The Origin of Vertebrates

by Walter Holbrook Gaskell

The evidence given by Braem, and it could be strengthened considerably, is conclusive against the morphological importance of the theory of the germinal layers, and transfers the fundamental importance of the early embryonic formation, from that of a three-layered embryo to that of a single-layered embryo—the blastula—from which, in various ways, the adult animal has arisen.

The derivation of both arthropod and vertebrate from such a single-layered animal is perfectly conceivable, even though the gut of the latter is not homologous with the gut of the former. We have seen that the teachings of embryology, as far as its later stages are concerned, afford one of the main supports upon which this theory rests. What, therefore, is required to complete the story is the way in which these later stages arise from the blastula stage; here, as in all cases, the ontogenetic laws must be in harmony with the phylogenetic; of the latter the most important is the steady development of the central nervous system for the upward progress of the animal race. The study of comparative anatomy indicates the central nervous system, not the gut, as the keystone of the edifice. So, also, it must be with ontogeny; here also the central factor in the formation of the adult from the blastula ought to be the formation of the central nervous system, not that of the gut.

Such, it appears to me, is the case, as may be seen from the following considerations.

The study of the development of any animal can be treated in two ways: either we can trace back from the adult to the very beginning in the ovum, or we can trace forward from the fertilized egg to the adult. Both methods ought to lead to the same result; the difference is, that in the first case we are passing from the more known to the less known, and are expressing the unknown in terms of the known. In the second case we are passing from the less known to the more known, and are expressing the known in speculative terms, invented to explain the unknown. What has just been said with respect to the germinal layers means that, however much we may study the embryo and try to express the adult in terms of it, we finally come back to the first way of looking at the question, and, starting with the adult, trace the continuity of function back to the first formation of cells having a separate function.

Let us, then, apply this throughout, and see what are the logical results of tracing back the various organs and tissues from the adult to the embryo.

The adult body is built up of different kinds of tissues, which fall naturally, from the standpoint of physiology, into groups. Such groups are, in the first place—

1. All those tissues which are connected with the central nervous system, including in that group the nervous system itself.

2. All those tissues which have no connection with the nervous system.

In the second group the physiologist places all germinal cells, all blood- and lymph-corpuscles, all plasma-cells and connective tissue and its derivatives—in fact, all free-living cells, whether in a free state or in a quiescent, so to speak encysted, condition, such as is found in connective tissue. In the first group the physiologist recognizes that the central nervous system is connected with all muscular tissues, whether striped or unstriped, somatic or splanchnic, and that such connection is of an intimate character. Further, all epithelial cells, either of the outer or inner surfaces, whether forming special sense-organs and glands, such as the digestive and sweat-glands, or not, are connected with the nervous system. Besides these structures, there is another set of organs as to which we cannot speak definitely at present, which must be considered separately, viz. all the cells, together with their derived organs, which line the body-spaces. Whatever may be the ultimate decision as to this group of cells, it must fall into one or other of the two main groups.

The members of these two groups are so interwoven with one another that either, if taken alone, would still give the form of the body, so that, in a certain sense, we can speak of the body as formed of two syncytia, separate from each other, but interlaced, of which the one forms a continuous whole by means of cells connected together by a fluid medium or by solid threads formed in such fluid medium, while the other does not form a syncytium in the sense that any cell of one kind may be connected with any cell of another kind, but a syncytium of which all the different elements are connected together only through the medium of the nervous system.