Form and Function: A Contribution to the History of Animal Morphology

by E. S. Russell

The idea that the cell-contents might show a characteristic and individual structure had hardly dawned upon botanists when Schleiden published his famous paper, Beiträge zur Phytogenesis.[240] Schleiden's theme in this paper is the origin and development of the plant cell, a subject then very obscure, in spite of pioneer work by Mirbel. A few years before, Robert Brown had called attention to the presence in the epidermal cells of orchids and other plants of a characteristic spot which he called the areola or nucleus.[241] Schleiden saw the importance of this discovery, confirmed the constant presence of the nucleus in young cells, and held it to be an elementary organ of the cell. He named it the cytoblast because, in his opinion, it formed the cell. It was embedded in a peculiar gummy substance, the cytoblastem, which formed a lining to the cellulose cell-wall. Within the nucleus there was often a small dark spot or sphere—the nucleolus. The nucleus, Schleiden thought, originated as a minute granule in the cytoblastem which gradually increased in size, becoming first a nucleolus (Kernchen), and then, by further condensation of matter round it, a nucleus. Several nuclei might be formed in this way in a single cell. New cells took their origin directly from a full-grown nucleus, in a peculiar way which Schleiden describes as follows:—"As soon as the cytoblasts have reached their full size a delicate transparent vesicle arises on their surface; this is the young cell, which at first takes the shape of a very flat segment of a sphere, of which the plane surface is formed by the cytoblast, the convex side by the young cell itself, which lies upon the cytoblast like a watch-glass on a watch" (p. 145). The young cells increase in size and fill up the cavity of the old cell, which is in time resorbed. Cell-development always takes place within existing cells, and either one or many new cells may be formed within the mother-cell. Schleiden's views on cell-formation were drawn from some rather imperfect observations on the embryo-sac and pollen-tube, but he extended his theory to cell-formation in general. Though wrong in almost all respects the theory had at least the merit of fixing attention upon the really important constituents of the cell, the nucleus and the cell-plasma. To Schleiden, too, we owe the conception of the cell as a more or less independent living unity, whose life is not entirely identified with the life of the plant as a whole. "Each cell," he writes, "carries on a double life; one a quite independent and self-contained life, the other a dependent life in so far as the cell has become an integral part of the plant" (p. 138).

So long as the definition of the plant cell embraced little more than the hardened cell-wall it was little wonder that "cells" in this sense were not recognised in animal tissues, except in a few exceptional cases—as in the notochord by Johannes Müller.[242] Careful observation of animal tissues discovered in some cases the existence of discontinuous units of structure, but these were not, as a rule, recognised before 1838 as analogous to plant cells. Von Baer, for example, observed that the young chick embryo was composed partly of an albuminous mass and partly of Kügelchen or little globules suspended in it (Entwickelungsgeschichte, i., pp. 19, 144). Since such Kügelchen disposed in a row formed the notochord (i., p. 145) it seems probable that his Kügelchen were really cells. Similarly A. de Quatrefages[243] in 1834 saw and figured segmentation spheres in the developing egg of Limnæa, but he called them globules and did not recognise their analogy with the cells of plants. According to M'Kendrick,[244] Fontana, so far back as 1781,[245] described cells with nuclei in various tissues, and used acids and alkalis to bring out their structure more clearly. But it was not till 1836-7-8 that a fairly widespread occurrence of cells in animal tissues was recognised. The pioneer in this seems to have been Purkinje, who described cells in the choroidal plexus in 1836,[246] and compared gland cells with the cells of plants in 1837.[247] Henle in 1837[248] and 1838[249] described various kinds of epithelial tissue, distinguishing them according to the kind of cell composing them; he also discovered the mode of growth of stratified epithelium. Valentin[250] appears to have seen cells in cartilage and epithelium even before Henle, and to have observed cells in the blastoderm of the chick. In his report on the progress of anatomy during 1838 Johannes Müller was able to refer to quite a number of papers dealing with the occurrence of cells in animal tissues. In addition to those already noted, he mentions work by Breschet and Gluge on the cells of the umbilical cord, by Dumortier on the cells in the liver of molluscs, by Remak and by Purkinje on nerve cells, by Donné on the cells of the conjuctiva, cornea and lens. He reports, too, that Turpin had compared the epithelial cells of the vagina with the cell-tissue of plants. Müller himself had not only recognised the cellular nature of the notochord, but had observed the cells of the vitreous humour, fat cells and pigment cells, and even the nuclei of cartilage cells. From Schwann (1839) we learn that C. H. Schults had followed back the corpuscles of the blood to their original state of nucleated cells, and that Werneck had recognised cells in the embryonic lens. A preliminary notice of Schwann's own work appeared in 1838 (Froriep's Notizen, No. 91, 1838), the full memoir in 1839, under the title Mikroskopische Untersuchungen über die Uebereinstimmung in der Struktur und dem Wachstume der Tiere und Pflanzen.[251]

Theodor Schwann was a pupil of Johannes Müller, and we know that Müller took much interest in the new histology. It is probably to his influence that we owe Schwann's brilliant work on the cell, which appeared just after Schwann left Berlin for Löwen. Schwann was himself, as his later work showed, more a physiologist than a morphologist; he did quite fundamental work on enzymes, discovering and isolating the pepsin of the gastric juice; he proved that yeast was not an inorganic precipitate but a mass of living cells; he carried out experiments directed to show that spontaneous generation does not occur. We shall see in his treatment of the cell-theory clear indications of his physiological turn of mind. Schwann was only twenty-nine when his master-work appeared, and the book is clearly the work of a young man. It has the clear structure, the logical finish, which the energy of youth imparts to its chosen work. So the work of Rathke's prime, the Anatomische-philosophische Untersuchungen of 1832 shows more vigour and a more reasoned structure than his later papers. Schwann's book is indeed a model of construction and cumulative argument, and even for this reason alone justly deserves to rank as a classic.

The first section of his book is devoted to a detailed study of the structure and development of cartilage cells and of the cells of the notochord, and to a comparison of these with plant cells. He accepts Schleiden's account of the origin and development of nuclei and cells as a standard of comparison; and he seeks to show that nucleus and nucleolus, cell-wall and cell-contents, show the same relations and behave in the same manner in these two types of animal cells as in the plant-cells studied by Schleiden. The types of cell which he chose for this comparison are the most plant-like of all animal cells, and he was even able to point to a thickening of the cell-wall in certain cartilage cells, analogous to the thickening which plays so important a part in the outward modification of plant-cells. The analogy indeed in structure and development between chorda and cartilage cells and the cells of plants seemed to him complete. The substance of the notochord consisted of polyhedral cells having attached to their wall an oval disc similar in all respects to the nucleus of the plant-cell, and like it containing one or more nucleoli. Inside the mother-cell were to be found young developing cells of spherical shape, lacking however a nucleus. Cartilage was even more like plant tissue. It was composed of cells, each with its cell membrane. The cells lay close to one another, separated only by their thickened cell-wall and the intercellular matrix, showing thus even the general appearance of the cellular tissue of plants. They contained a nucleus with one or two nucleoli, and the nucleus was often resorbed, as in plants, when the cell reached its full development. Other nuclei were in many cases present in the cell, round which young cells could be seen to develop, in exactly the same manner as in plants. These nuclei had accordingly the same significance as the nuclei of plants, and deserved the same name of cytoblasts or cell-generators. The true nucleus of the cartilage cell was probably in the same way the original generator of the mother-cell.

Having proved the identity in structure and function of the cells of these selected tissues with the cells of plants, as conceived by Schleiden, Schwann had still to show that the generality of animal tissues consisted either in their adult or in their embryonic state of similar cells. This demonstration occupies the second and longest section of his book.

His method is throughout genetic; he seeks to show, not so much that all animal tissues are actually in their finished state composed of cells and modifications of cells, as that all tissues, even the most complex, are developed from cells analogous in structure and growth with the cells of plants.

All animals develop from an ovum; it was his first task to discover whether the ovum was or was not a cell. It happened that, some years before Schwann wrote, a good deal of work had been done on the minute structure of the ovum, particularly by Purkinje and von Baer. Purkinje in 1825[252] discovered and described in the unfertilised egg of the fowl a small vesicle containing granular matter, which he named the Keimbläschen or germinal vesicle. It disappeared in the fertilised egg. As early as 1791 Poli had seen the germinal vesicle in the eggs of molluscs, but the first adequate account was given by Purkinje. In 1827[253] von Baer discovered the true ova of mammals and cleared up a point which had been a stumbling block ever since the days of von Graaf, who had described as the ova the follicles now bearing his name.[254] Even von Graaf had noticed that the early uterine eggs were smaller than the supposed ovarian eggs; Prévost and Dumas[255] had observed the presence in the Graafian follicle of a minute spherical body, which, however, they hesitated to call the ovum; it was left to von Baer to elucidate the structure of the follicle and to prove that this small sphere was indeed the mammalian ovum. His discovery was confirmed by Sharpey and by Allen Thomson. Von Baer found the germinal vesicle in the eggs of frogs, snakes, molluscs, and worms, but not in the mammalian ovum; he considered the whole mammalian ovum to be the equivalent of the germinal vesicle of birds—a comparison rightly questioned by Purkinje (1834). In 1834 Coste[256] discovered in the ovum of the rabbit a vesicle which he considered to be the germinal vesicle of Purkinje; he observed that it disappeared after fertilisation. Independently of Coste, and very little time after him, Wharton Jones[257] found the germinal vesicle in the mammalian ovum. Valentin in 1835,[258] Wagner in 1836,[259] and Krause in 1837,[260] added considerably to the existing knowledge of the structure of the ovum. Wagner in his Prodromus called attention to the widespread occurrence, within the germinal vesicle of a darker speck which he called the Keimfleck or germinal spot, known sometimes as Wagner's spot. He recognised the Keimfleck in the ova of many classes of animals from mammals to polyps. Frequently more than one Keimfleck occurred.

Schwann had therefore a good deal of exact knowledge to go upon in discussing the significance of the ovum for the cell-theory. There were two possible interpretations. Either the ovum was a cell and the germinal vesicle its nucleus, or else the germinal vesicle was itself a cell within the larger cell of the ovum and the germinal spot was its nucleus. Schwann had some difficulty in deciding which of these views to adopt, but he finally inclined to the view that the ovum is a cell and the germinal vesicle its nucleus, basing his opinion largely upon observations by Wagner which tended to prove that the germinal vesicle was formed first and the ovum subsequently formed round it. But the ovum was not, in Schwann's view, a simple cell, for within it were contained yolk-granules, one set apparently containing a nucleus, the others not. Even the second set, those composing the yellow yolk, were considered by Schwann to deserve the name of cells, because, although a nucleus could not be observed in them, they had a definite membrane, distinct from their contents—a conception of the cell obviously dating from the earliest botanical notions of cells as little sacs. The yolk cells were not mere dead food material but living units which took part in the subsequent development of the egg. The relation between the unfertilised egg and the blastoderm which arises from it is not made altogether clear by Schwann. According to his account the cells of the blastoderm are formed actually in the ovum. Round the nucleus of the egg appears a Niederschlag or precipitate which is the rudiment of the blastoderm (p. 68). When the egg leaves the ovary the nucleus disappears, leaving behind it this rudiment of the blastoderm, which rapidly grows and increases in size. The blastoderm of the chick before incubation is found to be composed of spherical anucleate bodies which Schwann considers to be cells, because they almost certainly develop into the cells of the incubated blastoderm, which are clearly recognisable as such after eight hours' incubation. The serous and mucous layers can be distinguished after sixteen hours' incubation, and it is found that the cells of the serous layer contain definite nuclei, though such seem to be absent in the cells of the mucous layer. Between the two layers other cells are formed belonging to the vessel layer, which is, however, in Schwann's opinion not a very definitely individualised layer.

Schwann's next step is a detailed demonstration of the origin of each tissue from simple cells such as those composing the incubated blastoderm.

"The foregoing investigation has taught us that the whole ovum shows nothing but a continual formation and differentiation of cells, from the moment of its appearance up to the time when, through the development of the serous and mucous layers of the blastoderm, the foundation is given for all the tissues subsequently appearing: we have found this common parent of all tissues itself to consist of cells; our next task must be to demonstrate not only in this general way that tissues originate from cells, but also that the special formative mass of each tissue is composed of cells, and that all tissues are either constituted by simple cells or by one or other of the manifold kinds of modified cells" (p. 71). Five classes of tissue can be distinguished, according to the extent and manner of the modifications which the cells composing them have undergone. There are first of all independent and isolated cells, such as the corpuscles of the blood and lymph, not forming a coherent tissue in the ordinary sense. Next there are the assemblages of cells lying in contiguity with one another, but not in any way fused; examples of this class are the epidermal tissues and the lens of the eye. In the third class come tissues the cells of which have fused by their walls, but whose cell-cavities are not in continuity, such as osseous tissue and cartilage. In the tissues of the fourth class, comprising the most highly specialised of all, not only are the cell-walls continuous but also the cell-cavities; to this class belong muscle, nerve and capillary vessels. A fifth class, of rather a special nature, includes the fibrous tissues of all kinds. This is the first classification of tissues upon a cellular basis, and it marks the foundation of a new histology which took the place of the "general anatomy" of Bichat. The exhaustive account which Schwann gives of the structure and development of the tissues in this section of his book constitutes the first systematic treatise on histology in the modern sense, and it is still worth reading, in spite of many errors in detail.