The Origin of Vertebrates

by Walter Holbrook Gaskell

We see, then, that the origin of all these free-living cells can be traced back to the very earliest of the Metazoa. Here between the dermal and gastral layers a gelatinous material, the mesoglœa is secreted by these layers. This material is non-living, non-cellular. In it live free cells which may either be germ-cells, amœbocytes, or 'collencytes' (connective tissue cells). If this mesoglœa were a fluid secretion, then we should have a tissue of the nature of blood or lymph; if it were solid, then we should have the foundation of connective tissue, cartilage, and bone.

From this primitive tissue it is easy to see how the special elements of the vascular, lymphatic, and skeletal tissues gradually arose, the matrix being provided by the cells of the syncytial host and the cellular elements by the archæocytes. In fact, we have no right to speak of these lowest members of the Metazoa as not being triploblastic, as possessing nothing corresponding to mesoblast, for in these free cells in the mesoglœa we have the origin of the mesenchyme of the higher groups. Thus Lankester, talking of mesenchyme, says: "I think we are bound to bring into consideration here the existence in many Cœlentera of a tissue resembling the mesenchyme of Cœlomocœla. In Scyphomedusæ, in Ctenophora, and in Anthozoa, branched fixed and wandering cells are found in the mesoglœa which seem to be the same thing as a good deal of what is distinguished as mesenchyme in Cœlomocœla. These appear to be derived from both the primitive layers; some produce spicules, others fibrous substance, others again seem to be amœbocytes with various functions. It appears to be probable that, though it may be necessary to distinguish other elements in it, the mesenchyme of Cœlomocœla is largely constituted by cells, which are the mother-cells of the skeletotrophic group of tissues, and are destined to form connective tissues, blood-vessels, and blood."

Thus we see that the earliest Metazoa were composed of a dermal and gastral epithelium, with a sub-epithelial nervous system connecting the parts together, which formed, as it were, a host, carrying around free living cells of varying function, all of which may be looked on as derived from archæocytes, i.e. germ-cells. From these the cœlomatous animals arose, and here also we find, according to present-day opinion, that the cœlom arose in the first place in the very closest connection with the germ-cells or gonads. Thus Lankester, in his review of the history of the cœlom, states:—

"The numerous embryological and anatomical researches of the past twenty years seem to me to definitely establish the conclusion that the cœlom is primarily the cavity, from the walls of which the gonad cells (ova or spermata) develop, or which forms around those cells. We may suppose the first cœlom to have originated by a closing or shutting off of that portion of the general archenteron of Enterocœla (Cœlentera), in which the gonads developed as in Aurelia or as in Ctenophora. Or we may suppose that groups of gonad mother cells, having proliferated from the endoderm, took up a position between it and the ectoderm, and there acquired a vesicular arrangement, the cells surrounding the cavity in which liquid accumulated.

"The cœlom is thus essentially and primarily (as first clearly formulated by Hatschek) the perigonadial cavity or gonocœl, and the lining cells of gonadial chambers are cœlomic epithelium. In some few groups of Cœlomocœla the cœloms have remained small and limited to the character of gonocœls. This seems to be the case in the Nemertina, the Planarians, and other Platyhelmia. In some Planarians they are limited in number, and of individually large size; in others they are numerous."

When Lankester says that "the lining cells of gonadial chambers are cœlomic epithelium," that is equivalent to saying that the lining cells of the cœlom form an epithelium which was originally gonadial, provided that, as seems to me most probable, his second suggestion, of the cœlom being formed from gonadial mother-cells which have taken up an intermediate position between endoderm and ectoderm and there acquired a vesicular arrangement, is the true one. It does not seem to me possible to conceive of the gonads arising from cells of the epiblast or of the hypoblast, in the sense that such cells are differentiated cells belonging to a layer with a definite meaning. When we consider that the gonad gives origin to the whole of a new individual, that in the protozoan ancestors of the Metazoa their ultimate aim and object was the formation of gonads, it seems a wrong conception to speak of the gonads as formed from cells belonging either to the gut-wall or to the external epithelium. The gonads must stand in a category by themselves; they represent a whole, while the other cells represent only a part; they cannot therefore be derived from the latter. They may, and indeed do, give rise to cells of a subordinate character, but they cannot rightly be spoken of as derived from such cells. The very fact mentioned by Lankester, that in the lowest cœlomatous Metazoa, the Platyhelminthes, the cœloms are limited to the character of simple gonocœls, strongly points to the conclusion that all the cœlomic cells were originally of the nature of gonadial cells, and therefore free-living and independent of the rest of the cells of the body. Whether the germ-cells appear, as in Hydra, to be derived from the ectoblast, or, as is usually stated, from the endoblast, in neither case ought they to be classed with the internal or external epithelium; they are germ-cells, and the epithelium which they form is neither epiblastic nor hypoblastic, but germinal, forming originally a simple gonocœle, afterwards, in the higher forms, the cœlom with its cells of various function. Thus, to quote again from Lankester, "The cœlomic fluid and the cœlomic epithelium, as well as the floating corpuscles derived from that epithelium, acquire special properties and importance over and above the original functions subservient to the maturation of the gonadial cells ... the most important developments of the cœlom are in connection with the establishment of an exit for the generative products through the body-wall to the outer world, and further in the specialization of parts of its lining epithelium for renal excretory functions."

Such exits led very early to the formation of cœlomoducts, which are true outgrowths of the cœlom itself (p. 14): "The cœlomoducts and the gonocœls of which they are a part, frequently acquire a renal excretory function, and may retain both the function of genital conduits and of renal organs, or may, where several pairs are present (metamerized or segmented animals), subserve the one function in some segments of the body, and the other function in other segments."

The origin of the cœlom and its derivatives from a germinal membrane, as suggested by Lankester, appears to me most probable, and, if true, it carries with it conclusions of far-reaching importance, for it necessitates that all the cells which line true cœlomic cavities, and their derivatives, belong to the category of free-living cells, and are not connected with the nervous system. The cells in question are essentially those which line serous cavities and those which form excretory glands such as the kidneys. In the latter organ we ought especially to be able to obtain a clear answer to this question, for is it not a gland which secretes into a duct and might therefore be expected to be innervated in the same way as other secretory glands? Although there is a strong primâ facie presumption in favour of the existence of renal secretory nerves, yet according to the universal opinion of physiologists no evidence in favour of such nerves has hitherto been found; all the phenomena of excretion of urine consequent on nerve stimulation are explicable by the action of nerves on the renal vessels, not on the renal cells. Not only is the physiological evidence negative up to the present time, but also, I think, the histological. On the one hand, Retzius has failed to find nerve-connections with kidney-cells; on the other, Berkley has obtained such evidence with the Golgi method, but failed entirely with methylene blue. I do not myself think that the evidence of the Golgi method alone is sufficient without corroboration by other methods, and, in any case, Berkley's evidence does not show the nerve-fibres terminating in the kidney-cells, in the same way as can be shown by modern methods to exist in the case of epithelial cells of the surface, etc. Quite recently another paper on this subject has appeared by Smirnow, who appears to have obtained better results than those given by Berkley.

Apart from these physiological and histological considerations, this question is also dependent upon the nature of the development of the excretory organs, for, according to Lankester, all excretory organs may be divided into the two classes of nephridial organs and cœlomostomes, of which the former are largely derived from epiblast. We should, therefore, expect to find secretory nerves to nephridial organs, though possibly not to cœlomostomes. The kidneys of the Mammalia are supposed to be true cœlomostomes, although, according to Goodrich's researches, the excretory organs in Amphioxus are solenocytes, i.e. true nephridia.

As to the lining epithelium of the peritoneal, pleural, and pericardial cavities—i.e. the mesothelium—there is no definite evidence that these cells are provided with nerves. Such surfaces are remarkably insensitive in the healthy condition, and the pain in such cavities is essentially a pressure phenomenon and referable to special sense-organs, such as Pacinian bodies, etc., rather than to the mesothelium itself.