The Origin of Vertebrates

by Walter Holbrook Gaskell

I have endeavoured in this chapter to make some suggestions upon the origin of the notochord and of the vertebrate gut in accordance with my theory of the origin of vertebrates. I feel, however, strongly that these suggestions are much more speculative than those put forward in the previous chapters, and of necessity cannot give the same feeling of soundness as those based directly upon comparative anatomy and histology. Still, the fact remains that the origin of the notochord is at present absolutely unknown, and that my speculation that it may have originated as an accessory digestive tube is at all events in accordance with the most widely spread opinion that it arises in close connection with an alimentary canal.

CHAPTER XIV

THE PRINCIPLES OF EMBRYOLOGY

The law of recapitulation.—Vindication of this law by the theory advanced in this book.—The germ-layer theory.—Its present position.—A physiological not a morphological conception.—New fundamental law required.—Composition of adult body.—Neuro-epithelial syncytium and free-living cells.—Meaning of the blastula.—Derivation of the Metazoa from the Protozoa. Importance of the central nervous system for Ontogeny as well as for Phylogeny.—Derivation of free-living cells from germ-cells.—Meaning of cœlom.—Formation of neural canal.—Gastrula of Amphioxus and of Lucifer.—Summary.

In a discussion upon this theory of mine, which took place at Cambridge on November 25 and December 2, 1895, it was said that such a theory was absolutely and definitely put out of court, because it contravened the principles of embryology, was opposed, therefore, to our surest guide in such matters; and the law was laid down with great assurance that no claim for genetic relationship between two groups of animals can be allowed which is based upon topographical and structural coincidences revealed by the study of the anatomy of two adult animals, however numerous and striking they may be, if there are fundamental differences in the embryology of the members of these two groups.

According to my theory the old gut of the arthropod still exists in the vertebrate as the tubular lining of the central nervous system, and the vertebrate has formed a new gut. According to the principles of embryology as held up to the present, in all animals above the Protozoa, the different structures of the body arise from three definite embryonic layers, the epiblast, mesoblast, and hypoblast, and in all cases the gut arises from the hypoblastic layer. In the vertebrate the gut also arises from the hypoblast, while the neural canal is epiblastic. My theory, then, makes the impossible assertion that what was hypoblast in the arthropod has become epiblast in the vertebrate, and what was epiblast in the arthropod has become hypoblast in the vertebrate. Such a conception is supposed to be so absolutely impossible that it only requires to be stated to be dismissed as an absurdity.

Against this opinion I claim boldly that my theory is not only not contrary to the principles of embryology, but is mainly based upon the teachings of embryology. I wish here not to be misunderstood. The great value of the study of embryology for questions of the sequence of the evolution of animals is to be found in what is known as the Law of Recapitulation, which asserts that every animal gives some indication in the stages of its individual development of its ancestral history. Naturally enough it cannot pass through all the stages of its past history with equal clearness, for what has taken millions of years to be evolved has to be compressed into an evolution lasting only a few months or weeks, or even less.

When in the highest vertebrate a vestigial organ, such as the pineal gland, can be traced back without leaving the vertebrate kingdom to a distinct median eye, such as is found in the lamprey, that rudimentary organ is evidence of an organ which was functional in the earliest vertebrates or their immediate ancestors. So it is generally with well defined vestigial organs found in the adult animal; they always indicate an organ which was functional in the near ancestor.

Passing from the adult to the embryo we still find the same law. Here, also, vestigial organs are met with, which may leave no trace in the adult, but indicate organs which were functional in the near ancestor. Thus, but for embryology, we should have no certainty that the air-breathing vertebrates had been derived from water-breathing fishes; the indication is not given by any close resemblance between the formation of the embryos in their earliest stages, but by the formation of vestigial gill-arches even in the embryos of the highest mammal.

For all questions of evolution the presence of vestigial organs in the embryo is the important consideration, for they give an indication of near ancestry; the early formation of the embryo concerns a much more remote ancestral period, all vestigial organs of which may well have been lost and obscured by cœnogenetic changes. Let us, then, consider the two things—the vestigial organs and the early formation of the embryo—separately, and see how far my opponents are justified in their statement that my theory contravenes the principles of embryology.