The Origin of Vertebrates

by Walter Holbrook Gaskell

First, I will take the teachings of vestigial organs and the arrangement of organs found in the vertebrate embryo. Here it is impossible to say that my theory is contrary to the teaching of embryology, for as the previous chapters have shown again and again, the argument is based very largely upon the facts of embryology. In the first place, the comparison which I have chiefly made is a comparison between the larval form of a very low vertebrate and the arthropod group, a comparison which exists only for the larval form, and not for the adult. The whole theory, then, is based upon a developmental stage of the vertebrate, and not upon the anatomy of the adult.

Throughout the whole history it seems to me perfectly marvellous how completely the law of recapitulation is vindicated by my theory of the origin of the vertebrate. The theory asserts that the clue to the origin of vertebrates is to be found in the tubular nature of the central nervous system of the vertebrate; in that the vertebrate central nervous system is in reality formed of two things: (1) a central nervous system of the arthropod type, and (2) an epithelial tube in the position of the alimentary canal of the arthropod.

Is it possible for embryology to recapitulate such a phylogenetic history more clearly than is here the case? In order to avoid all possibility of our mistaking the clue, the nerve-tube in the embryo always opens into the anus at its posterior end, while in the larval Amphioxus it is actually still open to the exterior at the anterior end. The separateness of the tube from the nervous system at its first origin is shown especially well in the frog, where, as Assheton has pointed out, owing to the pigment in the cells of the external layer of epithelium, a pigmented tube is formed, on the outside of which the nervous tissue is lying, and step by step the gradual intermingling of the nerve-cells and the pigmented lining cells can be followed out.

Consider the shape of the nerve-tube when first formed in the vertebrate. At the cephalic end a simple bulged-out tube with two simple anterior diverticula, which passes into a narrow straight spinal tube; from this large cephalic bulging a narrow diverticulum, the infundibulum, passes to the ventral surface of the forming brain. This tube is the embryological expression of the simple dilated cephalic stomach, with its ventral œsophagus and two anterior diverticula, which opens into the straight intestine of the arthropod. Nay, more, by its very shape, and the invariable presence of two anterior diverticula, it points not only to an arthropod ancestry, but to a descent from a particular group of primitive arthropods. Then comes the formation of the cerebral vesicles, and the formation of the optic cup, telling us as plainly as can be how the invasion of nervous material over this simple cephalic stomach and its diverticula has altered the shape of the original tube, and more and more enclosed it with nervous elements.

So, too, in the spinal cord region. When the tube is first formed, it is a large tube, the latero-ventral part of which presents two marked bulgings; connecting these two bulgings is the anterior commissure. These two lateral bulgings, with their transverse commissure, represent, with marked fidelity, the ventral ganglion-masses of the arthropod with their transverse commissure, and occupy the same position with respect to the spinal tube, as the ganglion-masses do with respect to the intestine in the arthropod. Then the further development shows how, by the subsequent growth of the nervous material, the calibre of the tube is diminished in size, and the spinal cord is formed.

Again, I say, is it possible to conceive that embryology should indicate the nature of the origin of the vertebrate nervous system more clearly than it does?

It is the same with all the other organs. Take, for instance, the skeletal tissues. The study of the vertebrate embryo asserts that the cartilaginous skeleton arose as simple branchial bars and a simple cranio-facial skeleton, and also that the parenchymatous variety of cartilage represents the embryonic form. Word for word, the early embryonic stage of the vertebrate skeleton closely resembles the stage reached in the arthropod, as shown by Limulus, and again records, unmistakably, the past history of the vertebrate.

So, too, with the whole of the prosomatic region; the situation of the old mouth, the manner in which the nose of the cephalaspidian fishes arose from the palæostracan, are all shown with vivid clearness by Kupffer's investigations of the early stage of Ammocœtes, while at the same time the closure of the oral cavity by the septum shows how the oral chamber was originally bounded by the operculum. Nay, further, the very formation of this chamber embryologically was brought about by the forward growth of the lower lip, just as it must have been if the chilaria grew forward to form the metastoma.

So, too, the study of the embryo teaches that the branchiæ arise as ingrowths, that the heart arises as two longitudinal veins, just as the theory supposes from the facts provided by Limulus and the scorpions. No indication of the origin of the thyroid gland is given by the study of its structure in any adult vertebrate, but in the larval form of the lamprey there is still preserved for us a most graphic record of its past history.

The close comparisons which it is possible to make between the eye-muscles of the vertebrate and the recti muscles of the scorpion group on the one hand, and between the pituitary and coxal glands on the other, are based upon, or at all events are strikingly confirmed by, the study of the cœlomic cavities and the origin of these muscles in the two groups. In fact the embryological evidence of the double segmentation in the head and the whole nature of the cranial segments is one of the main foundation-stones on which the whole of my theory rests.