The Origin of Vertebrates

by Walter Holbrook Gaskell

Any attempt to discover how vertebrates arose from invertebrates must be based upon the study of Comparative Anatomy, of Palæontology, and of Embryology. The arguments and evidence put forward in the preceding chapters show most clearly how the theory of the origin of vertebrates from palæostracans is supported by the geological evidence, by the anatomical evidence, and by the embryological evidence. Of the three the latter is the strongest and most conclusive, if it be taken to include the evidence given by the larval stage of the lamprey.

The stronghold of embryology for questions of this sort is the Law of Recapitulation, which asserts that the history of the race is recapitulated to a greater or less extent in the development of the individual. In the previous chapters such recapitulation has been shown for all the organs of the vertebrate body. In this respect, then, embryology has proved of the greatest value in confirming the evidence of relationship between the palæostracan and the vertebrate, given by anatomical and geological study.

There is, however, another side to embryology, which claims that the tissues of all the Metazoa are built up on the same plan; that in all cases in the very early stage of the embryo three layers are formed, the epiblast, mesoblast, and hypoblast; that in all animals above the Protozoa these three layers are homologous, the epiblast in all cases forming the external or skin-layer, the hypoblast the internal or gut-layer.

Such a theory, therefore, as is advocated in this book, which turns the gut of the arthropod into the neural canal of the vertebrate, and makes a new gut for the vertebrate from the external surface must be wrong, as it flatly contradicts the fundamental germ-layer theory.

Of recent years grave doubts have been thrown upon the validity of this theory, doubts which have increased in force year by year as more and more facts have been discovered which are not in agreement with the theory. So much is it now discredited that any criticism against my theory, which is based upon it, weighs nothing in the balance against the positive evidence of recapitulation already stated. If the germ-layer theory is no longer credited, upon what fundamental laws is embryology based?

In this chapter I have ventured to suggest a reply to this question, based on the uniformity of the laws of growth throughout the existence of the individual.

In the adult animal the body is composed of two kinds of tissues, those which are connected with or at all events are under the control of the nervous system, and those which are capable of leading a free life independent of the nervous system. These two kinds of tissues can be traced back from the adult to the embryo, and it is the task of embryology to find out how these two kinds of tissue originate.

The following out of this line of thought leads to the conception that, throughout the Metazoa, the body is composed of a host which consists of the master-tissues of the body, and takes the form of a neuro-epithelial syncytium, within the meshes of which free living independent organisms or cells live, so to speak, a symbiotic existence.

The evidence points to the origin of all these free cells from germ-cells, and thus leads to the conception that the blastula stage of every embryo represents two kinds of cells, the one which will form the mortal host being the locomotor neuro-epithelial cell, the other the independent immortal symbiotic germ-cell. Such conception leads directly to the conclusion that the blastula stage of every member of the Metazoa is the embryonic representation of a Protozoan ancestor of the Metazoa; an ancestor, whose nature may be illustrated by such a living form as Volvox globator, which, like a blastula, is composed of a layer of cells forming a hollow sphere. These cells partly bear cilia, and so form a locomotor host, partly are of a different character, and form male and female germ-cells. The latter leave the surface of the sphere, pass as free individuals into its fluid contents, form spermaries and ovaries, and then by the rupture of the mortal locomotor host pass out into the external medium, as free swimming young Volvox.

It is of interest to note that such members of the Protozoa are among the most highly developed of the members of this great group.