The Origin of Vertebrates

by Walter Holbrook Gaskell

We may then, I would suggest, look upon the adult as formed of a neural syncytium, which we may call the host, which carries with it in its meshes a number of free cells not connected with the nervous system. If, then, we confine our attention to the host and trace back this neural syncytium to its beginnings in the embryo, we see that, from the very nature of the neuro-epithelial couple, each epithelial moiety must approach nearer and nearer to its neural moiety, until at last it merges with it; the original neuro-epithelial cell results, and we must obtain, as far as the host is concerned, a single-layered blastula as the origin of all Metazoa. It follows, further, that there must always be continuity of growth in the formation of the host, i.e. in the formation of the neuro-epithelial syncytium; that therefore cells which have been previously free cannot settle down and take part in its formation, as, for instance, in the case of the formation of any part of the gut-epithelium or of muscle-cells from free-living cells.

Further, since the neural moiety is the one element common to all the different factors which constitute the host, it follows that the convergence of each epithelial moiety to the neural moiety, as we pass from the adult to the embryo, is a convergence of all outlying parts to the neural moiety, i.e. to the central nervous system, if there is a concentrated nervous system. Conversely, in the commencing embryo the place from which the spreading out of cells takes place, i.e. from which growth proceeds, must be the position of the central nervous system, if the nervous system is concentrated. If the nervous system is diffuse, and forms a general sub-epithelial layer, then the growth of the embryo would take place over the whole surface of the blastula.

Turning now to the consideration of the second group of tissues, those that are not connected with the central nervous system, we find that they include among them such special cells as the germinal cells, free cells of markedly phagocytic nature, and cells which were originally free and phagocytic, but have settled down to form a supporting framework of connective tissue, and are known as plasma-cells. In the embryo we find also in many cases free cells in the yolk, forming more or less of a layer, which function as phagocytes and prepare the pabulum for the fixed cells of the growing embryo; these cells are known by the name of vitellophags, and in meroblastic vertebrate eggs form somewhat of a layer known by the name of periblast. Such cells must be included in the second group, and, indeed, have been said again and again to give origin to the free-living blood-corpuscles of the adult. In other cases they are said to disintegrate after their work is done.

In the adult the free-living lymphocytes and hæmocytes reproduce themselves from already existing free-living cells, but as we pass back to the embryo there comes a time, comparatively late in the history of the embryo, when such free-living cells are not found in the fluids of the body, and they are said to arise from the proliferation and setting free of cells which form a lining epithelium. Such formation of leucocytes has been especially described in connection with the lining epithelium of the cœlomic cavities, as stated in Chapter XII., so that anatomists look upon the origin of these free cells as being largely from the cœlomic epithelium, or mesothelium, as Minot calls it.

Then, again, the free cells which form the germinal cells can be traced back to a germinal epithelium, which also is part of the cœlom. Thus the suggestion arises that in the embryo a cellular lining is formed to a cœlomic cavity (mesothelium) composed of cells which have no communication with the nervous system, and are capable of a separate existence as free individuals, either in the form of germinal cells or of lymphocytes, hæmocytes, and plasma-cells, so that these latter free cells may be considered as living an independent existence in the body, and ministering to it in the same sense as the germ-cells live an independent existence in the body. Again, the function of this mesothelium apart from the germ-cell is essentially excretory and phagocytic. It is the cells of the excretory organs as well as the lymphocytes which pick up carmine-grains when injected. It is the cells of the modified excretory organs, as mentioned in Chapter XII., which, according to Kowalewsky and others, give origin to the free leucocytes.

We see, then, that the conception of a syncytial neuro-epithelial host holding in its meshes a number of free cells leads directly to the questions: What is the cœlom? To which category does its lining membrane belong? and further, also, What is the origin of these free cells?

The Metazoa have been divided into two great groups—those which possess a cœlom (the Cœlomata; Lankester's Cœlomocœla) and those which do not (Cœlenterata; Lankester's Enterocœla). As an example of the latter we may take Hydra, because it is a very primitive form, and because its development has been carefully worked out recently by Brauer.

In Hydra we find a dermal layer of cells and an inner layer of cells separated by a gelatinous mass known as mesoglœa; in this mass between the dermal and inner layers scattered cells are found, the interstitial cells. Now, according to Brauer the position of the germ in Hydra is the interstitial cell-layer. One cell of the ovarium becomes the egg-cell, the others have their substance changed into yolk-grains, forming the so-called pseudo-cells, and as such afford pabulum to the growing egg-cell. Thus we see that in between the dermal and gastral layer of cells a third layer of cells is found, composed of free living germ-cells, some of which, by the formation of yolk-granules, become degraded into pabulum for their more favoured kinsfolk. These interstitial cells are said to arise from the dermal layer, or ectoderm, but clearly, as in other cases, germ-cells constitute a class by themselves and cannot be spoken of as originating from ectoderm-cells or from hypoderm-cells.

So also in Porifera, Minchin states: "In addition to the collared cells of the gastral layer, and the various cell-elements of the dermal layer, the body-wall contains numerous wandering cells or amœbocytes, which occur everywhere among the cells and tissues. Though lodged principally in the dermal layer, they are not to be regarded as belonging to it, but as constituting a distinct class of cells by themselves. They are concerned probably with the functions of nutrition and excretion, and from them arise the genital products." Further (p. 31): "At certain seasons some of these cells become germ-cells; hence the wandering cells and the reproductive cells may be included together under the general term archæocytes." Also (p. 51): "The mesoglœa is the first portion to appear as a structureless layer between the dermal and gastral epithelia, and is probably a secretion of the former."

He also points out that in these, the very lowest of the Metazoa, the separate origin of these archæocytes can be traced back to a very early period of embryonic life. Thus in Clathrina blanca the ovum undergoes a regular and total cleavage, resulting in the formation of a hollow ciliated blastula of oval form. At one point, the future posterior pole of the larva, are a pair of very large granular cells with vesicular nuclei, which represent undifferentiated blastomeres and are destined to give rise to the archæocytes, and, therefore, also to the sexual cells of the adult. Thus, as he says, from the very earliest period a distinction is made between the "tissue-forming" cells (my syncytial host) and the archæocytes.