Microscopical Researches/CLASS II. Independent cells united into continuous tissues

by Theodor Schwann

CLASS II.

Independent Cells united into continuous Tissues.

This class presents us with the greatest similarity between animal and vegetable structure, and, indeed, in so high a degree, that even an experienced botanist cannot distinguish some of the objects which belong to it from vegetable tissue. Most animal cells may be distinguished from the mature vegetable cells by their greater softness and delicacy; but those characteristics are in some measure wanting in this class, and it would be very difficult to distinguish microscopically between a thin layer cut off from the interior of the shaft of a feather and a portion of vegetable tissue. We shall, therefore, take the feather as our example, and endeavour to trace these cells, which correspond in so striking a manner with vegetable tissue, backwards to their primitive condition, explaining this transition by delineations, and in this way convince ourselves that, in their early stage, they also accord with the primitive cells of all other tissues. The tissues comprised under the term horny belong to this class, and the crystalline lens may also be included in it. The cells of these tissues generally remain independent, but more or less intimate blendings of the cell-walls with one another also occur in this class. Horny tissue may be reduced to two unessential subdivisions, viz.—1. Its membranous expansions, to which belong the Epithelium, in the extended sense of the term (including the Epidermis), and the Pigmentum nigrum, which must be enumerated here, in consequence of its intimate alliance with the epithelium. 2. The compact horny formations, including the Nails, Claws, Hair, Feathers, &c.

1. Epithelium.—It is very difficult to determine what this term ought to comprise. The cortical substance of the chorda dorsalis, which is composed of flattened hexagonal cells (in the larva of Rana esculenta, for example), cannot be regarded as epithelium, since it is made up of the same cells as those of the interior of the chorda dorsalis: the sole difference consisting in their being flattened. The serous layer of the germinal membrane also cannot well be considered to be epithelium, although it has the same structure, and yet it is difficult to give a definition of it which shall not comprise these structures. We shall not, however, enter upon this contention about mere terms, but proceed to the consideration of the structure of the epithelium.

The simplest form of epithelium is that of the round cells furnished with a nucleus which lies upon the inner surface of their wall, and encloses one or two nucleoli. When in connexion they assume a polyhedral form, but their free surface usually projects in the form of a segment of a sphere. Such is the appearance presented by the epithelium in many situations; I instance only that of the branchial rays of the fish by way of illustration. The cells are usually smaller and more granulous in mammal; but in the lower animals and in the fœtal stage of mammalia they are, in general, larger, smoother, and sometimes so transparent as to be visible by a subdued light only. I once had an excellent opportunity of observing the epithelium upon the mucous membrane of the stomach of a fœtal sheep, and its perfect resemblance to the parenchymatous cellular tissue of plants. A minutely granulous deposit may often be observed in the interior of the transparent epithelial cells; in those of the branchial rays of the fish, for instance, it appears to be formed in the neighbourhood of the nucleus. According to Henle, two nuclei never occur in an epithelial cell in mammalia; but I have several times observed that number in the external covering of the tadpole, and on one occasion I remarked that a perfectly developed epithelial cell furnished with a nucleus was enclosed within a larger cell. Changes in form from this rudimentary globular shape occur in the epithelial cells in two different manners; they either become flattened into tables, or prolonged into cylinders. The flattening out into tables takes place in such a manner that the nucleus forms the centre of one surface, as in the blood-corpuscle. I have observed the stages of transition from the globular to the tabular form in the epithelium of the external covering of the tadpole, which occasionally presented hexagonal flat columns or tables, the thickness of which was about equal to one third of their breadth. The thickness is so very slight in proportion to the breadth in the completely flattened epithelial cells, that it is no longer possible to distinguish the two lamellæ of the cell-membrane. It often occurs that the tabular epithelial cells are not regularly hexagonal, but represent flat elongated stripes, a fact which has been observed by Henle in the epithelium of the vessels.^[1] The cells which are prolonged into cylinders constitute the other modification in the form of the epithelial cells. They were discovered by Henle in the intestinal mucus-membrane. They likewise enclose the characteristic nucleus, and are arranged with their longest sides in apposition. Their blunt ends are turned outwards and free. The opposite end either terminates abruptly also, as in the chorion, or proceeds to a point. This tapering figure frequently commences at the upper part, so that the cells then have the form of a pointed cone, the base of which is turned towards the outside. Henle found that the cilia stand upon the free surfaces of the epithelial cylinders in those membranes which present the phenomenon of ciliary motion, a fact of itself sufficient to show that the epithelium ought not to be regarded as a mere inanimate covering to the organized structures.

With regard to the formation of the epithelial cells, Henle has already proved the rete Malpighii to consist of round nucleated cells, probably the young epidermal cells, and also that the diameter of the cells increases towards the outside, so that in the fœtal pig he was enabled to trace the gradual transition of the cells of the rete Malpighii into those of the epidermis. (Symbolæ ad anatomiam villor. intest., p.5.) An actual growth of the epithelial cells thus became very probable; I have likewise followed this process in the fœtal pig. The uppermost layer of the epidermis is there formed of large, tabular, hexagonal cells, furnished with a nucleus. Immediately beneath these lie nucleated cells, which are already much smaller, and almost round, so that the flattening must take place very rapidly. The farther you proceed from the surface the smaller the cells become, and the closer they encompass the nucleus. The size of the nucleus also diminishes in some degree, but by no means in the same proportion. In the lowest strata, the cells cannot any longer be distinguished, but the nuclei lie close together, with a small quantity of minutely granulous intermediate substance. It is, however, very difficult to obtain positive conviction of this fact, for the stratum of nuclei is too firmly connected with the cutis. We shall have an opportunity of observing this relation of the nuclei more distinctly hereafter in the feather. The mode of formation is probably this: cell-nuclei are formed, in the first place, immediately upon the surface of the cutis; and then around, and closely encompassing them, the cells. The cells and the nuclei (the latter, however, in a much less proportion) increase in size, and at length those in the uppermost layers become flattened in such a manner that the nucleus forms the centre of the table. This, then, is but a repetition of the same course of development observed in most other cells. Before I had proved the universal accordance between animal and vegetable cells, Henle thought that the original increase in volume of the epithelial cells might possibly be explained as taking place by imbibition. (1. c., p. 9.) As, however, we have observed this growth to be a phenomenon which occurs in all animal cells—as we have seen the formation of cells around the nuclei—as a chemical change in the cell-membrane may be proved to take place during the expansion of many of the cells, and as it frequently happens that not only does no thinning of the cell-membrane occur during expansion, but that an actual thickening takes place, all which are processes similar to those of plants—we must ascribe a peculiar vitality to the animal as well as to the vegetable cells, and explain this expansion of the epithelial cells, like as we did that of plants, as a growth by intussusception. The new epithelial cells, it is true, are formed immediately upon the cutis only, where the greatest vital energy prevails; but the cells expand independently, and grow by intussusception. I have brought forward an instance in which a young epithelial cell was formed within another in the tadpole. But this is certainly a very rare circumstance, and the majority of epithelial cells, in all the vertebrate animals, are certainly not formed as cells within cells, but on the outside of the cells in a minimum of cytoblastema, which is exuded from the cutis. It might be objected that this process of formation of the epithelium could not be possible, for the reason that, if the cells of the second stratum were twice as large as those of the first, then the whole layer of epidermis must be also twice as large as the first. But this objection may be easily set aside by the fact that the cells slide upon one another, and a double or triple layer of cells may thus originate from one stratum of nuclei.

2. The Pigmentum nigrum. The pigment is familiarly known as being usually contained in round or (in consequence of their close apposition) hexagonal cells, in the form of innumerable very minute granules, which exhibit a lively molecular motion. This motion may sometimes be observed even within the cells, so that the rest of their contents must be fluid. As it is also known, that the pigment-granules may sometimes be pressed out from the cells, no doubt can exist respecting the cellular nature of these bodies, formerly called pigment-globules. The wall of the pigment-cells exhibits a nucleus, which is already familiar to some observers. It may be seen in the fœtal condition of the pigment cells of the choroid coat in mammalia, at different points in that of the very young foetal pig for instance, quite distinctly; and it occasions the well-known white spot in the centre of the cells. It commonly contains one or two nucleoli. It sometimes happens that no pigment-granules are deposited around the nucleus, but that it is surrounded by a clear, transparent areola.

Some pigment-cells undergo a most remarkable transformation, and one which acquires an especial importance, from the fact that it serves as a type of formation for other more important classes of cells. This transformation consists in the cells being elongated on three or more sides into hollow fibres. These we shall name stellated cells. It has, indeed, been necessary to allude to them already when treating of bone. The characteristic contents of the pigment-cells render them best adapted for an accurate examination of this type of formation. The stellated pigment-cells, known under the name pigment-ramifications, are best observed in the skin of the tadpole. They exhibit varieties in form; we select for our description such of them as present the longest fibres. (See plate II, fig. 9.) Their appearance is that of separate black spots, from which slender black fibres issue on different sides. The black spots represent the bodies of the cells filled with pigment; the fibres are the prolongations of the cells filled with the same material. The separate pigment-granules may be distinguished in many situations. The body of the cell, which is sharply defined on its exterior, sometimes presents a clearer spot of a round or oval form, through which the cell-nucleus glimmers, and in some few instances can be distinctly perceived with its nucleolus. The diminution of the cell in various directions, in order to pass over into a fibre, is so gradual that there is no defined limit between them. The fibres pass between the cells of the epithelium, and are therefore frequently curved : they are in general thickest in the neighbourhood of the cell, and diminish as they proceed from it ; but they sometimes also swell out slightly at some distance from the cell. The presence These fibres give off others at different points. of the cell-nucleus, and the fact that all the stages of transition from indubitable pigment-cells to these bodies may be demonstrated, are sufficient evidence that these black spots, with the fibres proceeding from them, are actually cells, and that the fibres are hollow prolongations of them filled with pigment. These transitions are delineated in plate II, fig. 8, just as they existed close together in another part of the tail of a tadpole: a is an indubitable pigment-cell, scarcely differing from an The only circumstance ordinary one; it has also its nucleus. which distinguishes the majority of the primitive cells of these stellated pigment-cells from common pigment-cells is that they are generally smaller, and more closely filled with pigment. b is a smaller cell, which has commenced to taper; and c is distinctly elongated into a fibre. A slightly clearer spot is the d and e only indication of the nucleus in both instances. elongate at both ends into fibres, one of which (the upper end of d) terminates in a knob witha defined outline. At the spot where this knob unites with the body of the cell, a shading, indicating a cavity, may be clearly perceived, the pigment being more closely deposited in the neighbourhood of the cell wall than in the centre; and lastly, f is a cell which elongates into fibres on three sides. When a small piece of the skin of the tadpole is torn in water, separate portions of these pigment-fibres, or prolongations of the cells filled with pigment, may be observed to float about isolated. Instances sometimes occur in which one of these pigment-fibres passes uninterruptedly from the body of one cell to that of another; for example, fig. 9, a. We may imagine this to be effected by the prolongations of two cells meeting at one point. As the pigment does not move from one cell to another, we cannot accurately determine whether the partition-walls become absorbed at such a point or not. Such, however, may be supposed to be the case, otherwise an interruption of the pigment corresponding to the double thickness of the cell-wall must be seen at the spot where the prolongations are in contact. The fibres issuing from the cells often become very minute in the last part of their course, from which we learn that the delicacy of fibres does not preclude their being hollow.

3. Nails.—In order to investigate the structure of the nail we should make use of that of a child immediately after birth, or, what is better, that of a mature, unborn, human fœtus; such an one, when divided into delicate longitudinal sections, will be found to consist of laminæ deposited one upon another, surface to surface. This laminated arrangement, however, becomes more and more indistinct upon the under surface of the nail which lies upon the skin, the nearer we approach to that portion contained in the fold of skin at the root, and the posterior half of the part which is embedded in that fold exhibits no laminated structure whatever, but consists of small polyhedral cells, many of which present perfectly distinct cell-nuclei. When a small portion is cut or torn off from the surface of such a nail, the form of the margins, which present smooth angular projections, leads at once to the supposition that the laminae of the nail are not structureless, but produced by the junction of little scales resembling those of epithelium. When treated with acetic or concentrated sulphuric acid, the scales separate more readily, and in some rare instances an indistinct nucleus may be recognized in them. No such scales can be seen in the root of the nail after the adherent lamella of epidermis has been scraped off, but polyhedral cells, which are much smaller than the scales, are found in that situation. Now it is a well known fact that the nail increases from its root, and is constantly pushed forwards. The polyhedral cells of the root must thus, therefore, become transformed into those scales by flattening and extension of their superficies, a process which the independent vitality of the cells renders easily conceivable. The cells of the nail already formed increase in size from the same cause, and the growth of the nail by no means depends upon a mere apposition at its root, although it is probable that the formation of new cells takes place in that situation only where the nail is in connexion with the organized skin. The nail would certainly be pushed forwards by the extension of the superficies of those cells, and by their flattening in reference to its thickness, but the more the cells become flattened, the thinner must the anterior part of the nail become. This probably is compensated for, by a formation of epithelium-scales upon the under surface of the nail, and especially at its posterior part. If, for example, an epithelium-scale become attached to the most posterior part of its under surface, it will be advanced somewhat forwards by the flattening of the cells above, and the formation of new cells at the end of the nail. At that part, however, a new scale is next formed, and laid upon the former one, and as the advance forwards goes on, a third and fourth are formed, and so on, so that, by this means, a thickening of the nail must take place proportionate to its advance from behind forwards. I consider, therefore, that this thickening of the nail, in consequence of growth from the under surface, and the thinning consequent upon the flattening of the cells, compensate each other, and that the almost uniform thickness of the nail is produced by this means. The superficial laminæ of that part of the nail which lies external to the fold of skin at all events do not continue to grow. I marked several nails with two points, by boring them with a needle and colouring the spot with nitrate of silver; the marks were made at the root of the nail, some in the longitudinal, others in the transverse direction. In the course of two or three months they had advanced to the point of the nail, but their distance from each other had not altered in the least.

4. Hoofs. The horny tissue of hoofs, in the fœtus at least, consists entirely of the most beautiful vegetable-like cells. If a thin transverse lamella be cut off from the hoof of a large fœtal pig, the preparation will present the exact appearance of vegetable cellular tissue. The following facts prove that the cells are not flat: in the first place, when the side walls do not stand quite perpendicular, they may be traced down below the level of the section, and the depth to which they go may be estimated; and secondly, longitudinal sections of the horny tissue of hoofs present a similar appearance to those made in the transverse direction. They are, therefore, polyhedral cells, and some of them, at least, contain a distinct nucleus.

When the tissue is quite fresh, it is not possible to distinguish the particular wall of every cell. But when the fœtus has lain for a time in strong spirit, the horny substance of the hoof may be easily separated from the foot, in consequence of the connexion between the cells having become looser. The undermost layers of cells, however, remain attached to the foot. The interior of the layer of horny substance so separated, consists of a crumbling mass, somewhat resembling a boiled yelk. The particles cannot, however, be separated quite so readily from one another as those of the yelk are. With the aid of the microscope, this mass is found to be composed of irregularly angular bodies, resembling the yelk-substance when boiled. These bodies are the isolated cells, whose peculiar walls are distinctly perceptible, and some few of them have a nucleus, which lies upon the inner surface of their wall. A continuous firm layer of flat epithelium-scales, the immediate continuation of the outer lamellæ of the epidermis, consisting of flat cells, surrounds these polyhedral cells as an external covering to the entire hoof. This lamella exists in the fœtal pig at a very early age, the layer of polyhedral cells being at that time very slight; in a more advanced stage of development, however, the latter forms the chief mass of the horny substance of the hoof. In the recent condition these cells must also have somewhat firm contents, otherwise, with so delicate a cell-membrane, the substance could not be so firm. But its elasticity was such as to prevent my crushing one of the cells with the compressorium, my object being to observe whether the cell-contents would flow out, or be torn like a firm substance. As the cell-contents form a large portion of this horny substance, whilst the nails consist for the most part of flat cells without any discernible contents, almost entirely therefore of cell-walls, a chemical distinction may be presumed to exist between the two structures.

5. Feathers. The feather is composed of the quill, the shaft, and the vane, or beard. The elementary structure of these parts is, however, what most interests us at present; and in order to investigate it, at least in order to become acquainted with the relation which the different elementary formations in the feather bear to cells, we must take one in which a part of the shaft is in progress of formation. The feathers at that time are surrounded by a dense capsule, which is composed, throughout its entire thickness, of gigantic tabular epithelium. The feather is so placed in this capsule, that the shaft and vane are folded together to form a hollow cylinder, which is occupied by the so-called organized matrix of the feather (see an article on this subject by Fr. Cuvier, in Froriep’s ‘ Notizen,’ No. 317). According to Cuvier, a membrane lines the inner surface of the vane, and gives off septa, which penetrate between its separate barbs. This membrane, however, as well as the septa, is composed of epithelium.

The shaft of the feather consists of a loose medullary sub- stance (the pith), surrounded by a firm cortex. On making thin transverse or longitudinal sections of the pith, it is seen to be composed of beautiful polyhedral cells, which perfectly resemble the parenchymatous cellular tissue of plants,—as the substance of cork for example. (See plate II, fig. 10.) The cell-cavities which have moderately thick, dark partition-walls, are at first filled with a transparent fluid, but subsequently become dry, and in that state contain air. Notwithstanding, however, that this pith so precisely resembles vegetable tissue in its general appearance, it may be questioned whether these cells be actually cells in that sense of the word in which we receive it here, viz. elementary cells of organic structure, and whether they correspond to vegetable cells. It therefore becomes necessary to investigate whether each cell has its peculiar wall, and whether the course of development of each individual cell be the same as in plants. There is no structure, however, in which it is easier to follow the process of development than in the one before us, chiefly because the cells, even from the first, have no connexion with the organized so-called matrix, but remain attached to the fully-developed cells of the shaft, when the matrix, which terminates externally with a smooth surface, is taken away. The following description is taken from the large wing-feather of a raven: it applies however equally well to the feathers of the young chicken.

The pith, when in progress of formation, is soft and friable. When a small portion of it is examined, after the component particles have been separated asunder, it is found to consist of cells, in various stages of development. Those which are most completely developed resemble the cells which we have seen in the mature feather (see plate II, fig. 11, a) in every other respect, but that they lie less firmly connected together, so that they may readily be isolated, and the peculiar wall of each cell be distinctly seen. The walls are of sufficient thickness to prevent their losing their angular shape, even when in the isolated state. There are intercellular spaces between some of them, and such also occur between the fully-developed cells of the perfect feather. The cell-membrane is dark and smooth, and the cell-contents consist of a transparent fluid. A very distinct nucleus is also seen lying upon the wall of each cell. It is oval, and contains one or two nucleoli, which are large in proportion to its size (see the figure). There is no nucleus to be seen in the fully-developed cells, and it is only in very rare instances that its remains can be detected; it must therefore undergo absorption at some subsequent period. The process may, indeed, be followed; for example, the cells which form the stages of transition between those delineated in fig. 11 a, and fig. 10, are more closely connected together, the nucleus m them becomes smaller, and its outline more irregular, the nucleolus meanwhile remains; at length both disappear. The degree of development attained by the cells is generally proportionate to their distance from the matrix; and as that is situated on the inner side of the feather, at the part where the shaft exhibits a furrow, those cells which he immediately under the cortex, at the back of the shaft, are the most perfectly developed. Now the cells when traced from that part inwards towards the matrix, are found to become gradually smaller; the cell-membranes lose their dark outlines, and present a granulous aspect. The nucleus of the larger granulous cells has still the same form as in those previously described with a smooth cell-membrane; its size, however, diminishes with that of the cell. The cells in this granulous condition resemble most of the elementary cells of other tissues. Plate II, fig. 11, b, c, represents the stages of transition. Advancing still nearer towards the matrix, the cells are no longer recognizable; all that we can see being numerous nuclei, which he close together in a minutely-granulous substance (plate II, fig. 12).

The process of formation of the cells of the pith is, therefore, as follows: a minutely-granulous mass is present in the first instance, in which numerous cell-nuclei lie, some of them exhibiting a nucleolus. Around them the cells are formed, being at first not much larger than the nuclei, and having a granulous aspect. The cells gradually expand; the nucleus also grows, and soon reaches its full maturity. It remains eccentrical, lying upon the cell-wall. The cell-membrane retains its granulous aspect for a time; gradually losing it, however, as the expansion of the cells advances; at the same time the contents of the cell-membrane become darker, but the cell-walls are not at all diminished in thickness. The walls of the cells, in the next place, become more firmly united together, so that they cannot be separated from one another so readily, and at the same time the nucleus gradually dis- appears. The contents of the cells at last dry up, and they become filled with air. The development of these cells accords, therefore, entirely with the vegetable cells, the nucleus being their true cytoblast; it is present before the cell, and, as is generally the case in the cells of plants, afterwards becomes absorbed. The cell expands, growing by intussusception, and the membrane of the fully-developed cell might, without much danger of error, be assumed to be more than ten times heavier than that of the youngest one. The physical, and probably also the chemical, condition of the cell-membrane undergoes a change. The cytoblastema, in which the cell-nuclei are in the first place formed, consists of granules, analogous to the mucus-granules, in which, according to Schleiden (Müller’s Archiv, 1838, plate III, fig. 2), the cytoblasts of vegetable cells originate. According to Schleiden, those mucus-granules are deposited from a solution of gum within a parent-cell. The cells of feathers are not formed in parent-cells, but in the neighbourhood of the organized matrix. There can be no doubt, however, but that the matrix only exudes a fluid, which afterwards becomes transformed into a granulous substance. I have not investigated the mode in which the nuclei originate in the cytoblastema, whether by a junction of smaller globules, whether the nucleoli first exist, and so forth. The growth of the nucleus proceeds for a time with that of the cell; for the latter is formed around the nucleus before it has reached its full size. The cytoblas tema of the cells of the pith of the feather is supplied by the nearest contiguous substance provided with vessels, that is, by the so-called matrix. In the young feathers of the hen, however, I found a layer of very small, extremely pale, round cells without nuclei,—a sort of imperfect epithelium,—between the matrix and the granulous cytoblastema, so that not even so much as an immediate contact exists between the latter and the organized substance.

The cortical substance of the shaft of the feather is a fibrous structure. Here the Cell-theory seems, at first sight, to fail; but we are soon taught otherwise, when we examine the generation of the fibres as exhibited in the incompletely formed portion of the cortical substance of a feather, which is in progress of formation within the capsule. The cortex is then seen to consist of large flat epithelium-cells, each having a beautiful nucleus, with one or sometimes two nucleoli. Some of these epithelial tables are long flat stripes, others are of an irregular rhomboidal form. They are very firmly connected together. Each cell generates several fibres, and the transitions may be readily observed at different parts of the same preparation. Plate II, fig. 13, represents them. The cells at first are flat tables, having a smooth margin, a slightly granulous aspect, and containing a very dis- tinct nucleus (fig. 13, a). Upon their margins and surfaces indistinct fibres gradually become visible, which project out insulated from the edges, but are connected together upon the surface by the substance of the tables (fig. 13, b). At this stage the fibres are pale, and the nucleus of the cell still quite visible. The fibres afterwards become more sharply and darkly defined; the insulated portions projecting from the edges are larger, the part of the table connecting them together becomes more indistinct, and the nucleus begins to wane, although it is still distinctly perceptible, and the nucleolus especially so (fig. 13, c). At length all traces of the original cell and the nucleus disappear, and we see only dark, stiff, thin fibres, which are closely connected together but may still be recognized as being insulated for a space, the length of the original table (fig. 13, d). These fibres, therefore, also originate from cells, and that not so much by an elongation of the cells, as by their division into several fibres. As the fibres which lie close together in the first instance, do not, as it seems, continue connected with one another, a portion of the original table must be absorbed, and the following may therefore be conceived to be the mode in which the fibres originate. After the two laminae of the table are in part or entirely blended together, an absorption takes place at certain parts, in such a manner, that the portions not absorbed lie in longitudinal lines, and thus remain as fibres. The reality of an absorption is, moreover, distinctly shown by the disappearance of the cell-nucleus. We have no evidence as to whether the fibres are hollow or not; it is sufficient for our purpose to know that they originate by a transformation of cells.

The quill of the feather has a similar structure to that of the cortical substance of the shaft.

The vane is composed of separate barbs, and each barb is again a miniature feather. The following description is taken from the undeveloped wing-feather of a sparrow. Each barb contains a secondary shaft, on the side of which is placed a secondary vane. ‘The secondary shaft has the same structure as the principal one, and consists of a cellular medullary sub- stance (pith), and a firm cortex. The secondary vane is com- posed of a great many triangles, which lie with their surfaces close together, having very narrow bases by which they are fixed upon the secondary shaft. Each triangle is formed of flat epithelium-cells arranged with their angles overlapping each other, each having its nucleus. The separate epithelium- cells are broadest below, diminish more and more towards the point, and extend proportionately in length. The nuclei le in a row, near about the middle line of the triangle. The last cell, at the apex of the triangle, is contracted into a long fibre. The last cell but one, and all the others in succession, become elongated, at the point at which the next following cell is attached to them, into pointed processes, which vary in length, and are extended on both sides of the cells in the plane of the triangle.

6. The Crystalline lens. The mode in which the lens is nourished has always been an enigma. Having no vessels, it has either been regarded as a secretion of its capsule, or its mode of life has been considered as generally resembling that of vegetables. We shall find the latter to be the correct view, and the singularity of the mode of its nutrition disappears altogether, when we become acquainted with the fact, that the growth of the organized tissues resembles that of vegetables. The general statement, that the lens has the vitality of a vegetable, does not, however, express much, unless the relation of its elementary structure to the cells of plants be proved. The lens is known to be composed of concentric layers, made up of characteristic fibres, which, not to go into details, may be said to pursue a general course from the anterior to the posterior surface.

In order to become acquainted with the relation which these fibres bear to the elementary cells of organic tissues, we must trace their development in the fœtus. When the lens of a chick is examined after eight days’ incubation of the egg, no fibres are to be found; but it is composed of round, extremely pale, and transparent smooth cells. Some contain the characteristic cell-nucleus, in others it cannot be detected; and there are also many nuclei without surrounding cells. Some larger cells may be observed in the chick at a more advanced period, which contain in their interior one or two smaller ones (see pl. I, fig. 10, d, from a fœtal pig), and from the manner in which these cells become flattened against the wall of the parent-cell, as well as from the presence of the nucleus in other cells, we may conclude, that these pale globules are actually cells, although a cell-membrane be not distinctly recognizable. Werneck, who first observed them, likewise calls them cells.

The following conditions of the crystalline lens may be observed in Mammalia. In a fœtal pig, three and a half inches in length, the greater part of the fibres of the lens is already formed; a portion, however, is still incomplete; and there are many round cells awaiting their transformation. The perfected fibres form a sphere in the centre of the lens; but there is no laminated structure as yet perceptible in it. The fibres may readily be separated from each other, and proceed in an arched form from the anterior towards the posterior side of the lens. This sphere, composed of the perfected fibres, becomes surrounded, in the circumference of the lens, with a thick and broad zone of fibres, which are as yet imperfectly developed. They have much the same course as the others, that is, they form arches from the anterior towards the posterior surface. They do not, however, reach the axis either in front or behind, but the fibrous zone is thickest in the middle, gradually diminishes towards the anterior and posterior surfaces of the lens, and terminates altogether without the fibres meeting anywhere in front, or reaching the axis. No laminated structure can be perceived in the zone; but the fibres may be readily insulated throughout its entire breadth. When the ends of these fibres are examined, they are found to be either simply rounded off, or to terminate in a small round dilatation, or to pass over into larger globules (cells); or, on the contrary, it may be more correctly expressed by saying, that the larger globules or cells become elongated to these fibres (see pl. I, fig. 12). The transition from cells to fibres may either be very gradual or somewhat sudden; but even in the latter case, the contour of the cell passes immediately over into that of the fibre, so that the latter is not merely affixed to the globule, but is a true continuation of it. Now, these cells which become elongated into fibres, perfectly accord with other neighbouring cells which are as yet quite round; and these again accord with the cells forming the greater portion of the lens in the embryo chick. They are round, extremely pale, smooth, transparent cells of very various size (see pl. I, fig. 10). Some have a very beautiful, sharply-defined, oval nucleus, which, in most instances, is not flattened, and which lies upon their wall, and encloses one or two nucleoli. Some cells are scarcely larger than the nucleus which they contain, fig. 10, b, for example. Some of these enclose young cells (fig. 10, d), and as they may be observed to flatten against the wall of the parent-cell, there would seem to be no question about the existence of a special cell-membrane for the latter, and thus the true cellular nature of these globules appears indubitable. The presence of the nucleus, and the fact of the outlines of the cells being too sharply defined for mere shadows, would, however, have been sufficient to render their cellular character probable. The very distinct nucleoli contained in the nuclei, which are not flattened, lie upon the inner surface of the wall and not in the centre, as represented in fig. 11.

Since, then, the round cells, as we have seen in the chick, form the primary structure of the crystalline lens, and no fibres can be detected in the early stage, and since the more fully-developed lens of the fœtal pig exhibited many fibres and fewer round cells, and at the same time cells which became elongated into fibres, we cannot but regard the fibres generally as elongated cells. It is true that a cell-membrane cannot be distinguished on the fibres, nor can it be distinctly recognized on the round cells. If, however, the arguments above cited rendered its existence in the round cells certain, they must avail equally in the case of the fibres. Nuclei are also frequently found upon the fibres of the fœtal pig. Some of the fibres are flat. I have, also, several times observed an arrangement of the nuclei in rows; but I do not know what signification to attach to the fact. A blending of several cells to form a fibre may also possibly occur; but I have no observations decisive of the point. In fishes also, in a young pike for instance, the elongation of the cells into fibres may often be very distinctly seen.

Brewster found that many fibres of the crystalline lens, especially in fishes, exhibit the remarkable peculiarity of having their margins serrated. Pl. I, fig. 13, represents such a fibre taken from the innermost lamina of the lens of a pike. The fibres are flat, and their sharp margins furnished with long teeth, which are so disposed, that two neighbouring fibres lock into each other by them. We have here an instance of perfect analogy to a form of vegetable cells, which is delineated in fig. 14: it is an epidermal cell of a species of grass. It is very much elongated, quite flat, and furnished on the sides with teeth precisely similar to those of the fibres of the lens, which, in like manner, fit in between the denticulations of the contiguous cells. The fibre-cells of the crystalline lens which are delineated, have somewhat longer teeth in comparison with the breadth of the cell; they represent, however, some of the most strongly denticulated fibres. On pursuing the examination from the external towards the internal laminæ, the same lens will be found to present all possible stages of transition in this serration, from the smooth or only minutely-notched cells, to such as are strongly denticulated like those in the figure. This striking accordance of so remarkable a form of animal structure with a similar modification of vegetable cells, is a brilliant confirmation of the correctness of the view, that the fibres of the crystalline lens are really cells, however much they may deviate from the fundamental type of the cellular form.

There is no longer, therefore, any more difficulty in explaining the process of nutrition in the lens, than there is that of plants. The cells grow by their own independent force, and blood-vessels are unnecessary, as the nutrient fluid can be conducted from one cell into another. A morbid change of the cell-vitality, rendering the cell-contents opaque, is also possible.

The structures included in this class, notwithstanding the strong general resemblance which they bear to each other, have furnished us with far more varied modifications of the cellular form than the previous class exhibited; indeed, these so-called unorganized tissues have already prefigured the type of all the changes by which the organized tissues are developed from simple cells. Here, also, the fundamental form of the cells is that of a sphere, which, in consequence of their close contact, passes over, from mechanical causes, into a polyhedral figure. Two different modifications of this fundamental form occur, which cannot be explained mechanically; they are the flattening of the cells on two opposite sides to form tables, and their elongation in two directions into cylinders or fibres. We have already seen an instance of flattening of the cells in the blood-corpuscles of the previous class. It is not only more strongly marked here in the tabular epithelium, where the cell-cavity is quite obliterated, but a modification even of this form is presented to us in the elongation of these tables on two sides into flat stripes, as seen in the epithelium of the internal coat of veins for example, and still more distinctly in the cortical substance of the shaft of the raven’s feather. The epithelium of many of the mucous membranes, that of the intestine for instance, which Henle describes as consisting of little palisade-like cylinders placed close to one another, furnishes us with a rudimentary form of the elongation of cells into cylinders and fibres. Sometimes these little cylinders become acuminated at their lower extremity, or they may diminish throughout their entire length from above downwards, and thus become small cones. This prolongation of cells into long cylinders (called fibres) is much more remarkable in the crystalline lens. The fibres or cylindrical cells of the lens, however, themselves undergo very important modifications, inasmuch as they often become flattened on two sides into bands, and then the margins of these bands become denticulated. This serration is probably produced by a more forcible expansion, and therefore bulging-out of the walls of these bands at different points, which follow each other at pretty regular distances, whilst the intervening points, situated close to them, remain stationary. All the different stages of this serration, may be observed in the lens of the fish, if the fibres are examined from the exterior towards the centre of the structure. Now, in this flat and serrated condition, the cells of the crystalline lens perfectly resemble those of the epidermis of some grasses, and this accordance with indubitable vegetable cells is a proof that, despite the modifications which they undergo, they do not lose their cellular character. If the explanation I have given of the mode in which the serration is produced be correct, it will not materially differ in principle from the elongation of the cells into cylinders and fibres. For, in the latter case, a more forcible expansion of the cells is likewise presumed to take place in certain situations: the sole difference being, that in the latter case it takes place only at one or two opposite points of a cell, whereas with the serration it occurs at many separate ones. At this stage of our inquiry, we are reminded of the form of many pigment-cells, in which this expansion of the cell, at certain spots, takes place on several sides, and in a far higher degree, causing the cell to assume an irregular stellated form. The prolongations of these cells, however, retain their character as hollow processes, even when almost as minute as the fibres of cellular (areolar) tissue.

The distinction between cell-membrane and cell-contents is nowhere more distinctly defined than in the fully-developed cells of this class. In the perfected cells of the pith of feathers, for example, it is as marked as we ever find it to be in plants. When traced backwards to their earliest stages of development, their true cellular formation scarcely admits of a doubt, although the cell-membrane, for reasons given at page 36, cannot be so clearly distinguished. The elementary cells of the tissues of the following classes, in most instances, do not advance beyond this early stage in the development of the feather-cells, but the changes necessary to the formation of the subsequent tissues occur at this period; their cellular nature is, however, quite as certain as is that of the young feather-cells, although it be not possible to recognize their cell-wall so clearly as in their perfectly-developed condition.

The matter contained in the cells is either a transparent fluid, as in the cells of the pith of feathers previously to their becoming dry, or in the crystalline lens, when it contains albumen ; or, a minutely-granulous mass, as in many epithelium-cells, or pigment-granules; or, it is altogether absent, and the cell-walls, in consequence of their flattening, are in immediate contact. The air in the cells of the pith of mature feathers simply penetrates from without, during the process of their desiccation. With the exception of some of the cells of the lens, all the cells of this class are invariably furnished with a nucleus of the characteristic form. It is not, however, a persistent structure, as in the previous class, but in very many instances becomes absorbed when the cells have reached maturity; such is the case in the pith of the feather, the superior laminae of the epidermis, the nails, crystalline lens, &c. &c.

As a general rule the cells remain independent during all these changes, that is to say, each cell retains its especial wall, and its own peculiar closed cavity. More or less complete blendings of the cell-walls, and even of their cavities also, occur, however, as exceptions even in this class. The epithelial scales of the nail are so intimately connected together, that it is rarely possible to trace the contour of one of them in its entire circumference; and the same appears to be the case with the epithelium in the vessels of the adult. The coalescence, however, does not appear to be perfect, for, by the employment of concentrated acids, the scales of the nail may be separated somewhat more readily from each other. A union of the cavities of several cells seems to occur in the pigment-cells. A prolongation of a cell filled with pigment may be seen to pass uninterruptedly to the cavity of another cell (plate II, fig. 9, a). In such an instance, probably, the prolongations of two cell-cavities join at a certain point, the cell-walls unite together there, and the partition-wall becomes absorbed, and thus an uninterrupted passage from one cell-cavity into the other is produced. I am not certain as to whether a similar process does not take place in some fibres of the crystalline lens.

The transformations which the cells undergo are not, however, restricted to those already mentioned. A completely opposite process occurs in the cortical substance of the shaft of feathers, viz. a division of the cells into fibres. By this process, out of a single cell several fibres are generated, which, in the first instance, are united together by the rest of the substance of the cell, but at a later period of development may be insulated to a considerable extent. An elongation of the cells into these fibres takes place, indeed, at the same time, but the major portion of each fibre is formed by the division of the bodies of the cells.

With respect to the formation of the cells of this class, we find it to be a constant rule, that their size increases in proportion with their age, a fact which Henle has already pointed out with. regard to the epithelium. We have seen in the different tissues, that the nucleus is first present, that the cell is then formed around it, the nucleus, therefore, being the true cytoblast, and that it holds the same position in these cells that it does in those of plants, being fixed eccentrically upon the internal surface of the wall. Cell and nucleus advance in growth for a time, the former, however, much more vigorously than the latter. The nucleus is generally absorbed after the formation of the cell is completed. The generation and growth of the cells and all the phenomena connected with the nucleus resemble those of the vegetable cells, and we may unhesitatingly draw a parallel between them. In no class is the quantity of the cytoblastema smaller than in this. In the immature state the walls of the cells lie close together, with at the most, but a minimum of intercellular, substance between them at points where three cells are in contact, and it is only between those nuclei, around which no cells have as yet formed, that a somewhat larger quantity of cytoblastema is present.

The class of cells now treated of, and the teeth which will be examined in the following class, have been comprised under the term unorganized tissues, and their growth represented as dependent upon a secretion of the so-called matrix. If by this it is meant that the substance of horn is secreted by the matrix and hardened in the air, the view is manifestly an erroneous one; what we call horny substance being either merely the cell-walls, when, for example, the cells are flat, and there are no cell-contents, or the cell-walls and cell-contents together, when the cells are polyhedral, as in hoofs. All these cells are independent structures, which grow organically. But if, by the above description, it is meant that the organized matrix only furnishes (or secretes) the cytoblastema, no important objection can be raised. The cells of the horny tissue require a nutritive fluid for their growth. This is supplied to them by the blood, as it is in all tissues. As, however, the blood-vessels themselves do not pass between the cells of the horny tissue, the nutritive fluid must be furnished by the nearest substance in which blood-vessels exist, and in this sense the nearest organized substance may be called, matrix. But whether this cytoblastema which exudes from the matrix have a specific character, and on that account horn-cells are formed in it—or whether their formation take place in it for the same reason that the muscle-cells, those of areolar tissue, and so on, originate in other parts of the body, that is to say, whether it is determined by the plan of the entire organism, —is a question which does not as yet admit of a decision. It is, however, a characteristic of all the cells of this class (with the exception of the crystalline lens, which I have not examined in reference to the point), that the new cells are not generated between those already formed, but only in the cytoblastema nearest to the organized substance, if not, indeed, always in immediate contact with it. The teeth were necessarily separated from this class, because, as we shall see hereafter they present quite a different relation of the cells. The new cells of cartilage, so long as it does not contain any vessels, are not only formed upon the surface of the tissue, but also between the most recently-formed cells.

The chorda dorsalis forms the transition from this class to the following one. The cell-walls remain separate in the highest stage of their development, and it is only in their rudimentary forms, in the osseous fishes for example, that they coalesce and exhibit fibres between the cell-cavities. It does not appear to possess any vessels. The formation of new cells goes on at the extremities, for instance, at the point of the tail of the tadpole; it is not, however, limited to the surface, but appears to take place between the most recently-formed cells, for cytoblasts may be observed in the intercellular substance between the cells which have reached maturity. In this respect the chorda dorsalis resembles cartilage, but differs again from it, in that, as Müller discovered, it undergoes no change in boiling water, and also, in that, the nuclei are flat, while those of cartilage-cells are round or elliptical.

If the chorda dorsalis be reckoned in this class, it affords, as we have seen, an example of the generation of cells within cells. A different signification might, however, be ascribed to these young cells within the true cells of the chorda dorsalis, for they do not seem to be formed like their parent-cells, from cytoblasts. A generation of cells with cells takes place also in the lens. In all the other tissues of this class, with few exceptions, the formation of new cells takes place only on the outside of those already existing.

1. During several years past I have occasionally observed an innermost apparently structureless layer in different parts of the vessels, and as the elastic fibres of the middle coat of arteries become gradually more and more minute towards the interior of the vessel, and at length are scarcely perceptible, I regarded the layer above described as analogous to the middle arterial coat, in every respect but the possibility of discovering fibres in it. I explained certain scattered spots which occurred in it, by analogy with the middle and external coats of vessels. Lamellæ, for instance, were occasionally present, in which the elastic fibres had coalesced more or less intimately, and only a trace of a fibrous arrangement remained. In such instances there is seen a table composed of elastic tissue, perforated at different spots; I regarded those spots as openings which might perhaps be filled with some foreign substance. Purkinje and Räuschel (de Arter. et Venar. Structurâ) acknowledged the accordance of this membrane with the middle arterial coat, but distinguished it as a separate layer. Valentin denied that accordance, and described it as a peculiar structureless membrane. Henle was the first to explain its true relations. By his mode of scraping the internal surface of the vessels he obtained scales, which, from our present more accurate knowledge, we now recognise as epithelium. They were sometimes converted into lamellæ. There cannot in fact be a doubt about the correctness of this explanation, when the vessels of the fœtus are examined. I obtained by scraping, both from the larger veins and heart of a fœtal pig, large lamellæ of the most beautiful epithelium, consisting of flat stripes, which were nearly as long again as broad, and contained a very distinct and, in proportion to the size of the scales, large nucleus, with one or two nucleoli. I could not succeed so well in the few attempts which I made on arteries; probably the scales separate more readily from one another in them, and can then no longer be distinguished from the primitive cells of the elastic coat. The cells probably coalesce more or less intimately at a subsequent period, so as to form what is then a partially structureless layer, and the nuclei also disappear in part. I now conjecture that the above-described spots upon the inner coat may probably be persistent nuclei; I have not, however, made any new investigation upon the subject. With respect to the situation in which the one or other form of epithelium occurs, I refer to Henle’s very complete treatise (Müller’s Archiv, 1838, Heft 1). In addition to the parts mentioned by Henle, I have found epithelium upon the internal surface of the amnion in the fœtus of mammalia and man, where the hexagonal scales were very large and beautiful, enclosing a very distinct nucleus and nucleolus. Amongst those in the fœtal pig were some larger round cells, furnished with a larger nucleus without a nucleolus. The inner surface of the portion of the allantois projecting from the chorion in the same fœtus was also lined with tesselated (tabular, scaly) epithelium consisting of small scales. The external surface of the chorion was formed of cylindrical cells closely packed together, and provided with a nucleus, being similar to the epithelial cylinders of the intestinal mucous membrane discovered by Henle.