, of no other explanation than that previously given; and the history of the period of formation deficient in my researches (from the cause before stated) may be completed from his. According to him, “globules of the primitive mass, arranged longitudinally, in a linear series, are present previous to the muscular fibres. The granules, then, seem to draw somewhat nearer together, and to coalesce, at some parts completely, at others, on the contrary, only on the one or other side. In this manner threads are formed, which present at some spots the appearance of strings of pearls, whilst at others they are less sharply indented; they are also often seen to be still wrinkled on one side, while on the other their margin is already nearly a straight line. The expression "granules of the primitive mass" (Urmasse), or other similar terms, have been hitherto used to denote either the elementary cells themselves or their nuclei, indiscriminately; in consequence of the distinction between them, and their relation to each other being unknown. In the passage quoted, Valentin cannot have meant the nuclei, for, as we have seen, they do not coalesce. What he calls globules of the primitive mass must, therefore, be the elementary cells furnished with their nuclei, and in their earliest stage of development; that is, before they have undergone any transformation. The following arguments may likewise be adduced in favour of the correctness of the explanation which assumes these “ globules of the primitive mass” to be cells. In the first place, the structure formed by their coalescence, namely, the primitive fasciculus of muscle, is hollow; and, secondly, in the early stage of development of the fasciculi, the cell-nuclei le just so closely together, as they would if each nucleus had pertained to a previously round cell. If these nuclei were subsequent formations, generated in the primitive fasciculus of muscle, as in a cell, they ought to be more numerous in old than in young muscles.
It, therefore, seems scarcely to admit of a doubt, that each primitive muscular fasciculus is a secondary cell, formed by the coalescence of primary round cells, each furnished with a nucleus, and which were arranged together in a row. After the coalescence of the contiguous portions of the cell-walls has taken place, an absorption of the septa remaining between the cavities of the two neighbouring primary cells must commence, since no such septa can be perceived within the secondary cell at a later period. If the little transverse striae, by which the cavity of the fibres is sometimes divided, be actually nuclei placed transversely upon their edges, they are probably such as lay upon that part of the wall of the cells which was ab- sorbed. It seems that the coalescence of the cells, however, is not sufficiently complete to prevent a separation taking place more readily at the points of junction than elsewhere, and on this the phenomena of the artificial division of muscle before mentioned probably depend. [1]
When I made my first communication upon the formation of the primitive fasciculi of muscles by the coalescence of cells (Froriep’s Notizen, No. 103), the only corresponding instances known to exist among vegetable cells were those of the spiral and lactiferous vessels. The interest attached to the subject has very much increased since Meyen’s discovery of a much more striking analogy in the cells of the lber or imner bark —(bastzellen). (Wiegmann’s Archiv, 1838, p. 297.) He found that these long-extended cells, when boiled in hydrochloric acid, fell into small particles of nearly equal length; and investigation into the development of the cells of the liber in buds showed, that in the early period a corresponding quantity of distinct, somewhat longitudinally extended, prismatic, pa- renchymal cells are present, which are placed with their extremities accurately arranged one upon another, that they unite together at those parts, and that their septa are after-wards absorbed.
The secondary muscle-cell passes subsequently through all the changes incident to a simple cell. Its wall is at first thin, and it contains many small granules in its cavity in addition to the nucleus. A transformation of the cell-contents then takes place, the granules gradually disappearing ; the wall of the cell at the same time becoming thicker at the expense of the cavity, so that eventually the latter completely disappears, and the entire secondary cell is converted into a solid cord. The cell-nuclei at first remain whilst this thickening of the cell-wall is going on, and become enclosed by it, rather than pushed into the cavity of the cell. They are at length entirely absorbed. Is, then, the thickening of the wall of the secondary muscle-cell a thickening of the cell-membrane itself, as appeared to be the case in cartilage? or is it a secondary deposit upon its inner surface, so that the cell-membrane is chemically and microscopically distinct from the substance, by means of which the secondary cell becomes converted into a solid cord? The latter is the more usual case in vegetables. The position of the cell-nuclei affords important evidence for the solution of the above question ; for as those bodies, gene- rally at least, lie firmly attached to the inner surface of the cell-membrane, they would be pushed towards the interior by a thickening of the cell-membrane itself, whilst a secondary deposit upon its inner surface, must enclose and fix them there, unless they should become separated altogether from the cell-wall. Now, in muscle, they actually remain lying in the circumference of the fasciculus, as represented by pl. IV, fig. 3, 6. This fact, then, renders it probable that the thickening of the wall of the secondary muscle-cells is due only to a secondary deposit. Such a supposition must, however, have been adopted, independent of the argument just raised, since the muscular fasciculi are, as it seems, enclosed by a structureless membrane. ‘The fasciculi have been long described as invested by a sheath, but that investment has been considered to be composed of cellular tissue, and to correspond in the primitive fascicul to the cellular tissue, by which the larger fasciculi are separated from one another. This membrane seems, however, to have quite a different signification, and to be the cell-membrane of the secondary muscle-cell. It is structureless, very transparent, and appears as a very narrow and sharply-defined border around each primitive fasciculus. I well know how readily such an appearance is produced by a mere optical deception, and that one can never be positive with respect to it unless it be observed that the margin in question does not accurately follow every bend of the fasciculus. It is, therefore, difficult to be convinced of this in mammalia; but in all those larve of insects which present the broad transverse striae of the fasciculi, discovered by Müller, the membrane, when the continuity of the proper muscular substance of a primitive fasciculus has been broken at a certain point, may be distinctly observed passing over uninterruptedly from the one portion to the other. Pl. IV, fig. 4, represents such a fasciculus ; the membrane encompasses it so loosely (this larva had been preserved in spirits of wine) that a portion of the muscular substance could even change its position within the cavity. The membrane, where entirely isolated from the other parts of the preparation, shows itself to be quite structureless, and, indeed, the sharply-defined external contour renders it very improbable that it should be composed of areolar tissue. I, therefore, consider it extremely probable that it represents the cell-membrane of the secondary muscle-cell. It thus not only serves to isolate the fasciculi, but forms an essential constituent part of them. Pl. IV, fig. 5, exhibits this structureless membrane upon a muscular fasciculus of the pike; this preparation, however, was not quite convincing, inasmuch as the inferior edge of the fasciculus was covered by muscles lying above it. By means of this mem- brane, the muscular fasciculus remains, throughout its entire existence, a cell with a closed membrane and a cell cavity, the latter being filled with a firm substance, the peculiar muscular substance. It, therefore, clearly follows from the above that nervous fibres cannot pass between the primi- tive fibres (fibrils) of muscle ; and that the latter cannot separate from their fasciculi, so as to pursue a more extended and independent course, as is common with fibres of areolar tissue ; since, in either case, the cell-membrane must be ruptured.
The true muscular substance, which is thus, in the first place, formed as a secondary deposition upon the inner surface of the secondary muscle-cell, and continues to be so deposited until the entire cavity is filled, is composed in its mature con- dition, of very minute longitudinal fibres, the so-called primi-e tive fibres (fibrils) of muscle. These longitudinal fibres do not appear to represent the original condition of the secondary deposit, but the latter is structureless at first, and its trans- formation into fibres takes place subsequently. The change seems, however, to commence at a very early period, and indeed before the cavity is completely filled. The transverse strize of the muscular fasciculi, which, according to my mode of explanation, are produced by the peculiar form of the primitive fibres, likewise make their appearance before the complete filling up of the cell-cavity, as pl. IV, fig. 3, c, exhibits.
According to the observations of Meyen on the formation of the cells of the liber, after the coalescence of the cells and absorption of the septa, a secondary deposit also takes place upon the common cell-membrane in the same way that we have observed to take place in muscle; but I know of nothing amongst vegetables analogous to a secondary deposit consisting of longitudinal fibres. On the contrary, according to Valentin, such deposits appear to take place in plants universally in spiral lines. The beaded appearance which the primitive muscular fibres here and there present, might perhaps be regarded as the result of this tendency to a spiral formation, the intumescences (beads) being so placed, as to produce the transverse striz, and the latter may perhaps be spiral and not circular. This is, however, a mere conjecture, and requires further research.
The involuntary muscles, such as do not present the transverse strie, appear to originate in a manner similar to that just described. They differ, however, from the voluntary or striated muscles, in their fibres being generally shorter than those of the latter; probably, therefore, fewer primary cells arrange themselves together to form a secondary cell, and their fibres are commonly thinner and flat. I found in a human uterus, which contained a mature foetus, some long muscular fibres of the breadth of the common primitive fasciculi of voluntary muscles, which were so flat as scarcely to amount to 0:0010 to 0:0015 of a line in thickness. The involuntary muscles, likewise, have cell-nuclei, proving that the fibres composing them do not correspond to the primitive fibres (fibrils), but to the primitive fasciculi of the voluntary muscles. An opposite view of the matter might be taken from the circumstance of their frequently exhibiting no trace of longitudinal striz, and that probably the greater portion of them do not contain other more minute primitive fibres, or at least only such as are imperfectly developed. In this respect they are not so highly developed as the voluntary muscles. Perhaps the peculiar secondary deposit upon the cell-membrane of the secondary cell is all that is essential to the contraction of muscle; and it may not be important that that substance should consist of minute longitudinal fibres.
In order briefly to recapitulate our researches into the generation of muscle, the process may be thus stated. Round cells, furnished with a flat nucleus, are first present, the primary cells of muscle. These arrange themselves close together in a linear series; the cells thus arranged in rows, coalesce with one another at their points of contact ; the septa, by which the different cell-cavities are separated, then become absorbed, and thus a hollow cylinder, closed at its extremities, the secondary cell of muscle, is formed, within which the nuclei of the original cells, from which the secondary cell has been formed, are contained, generally lying near together on its wall. This secondary cell, then, passes through all the stages of a simple one. It expands throughout its entire length, whereby the nuclei are farther removed from one another, and sometimes even become elongated in the same direction. A deposit of a peculiar substance, the proper muscular substance, takes place at the same time upon the inner surface of the cylinder, by which the cavity is at first narrowed, and at length completely filled. The cell-nuclei lie external to this substance, between it and the cell-membrane of the secondary cell.
The transverse striae in the voluntary muscles become more manifest, and the deposited substance is more distinctly seen to be composed of longitudinal fibres, as the foetus advances in age, The nuclei are gradually absorbed. The cell-membrane of the secondary muscle-cell remains persistent through- out life, so that each primitive muscular fasciculus is always to be regarded as a cell.
2. Nerves. The nervous system presents two forms of elementary structure: 1st, fibres, nervous fibres in the extended sense of the term, including the fibres of the brain and spinal cord: 2d, globules, ganglion-globules, in addition to the ganglia occurring in the brain and spinal cord. Our task is to point out the relation which these two forms of elementary structure bear to the elementary cells.
Nervous Fibres.
Of these, there are two different forms: a, the common white nervous fibres ; b, the gray, so-called organic fibres.
a. White nervous fibres. They have the appearance of fibres, which, when examined microscopically, exhibit very dark margins, and these margins are produced by a substance apparently identical with that which gives them their white colour when examined with the unaided eye. Since the cause of this colour does not appear to be situated in the whole fibre generally, but to be confined to its external portion, this latter may be termed the white substance of the nervous fibres. The margin of a fibre generally presents a double outline on both sides, so that it has the appearance of a hollow tube, and the distance between the two outlines, then, denotes the thickness of the white substance. According to the researches of Remak, the white substance of every nervous fibre may be removed by pressure, and an extremely pellucid, pale band, which was previously surrounded by the white substance, then remains, corresponding to that which, previous to the manipulation, seemed to be the contents of the tube. (See R. Remak, Obss. Anat. et Microsc. de Syst. Nerv. Struc., Berol. 1838.)
Two opinions with respect to the nervous fibres may be deduced from the above observations ; either this pale band is the proper nervous fibre, and the white substance only a sheath (cortex) around it (this is the view taken by Remak), or the nervous fibre is actually a hollow fibre, the wall of which is formed by the white substance, the contents of which, however, are not fluid, but composed of a tolerably firm substance, namely, the above-mentioned band.
The history of the development of the nervous fibres must explain the relation which they bear to the cells. Remak [2] describes the early condition of the nerves in the following manner: “The substance of the cerebro-spinal nerves of the rabbit, in the third week of embryonal existence, consists of corpuscles, some of which are irregularly spherical, others slightly elongated, having a very delicate filament adhering to them; they are mostly transparent, and arranged in rows without, however, presenting any distinctly perceptible fibrous structure.” And 1. c. page 153, he says, “ A structureless and general globular mass is the original form, from which the primitive fibres of the cerebro-spinal nerves are developed. These primitive fibres are at first varicose, and contain no medulla; most of them pass into the cylindrical form, through the intermediate stage of transitional fibres.”
I have investigated the development of nerve in the foetal pig. The nerves of the foetus have not the shining white colour, presented by those of the adult animal, but are gray and transparent, and the younger the embryo the more striking are these appearances. We are, therefore, quite prepared to find that microscopic investigation shows the white substance of the fibres to be less perfectly or not at all developed. If a nerve, taken from a foetal pig of about six inches in length, be spread out, in the usual mode of preparation by tearing it under water, some fibres are seen which very much resemble those of the adult animal, and which are furnished with outlines almost as dark. The greater part of the substance, however, does not form connected fibres, but consists of separate round globules, or more or less long, irregular little cylinders, arranged with their long axes in the direction of the course of the nerves, having outlines, however, quite as dark as those of the nervous fibres. These appear to be what Remak refers to in the description previously quoted. In addition to them, however, a substance of quite another appearance is seen, which has not the dark outline, does not appear pellucid but granulated, and in which the cell-nuclei are distinctly recognisable.
When the other constituent parts predominate, the nuclei may very probably be overlooked, or possibly be regarded as extraneous substances. But they are in fact the primitive structure of nerve, for the younger the foetus the greater is their relative quantity, and in a pig’s foetus of three inches in length, I found them the sole constituent of nerve, none of the fibres furnished with the dark margins, nor any of the cylinders or globules being visible at that period of development. The development of nerve, however, does not appear to proceed uniformly in all individuals; for the dark globules and cylinders were already present in some other pigs’ em- bryos, which were scarcely any larger. Pl. IV, fig. 6, represents a portion of the ischiatic, and fig. 7, of the brachial nerve of such a foetus. We observe a palish, and very minutely-granulated cord, which, in consequence of certain longitudinal shadings, such as the delineation exhibits, presents the appearance of a coarse fibrous structure. Round or for the most part oval corpuscles, which are immediately recognised as cell-nuclei, and which sometimes also contain one or two nucleoli, are generally seen in the course of these shaded parts, throughout the entire thickness of the cord. Sometimes a fibre separates from such a cord, and stands out isolated, as at a in both the figures, and the nuclei are then seen to lie in the course of the fibres. A single fibre presents several nuclei in its course, as was also observed in secondary muscle-cells (see fig. 8, 5), but I have never remarked it in the cells of the fourth class, the fibre-cells. Although the (nervous) fibres cannot at this early period be distinctly perceived to be hollow, the wall not being distinguishable microscopically from the contents, yet we shall see that the progress of development renders it highly probable that they are so. If then these (nervous) fibres are so far analogous to the early condition of secondary muscle-cells, that they are hollow, and in various parts of their course contain nuclei, whose form shows them to be ordinary cell-nuclei, it is probable that they are generated in a similar manner to muscle; that is, that they are formed by the coalescence of primary cells, to which the nuclei, just noticed as present upon the fibres, have pertained; so that thus the nervous fibres would be secondary cells, cor- responding to the secondary muscle-cells, or primitive muscular fasciculi. The actual observation of the primary cells of nerve in their independent state, is very difficult, from the circumstance of our being unable at that period to distinguish between them and the surrounding tissues; for a whole organ is then composed entirely of independent cells, which have not as yet undergone any transformation. It is true I saw an independent cell, furnished with a nucleus, which seemed to have separated from the nervous cord, in one of the preparations alluded to, fig. 6 b; but I cannot positively assert that it had actually separated from that particular part, nor that it was a primary nerve-cell, for the cells in that preparation had not as yet undergone any change. In this instance, therefore, we must content ourselves, for the present at least, with the analogy to muscle.
These fibres, or secondary nerve-cells, differ very much in their appearance from the subsequent nervous fibres, which are furnished with distinct but not dark outlines; they have a pale, granulated aspect. By progressive development, however, they become converted into the white fibres, and pl. IV, fig. 8, d, represents the transition. The part of the figure to the right hand exhibits the fibre yet in the early condition, pale, granulated, and furnished with a cell-nucleus; in the portion to the left, it has completely assumed its subsequent form: it has a dark outline, is not granulated, and the one portion passes immediately into the other. The identity be- tween these pale fibres and the subsequent white nervous fibres is thus established.
In what then does this transformation of the pale granulated fibres into the white fibres consist? Clearly in the development of the white substance; we may, however, imagine three different modes in which this development may take place. It may take place, 1stly. By the white substance form ing as a sheath (cortex), around each fibre, and in this manner enclosing it. By this mode of explanation the fibre would be identical with the pale band discovered by Remak, which would therefore be the cell-membrane itself. 2dly. The white substance might be regarded as a transformation and thickening of the cell-membrane of those fibres, or secondary nerve-cells. According to this view, the white substance would be the cell-membrane, and Remak’s band the firm contents of the secondary cell. 3dly. The white substance may be formed as a secondary deposit upon the inner surface of the cell-mem- brane, being chemically distimct from the latter, and the remainder of the cell-cavity may then, and not until then, become filled up by Remak’s band.
It will be seen that the above question is analogous to that raised when we were treating of muscle, viz., whether the proper muscular substance be a thickening of the original cell-membrane itself, or a secondary deposit upon it. The reply is not, in either instance, essential to the proof of the origination of nerves or muscle from cells, but it is of so much the more importance for the explanation of the structure of a perfectly-developed nerve. If any conclusion may be drawn from the few observations which I have made on this point, the latter view appears to me the most probable, viz., that the white substance is a secondary deposit upon the inner surface of the cell-membrane. The white substance of each nerve is surrounded externally with a structureless and peculiar membrane, which appears to be minutely granulated. This membrane presents itself as a narrow, clear border, which is readily distinguished from the dark contours of the white substance. This membrane seems hitherto to have been included with the neurilema or with the cellular tissue, which surrounds the nervous fibre, and although its external outline is generally very sharply defined in the nerves of the frog, it would be difficult, on examination of the entire nerve of a mammal, to arrive at any conviction of its distinct and separate existence, did not opportunities of observing it in an isolated state present themselves. Pl. IV, fig. 9 a, represents such a preparation, taken from the cranial portion of the nervus vagus of a calf. The continuity of the white substance has here been broken by the process of preparation; but where it still exists, the double contours, (and thus the thickness of the white matter), may be clearly distinguished. But the nerve still exists at the part where the white substance is separated, its sharply-defined external margins may be seen, although their contours are but pale, and it may be observed that this pale outline does not pass into the external dark one of the white substance, but is continued on the outside of it as a narrow border, parallel to the two outlines of the white substance. The white substance of nerve is, therefore, surrounded externally with a thin, pale membrane, which has a sharply-defined external margin. If the membrane be very thin, it cannot be recognised as the pale border round the nervous fibre; it is still, however, distinctly visible at situations where the white substance is destroyed. (See fig. 9 b.) The mere fact of the membrane possessing a defined external border, is evidence against its being composed of areolar tissue ; and even the portion which does not contain any white substance, presents no appearance of a fibrous structure ; it simply appears to be somewhat minutely granulated. If this be correct, the membrane can have no other signification than that of cell-membrane of the nervous fibre, or secondary nerve-cell. The white substance is then a secondary deposit upon its inner surface. The position of the cell-nuclei is also favorable to this view. Most of the cell-nuclei, presented by the nervous fibres in their earliest and as yet pale condition, disappear during the formation of the white substance, a circumstance which is common to most other cells. Some, however, appear to remain for a longer period; occasionally, although rarely, a cell-nucleus is here and there seen upon the side of a nerve, (the white substance of which is completely developed), lying in the pale border, which surrounds the white substance. Fig. 9, c and d, exhibits them from the nervus vagus of a calf. At c the white substance, corresponding to the nucleus, even forms a slight projection into the cavity of the fibre. This nucleus seems therefore actually to belong to the fibre, and to lie upon the inner surface of the cell-membrane, while the white substance is so deposited, that the nucleus remains situated external to it. The band discovered by Remak would then be the proper cell-contents. Meanwhile I beg that the above may be regarded simply as an attempt at an explanation, the accuracy of which must be decided by further researches, for much more extensive investigations and a separate and distinct consideration are absolutely necessary for accurate decision of so important a subject.
According to the foregoing explanation, therefore, each nervous fibre is, throughout its entire course, a secondary cell, developed by the coalescence of primary nucleated cells. With respect to these cells, we remark, 1stly. An external, pale, thin cell-membrane, having a granulated but not a fibrous aspect, the inner surface of which constantly exhibits cell- nuclei in the very early period of the development of nerve ; but in the somewhat more advanced stage, when the white substance is developed, they are only occasionally found. 2dly. That the white, fat-like substance to which the peculiar appearance and distinct outline of the nerves are chiefly referable, is deposited upon the inner surface of this cell-membrane. When this deposit is thick, its double contour (to which the nerve is indebted for its tubular appearance), may be recognised; this, however, escapes observation when only a thin stratum of white substance is present. Morphologically considered, it therefore corresponds to the peculiar substance of muscle, for that is likewise developed as a secondary deposit upon the membrane of the secondary muscle-cell. 3dly. That the rest of the cell-cavity appears to be filled up by a firm substance, namely, the band discovered by Remak. There seems to be no structure analogous to this band in perfectly-developed muscles, for there, the secondary deposit, that is, the formation of the proper muscular substance, proceeds until the cavity of the secondary cell is completely filled.
We have thus traced the development of nerve to its perfect state, without those irregular globules and little cylinders with the dark outlines, (which were mentioned at page 143, as occurring at a middle stage of the development of nerve in addition to the pale fibres and the matured nervous fibres), having proved to be a transitional step in the process. I am inclined to regard them as an artificial product, caused by pressure and the action of water upon the as yet very delicate nerve. If, for example, water penetrate through the cell-membrane by imbibition, the oil-like white substance retracts into separate rounded bodies, and the facility with which this takes place is proportionate to its slight degree of consistence. This is often seen even in fully-developed nerves; an entire nerve frequently separates from this cause into separate globules or little cylinders, which have sharply-defined outlines, so that merely the cell-membrane proceeds uninter- ruptedly, in the form of a pale stripe, from the external wall of one of the dark portions to that of the other. Valentin has given a delineation of such a state of the nervous fibre, (Acta Acad. Leopold. Nat. Curios. vol. xviii, pl. III, fig. 7). As the white substance is less consistent in the foetus, it separates the more readily, and the artificial generation of such globules is very easy of observation in foetal nerves.
The growth of nerves neither proceeds from the circumference towards the central organs, nor vice versd, but their primary cells are included amongst those from which every organ is formed, and which, so far at least as their appearance is concerned, present no marks by which they can be distinguished from other cells. They are first characterized as nerves, when they become arranged in rows and coalesce to form a secondary cell. After that coalescence each nervous fibre forms a separate cell, which pursues an uninterrupted course from the organ, in which its peripheral extremity is situated, to the central organ of the nervous system. The white substance of nerves does not appear to be formed at so early a period in their peripheral extremities, as it is in their trunks. The Medizinischen Zeitung for August 1837, contains a description which I gave of some nerves from the tail of frog’s larve, which presented an appearance quite different from ordinary nerves, inasmuch as they had a pale contour-and no perceptible cavity. They were nerves in an early stage, previous to the development of the white substance. They represent the only form of nervous matter which we find in the tail of very young larve. Some isolated nerves, having the ordinary appearance of the dark contours, gradually make their appearance, and afterwards increase in quantity; they were first observed in the neighbourhood of the muscular fas- ciculus which traverses the middle of the tail. The development of the white substance appears therefore to advance from the trunks towards the circumference. These white fibres become more minute and paler towards the periphery. Sometimes such a fibre seems to terminate suddenly with even an incomplete acumination. But, on a more accurate observation, some extremely delicate, very thin filaments are generally seen going off from it. The pale immature fibres in the tail of the frog’s larvee also subdivide. A question now arises are those more minute fibres (which at least present an appearance of subdivision) already prepared within an ordinary white primitive nervous fibre, or are they actual subdivisions? Since each nervous fibre is a secondary cell, and retains its character as a simple cell, and since the simple cell-membrane continues to exist distinct from its secondary deposits, and from the cell-contents, it is quite conceivable that fibres may be generated in the secondary deposits or in the cell-contents, as they are in muscle, although we have as yet no evidence of the fact; but these fibres could no more issue out free from the white nervous fibre, than the primitive fibres of muscle could from secondary muscle-cell, because, in order to do so, they must necessarily rupture the cell-membrane of the secondary cell. These subdivisions, therefore, so far as the structure from whence they issue corresponds to an ordinary nervous fibre, and is not merely a fasciculus of very minute secondary nerve-cells, cannot be a mere appearance, nor anything but actual divisions, a simple secondary nerve-cell becoming eiongated into several minute fibres, in a manner analogous to that which we have witnessed in the fibre-cells, (see page 115.) The nerves in the tail of the tadpole may therefore be described as terminating by the nervous fibres, that is, the secondary cells becoming split in different directions after the manner of fibre-cells or stellate cells. In the memoir before alluded to, I have noticed some swellings upon the pale nervous fibres in the tail of the tadpole. They have a double signification; some which are marked off from the rest of the fibre by a sharply-defined outline are the nuclei of the cells, from which the fibres have been generated; the majority, however, which pass into the fibre without a well-defined contour, as generally occurs at situations where the fibres divide and diverge towards different sides, are the bodies of the original cells, which (especially when they become elongated at different parts into fibres) remain somewhat thicker than the prolongations themselves; the pigment-cells, pl. II, fig. 9 a, exhibit this appearance.
b. Gray or organic nervous fibres. The gray cords, which, according to the researches of Retzius and J. Müller, are derived from the sympathetic nervous system, and mingled with the cerebrospinal nerves in which they sometimes pursue a long isolated course, owe their gray appearance, according to the investigations of Remak, “to the peculiar structure of the primi- tive fibres, which arise in the ganglia. They are not tubular, that is, surrounded with a sheath, but naked, being transparent, almost gelatinous, and much more minute than most of the primitive tubes. They almost always exhibit longitudinal lines upon their surface, and readily separate into very minute fibres. In their course they are very frequently furnished with oval nodules, and covered with certain small oval or round, more rarely irregular, corpuscles, which exhibit one or more nuclei, and in size almost equal the nuclei of the ganglion-globules.” (Observationes anat. et microsc. de system. nervos. structura. Berol., 1838, p. 5.)!
These corpuscles may at once be recognised, both in Remak’s delineations, and when examined in the natural state, to be cell-nuclei, which are round or oval, and frequently furnished with one or two nucleoli. They are attached to the most minute fibres, and as they are thicker than the fibres, they often appear to be situated only on their outside. Observation, however, does not warrant the conclusion that such is actually the fact. In the secondary muscle-cells (in which the nuclei decidedly lie within the cell) it frequently appears, and especially in the later periods of development, previous to the disappearance of the nuclei, as if the nuclei lay externally to the cell, inasmuch as they become pushed towards the outside. But no doubt the cell-membrane is at the same time elevated upon them, as we saw to be so distinctly the case in the fat-cells. (Pl. III, fig. 10.) Now, these most minute organic fibres, furnished with nuclei, precisely resemble the earlier condition of the white nervous fibres, as they were represented in pl. IV, fig. 8, a 6. Both have the same pale, minutely-granulated appearance, and both present cell-nuclei in their course. The only difference is, that the organic fibres are much more minute and the nuclei smaller. Each single nucleated organic fibre (I do not mean an entire fasciculus of them) corresponds to a white primitive fibre, and is probably, like it, a secondary cell, which has been generated by a coalescence of primary cells, whose nuclei are the nodules described by Remak’s discovery of the peculiar structure of the organic nervous fibres explains an observation previously communicated by me upon some extremely minute, pale, nervous fibres, which did not appear tubular, and were nodulated at different spots, and which I discovered in the mesentery of frogs. No doubt they were organic fibres. Remak as existing upon these fibres. The similarity between the organic fibres and that which I have described as the varlier condition of the white nervous fibres, might be adduced as an objection to my description of the formation of nerves, and it might be said, that that form seemed to be the earlier form of the white nervous fibre, because the organic nerves were developed earlier than the white, and, therefore, organic fibres were the only ones present in the first instance. Ob- servation of the actual transition, as represented in pl. IV, fig. 8, c d, would, however, refute this argument. Each pale, nucleated fibre becomes a white nervous fibre, as an immediate consequence of the formation of the white substance, which is probably a secondary deposit upon the internal surface of the hollow fibre. The formation of this white substance, which, according to analogy, must occur in every one of the minutest fibres, either does not take place at all in the organic fibres, or does so at a much later period, and their peculiarity therefore consists in their remaining stationary at an earlier stage of development, and either never attaining to the higher development of ordinary nerves, or only at a much later period, (a point which might be decided by comparing their numbers in old and young individuals.) One can conceive that the function of the organic nerves, whether it be actually a chemico-vital one, or consist merely in the production of involuntary motion, requires less-developed nerves, in the same way that the involuntary muscles do not attain the same de- gree of development as the voluntary.
2. Ganglion-globules.
These occur in the gray substance of the brain and spinal cord and in the ganglia, having generally the appearance of comparatively large granulous globules, enclosing a round vesicle, placed eccentrically, and which again exhibits in its interior one or two small dark points. According to Remak, two of these vesicles sometimes occur in one globule. Valentin (Nov. act. Acad. Leopold. xviii, p. 196), calls attention to the similarity between their composition and that of the egg, he compares the vesicle of the ganglion-globules to the germinal vesicle, their parenchyma to the yelk-substance, and ascribes a protecting investment of fibres resembling areolar tissue to both structures. This is certainly a very striking comparison, but the external investment must not in either instance be regarded as a something unessential, as a structure composed of other elementary parts, for the ganglion-globules, like the yelk, are true cells, and their external covering is an essential component part of them; it is the cell-membrane. The vitelline membrane of the bird’s egg, while contained in the ovary, is perfectly structureless, not composed of more minute ele- mentary parts; the same is the case with the investment of the ganglion-globules. They are both of them true simple cells. The parenchyma of the ganglion-globules forms the cell-con- tents, and the vesicle in their interior is the cell-nucleus ; the small corpuscles which it contains are the nucleoli. The vesicle of the ganglion-globules lies, as in other cells, eccentrically upon the internal surface of the cell-membrane. This cell- membrane may be most distinctly observed in the ganglion- globules of the sympathetic nerves of the frog, previous to their junction with the sacral plexus. (See pl. IV, fig. 10, a.) It there appears comparatively dark, and sharply defined, both externally and internally, so that its thickness may be readily measured. Valentin has already remarked, that the capsule of the gan- glion-globules is thicker in the lower animals. In the situation before mentioned in the frog, it seems as though a ganglion- globule were sometimes formed within another cell. (See fig. 10, b.) The ordinary contents of these ganglion-globules is a minutely-granulous, yellowish substance. On one occasion, however, I saw a ganglion-globule from the head of an ox (I do not precisely know from what part it was taken), in which the granulous appearance was confined to the surface, the interior being clear,—a fact which was rendered distinctly perceptible by causing the globule to roll about. It is nothing remarkable that two nuclei should sometimes occur in one ganglion-globule; we have observed this already in several cells, in those of cartilage for instance. In those instances, however, only one of them was the true cell-nucleus, the cytoblast of the cartilage-cell, the other being a subsequent formation within the cell. 3. Capillary vessels.
Plate II, fig. 9, represents two stellate pigment-cells, which have coalesced at a. In that instance two cells had been gene- rated at some distance from one another, their bodies may still be distinguished as two spots somewhat thicker than the rest of the structure. These cells became elongated on different sides into hollow processes, which, like the cavities of the bodies of the cells, are filled with pigment. Two processes of the two cells came into contact at a, and then coalesced, the separation at the point of union appears to have been absorbed also at the same time, so that the cavities of the two cells communicate immediately with one another; at all events there is no apparent interruption to the pigment, which forms the contents of the cells and their prolongations. (See page 78.) Now, if we imagine several such stellate cells to be developed on a large surface at similar distances from one another, and the several prolongations issuing from each separate cell to coalesce with those issuing from the other cells, in the manner represented in the figure at a, the result will be a network of canals ex- tending over the entire surface, and all communicating with each other. The size of the meshes of the network is determined by the distance of the cells from each other, and by the number of the prolongations issuing from each cell. Such, then, appears to be the process by which the capillary vessels are formed.
The observations, on which this mode of formation of the capillary vessels is based, were made partly on the tails of very young tadpoles, and partly on the germinal membrane of the hen’s egg. They are as follows:
1. The capillary vessels, in the tail both of the fully-deve- loped and young tadpoles, are seen to be surrounded by a thin, but distinctly perceptible membrane, which does not exhibit any fibrous arrangement. (See pl. IV, fig. 11.) The variety in the thickness of this membrane in different im- stances sufficiently explains why we cannot distinguish it in all capillary vessels, just as we cannot detect the cell-membrane even in the blood-corpuscles, although there can be no doubt of its existence. Where the capillary vessels exhibit a fibrous structure, they have arrived at a more complicated stage of their formation, and I regard such fibres as distinct from their cell-membrane.
2. Very distinct cell-nuclei occur at different spots upon the walls of the capillaries, both of the young and fully-developed tadpole. They appear to lie either in the thickness of the wall, or on the internal surface of the vessels, on which they often form a projection. (See fig. 11.) They admit of a double explanation. They are either the nuclei of the primary cells of the capillaries, or nuclei of epithelial cells, which invest the capillary vessels. It is true that epithelial cells occur in vessels which have a great resemblance to capillary vessels, if they are not actually such, as may be very distinctly seen in the vessels of the membrana capsulo-papillaris in a foetal pig of from four to six inches long, where some of them project, in the form of half-spheres, into the cavity of the vessel ; but there were no epithelial cells perceptible surrounding the nuclei in the capillaries of the tadpole’s tail. On the contrary, these nuclei frequently seemed to lie free upon the internal wall of the vessel, and must have been much more abundant had they been nuclei of epithelial cells. That these are the nuclei of the primary cells of the capillaries is, therefore, most probable, although this exclusive argument by no means decides the question.
3. In the tail of very young tadpoles, the capillary network presents, besides the ordinary cylindrical canals which have an equal diameter, and in which the blood flows in a regular current, other vessels of an irregular form. Unfortunately I neglected to make a drawing of them; they accord, however, in all essential particulars with the capillaries of the germinal membrane of the hen’s egg represented in pl. IV, fig. 12, except that the meshes of the vascular network are much larger in the tail of the tadpole. They are not regularly cylindrical. They are generally widest in situations where branches are given off, sometimes wider even than the ordinary capillary vessels. (See a, 6 in figure 12.) The branches diminish very rapidly as they leave those broad parts, and widen again as they approach another dilated portion. They present every degree of narrowing from vessels in which it could scarcely be remarked, to those which are reduced so much as to be scarcely thicker than a fibre of areolar tissue (as in c). Branches are also sometimes given off from these wider parts, which likewise diminish very rapidly to the same degree of minuteness, without reaching another dilated part (as at d e), and which are, therefore, blind ones. According to the above view of the development of the capillaries, these appearances may be explained in the following manner: the wider portions, a, b, &c., are the bodies of the primary cells. Hollow processes, as at d, are sent out from the bodies of the cells as the result of a more vigorous growth in different situations, precisely as is the case in all stellate cells. These prolongations meet with similar ones from other cells, and thus produce the form c. But being hollow, they are capable of expansion during their growth, and thus the canal c becomes converted into f, and at length into g, which is as wide as an ordinary capillary vessel. A more accurate analysis of the observations, however, is necessary to enable us to judge of the correctness of this explanation. It might be doubted, in the first place, whether these were really capillaries. The blood flows uninterruptedly through the ordinary capillaries, but there are no blood-corpuscles in these canals, at least in the more minute ones; they are, therefore, more difficult to discover, and readily give rise to a doubt whether they are canals. But their direct continuity with the ordinary capillaries may be clearly demonstrated, and blood-corpuscles actually enter the wider ones. If they be true capillary vessels, they may either be ordinary ones in a state of contraction, or they must represent a certain stage of their development. But if it be difficult to conceive that a capillary vessel can have the power to contract itself almost to the minuteness of a filament of areolar tissue, such an assumption cannot be supported at all in respect to the blind branches, which do not join any other vessel, as at d. This form might, indeed, be admitted to be a certain stage of development, although not of the kind de- scribed above; but branches might be sent off from the capillaries already existing, which again might give off others. The objection, that such an explanation does not account for the varying width of these capillaries, might be met by assuming that circumstance to depend upon the surrounding substance. It is, therefore, necessary to see the primary cells previous to their union with the actual capillaries. Now it is certain that a great many stellate cells are found in the tail of the tadpole. They lie beneath the epithelium and pigment-cells on the same plane with the capillary vessels; are smaller than the pigment-cells, and contain a colourless or palish yellow substance; they send off processes on different sides, which vary in number very much in different instances, but are generally short, and for the most part do not join with processes from other cells. Their shape has no sort of connexion with that of the pigment-cells which le above them, for when, as is the case in many larvae, the latter only send off prolongations on two sides, these cells exhibit several pro- cesses on different sides. They cannot, therefore, be young pigment-cells. Such branches of the capillaries, as those at d, sometimes appear to be connected with one of those stellate cells, and the others might, therefore, be regarded as young cells of capillary vessels which had not as yet begun to anastomose. ‘These anastomoses, however, are not sufficiently evident to enable me positively to assert their existence. The great number of these stellate cells, and their presence at all ages of the tadpole, are also circumstances unfavorable to the supposition that they are primary cells of capillaries. They might, indeed, be conceived to indicate a lower stage of development, as not having yet undergone any change, and that eventually capillary vessels may be developed from some, whilst others continue their existence without such a transformation, and fill the place of cells of areolar tissue. That, however, would be somewhat too hypothetical, and I shall, therefore, not adduce these cells as proof of the existence of primary cells of capillary vessels. The uncertainty which attaches to the observations on this point in the tail of the tadpole appears, however, to be removed when we examine the incubated hen’s egg.
4, When the germinal membrane of an hen’s egg which has been subjected to thirty-six hours’ incubation (at which period the formation of red blood has commenced, and is distinctly perceptible), is placed under the microscope, and the area pellucida examined with a magnifying power of 450, the capillary vessels are readily distinguished in it by their yellowish-red colour. Notwithstanding repeated endeavours, I cannot succeed at this season of the year when the hens are moulting, in subjecting eggs to incubation for so long a period, I can, therefore, only give a representation of these vessels from a recollection of what I observed in the early part of this year. (See pl. IV, fig. 12.) In some situations the capillaries are perfect, and connected with the larger vessels; at others they have the appearance represented in the figure, and illustrated previously by observations on the tail of the tadpole. In addition to these capillaries, which form a network of canals of irregular caliber and give off blind branches, some separate irregular corpuscles are seen, such as h and i, which do not appear to be connected with the vascular network. These bodies send off blind processes of various forms in different directions, and have the appearance, therefore, of stellate cells. They have a yellowish-red colour, like that of the bone-capillaries, which circumstance is alone sufficient to suggest the supposition that they are cells of capillary vessels in progress of development. This becomes much more probable, when we observe some of these corpuscles, such as k, already connected with the true capillaries. We may, therefore, with a high degree of probability at least, regard them as the primary cells of capillary vessels; and in that case the description of the formation of these vessels, previously given, would be the correct one. The following would, therefore, be the mode in which the formation of the capillaries and of the blood takes place in the germinal membrane: among the cells which compose the germinal membrane, some which are deposited at certain distances from one another, are developed into the primary cells of capillary vessels by becoming elongated on different sides so as to form stellate cells. The processes of the different cells come into contact and coalesce, the septa are absorbed, and in this manner a network of canals of very irregular caliber is produced, the prolongations of the primary cells being much thinner than the bodies of the cells. These processes of the cells or passages of communication undergo expansion until they and the bodies of the cells all attain one equal width, until, in fact, a network of canals of uniform caliber is formed. The fluid portion of the blood constitutes the contents of the primary cells, as well as of the secondary ones—the vessels produced by their coalescence; and the blood-corpuscles are young cells which are developed in their cavities.
Thus this last class, comprising tissues, which, in their functions, are the most characteristic of the animal kingdom, exhibits the same principle of development that we have met with in all the others; namely, that cells originate in the first place, and that. these become transformed into the elementary parts of the tissues. The elementary cells in this class, however, undergo more essential changes during their transformation than those of any previous one. They not only do not remain, as in the first two classes, independent, that is provided with a special cavity and particular wall; not only does a coalescence of the walls of neighbouring cells take place, as in the third class, but the cavities of the different cells also unite together in consequence of the absorption of the coalesced partition-walls of the several cells, so that the primary cells cease to exist as distinct objects. It is to a certain extent the opposite process to that which occurred in the fourth class, where, in addition to the prolongation of the cells, a splitting of them into several, probably hollow, fibres, a sort of division of the cells took place. The type of the transformation of the primary cells, as presented by nerve, muscle, and capillary vessels, is not, however, altogether limited to this class, but has been already exhibited by previous classes, and even in plants. Some of the pigment-cells have been cited before as examples, and the generation of the cells of the liber observed by Meyen was brought forward as an instance of perfect analogy in vegetables.
The independent existence of each separate primary cell is, no doubt, lost as a consequence of this perfect coalescence of several cells; not so, however, its character as Cell in general. On the contrary, several primary cells contribute to form one secondary cell, having the full signification of one independent cell. Each secondary cell in muscle and nerve forms a closed Whole, and the distinction between cell-membrane and cell-contents or secondary deposit seems to continue throughout life. In this way the nerves bring every part of the body into connexion with the central portions of the nervous system by means of a single uninterrupted cell. The different parts of the body, however, are connected together by another kind of uninterrupted secondary cell, namely, the capillaries. The capillary system, generated from several primary cells, forms one single secondary cell. The cavity of the secondary cell communicates with that of the large vessels. Researches are still required to decide the question whether these latter are mere dilatations of the capillaries, or whether they are formed simply by the junction of other elementary parts. In the latter case the capillary vessels would open into a cavity altogether distinct from their own, just as a vegetable cell opens into an intercellular space. It sometimes occurs that the cavities of certain vegetable cells open directly outwards, but such instances are very rare.
As a primitive muscular fasciculus, a nervous fibre and a capillary vessel are corresponding formations in this class; we may also compare these structures with the elementary parts of other tissues. The elementary cells of all tissues correspond with one another, beg formed universally according to similar laws. A blood-corpuscle, an epithelial cell, a cartilage-cell, an elementary cell of areolar tissue (therefore, also a fasciculus of areolar tissue formed from it), correspond to an elementary cell of muscle, &c. There is no structure analogous to an entire primitive fasciculus of muscle or a secondary muscle-cell or a nervous fibre amongst the principal component parts of the tissues previously discussed, because with them the formation of secondary cells only occurs as an exception. A muscular fasciculus differs, therefore, from a fasciculus of areolar tissue, and a primitive fibre of areolar tissue has no analogy with a primitive muscular fibre.
- ↑ It might be important to examine whether the zigzag plications of muscles, during contraction, have not perhaps some connexion with the length to which the portion of a muscular fibre generated from one single cell has become expanded, so that probably the angle of each flexion coincides with the point of junction of two cells.
- ↑ Müller’s Archiv, 1836, p. 148. Respecting the microscopic structure of the brain and spinal cord of the foetus, see Valentin, Entwickelungsgeschichte, p. 183.