The Origin of Vertebrates

by Walter Holbrook Gaskell

The whole argument, so far, has in every case ended with the conclusion that the original scorpion-like form with which I have been comparing Ammocœtes resembled in many respects Limulus rather than the present-day scorpions, and therefore in the case also of the generative organs, with which the thyroid gland or palæo-hysteron was in connection, it is more probable that they were cephalic in position rather than abdominal. If this were so, then the duct on each side, starting from the median ventral uterus, would take a lateral and dorsal course to reach the huge mass of generative gland lying within the prosomatic carapace, just as I have represented in the figure of Eurypterus (Fig. [79]), a course which would take much the same direction as the ciliated groove in Ammocœtes.

We ought, therefore, on this supposition, to expect to find the remains of the invertebrate generative tissue, the ducts of which terminated in the thyroid, in the head-region, and not in the abdomen.

Upon removal of the prosomatic carapace of Limulus, a large brownish glandular-looking mass is seen, in which, if it happens to be a female, masses of ova are very conspicuous. This mass is composed of two separate glands, the generative glands and the hepatico-pancreatic glands—the so-called liver—and surrounds closely the central nervous system and the alimentary canal. From the generative glands proceed the genital ducts to terminate on the posterior surface of the operculum. From the liver ducts pass to the pyloric end of the cephalic stomach, and carry the fluid by means of which the food is digested, for, in all these animals, the active digesting juices are formed in the so-called liver, and not in the cells of the stomach or intestine.

It is a very striking fact that the brain of Ammocœtes is much too small for the brain-case, and that the space between brain and brain-case is filled up with a very peculiar glandular-looking tissue, which is found in Ammocœtes and not elsewhere. Further, it is also striking that in the brain of Ammocœtes there should still exist the remains of a tube extending from the IVth ventricle to the surface at the conus post-commissuralis, which can actually be traced right into this tissue on the outside of the brain (see Fig. [13], a-e, Pl. XXVI., in my paper in the Quarterly Journal of Microscopical Science). This, in my opinion, is the last remnant of one of the old liver-ducts which extended from the original stomach and intestine into the cephalic liver-mass. This glandular-looking material is shown surrounding the pineal eye and its nerve, in Fig. [31], also in Fig. [22], and separately in Fig. [92]. It is composed of large cells, with a badly staining nucleus, closely packed together with lines of pigment here and there between the cells; this pigment is especially congregated at the spot where the so-called liver-duct loses itself in this tissue. The protoplasm in these large cells does not stain well, and with osmic acid gives no sign of fat, so that Ahlborn's description of this tissue as a peculiar arachnoideal fat-tissue is not true; peculiar it certainly is, but fatty it is not.

Fig. 92.—Drawing of the Tissue which surrounds the Brain of Ammocœtes.

This tissue has been largely described as a peculiar kind of connective tissue, which is there as packing material, for the purpose of steadying a brain too small for its case. On the face of it such an explanation is unscientific; certainly for all those who really believe in evolution, it is out of the question to suppose that a brain-case has been laid down in the first instance too large for the brain, in order to provide room for a subsequent increase of brain; just as it is out of the question to suppose that the nervous system was laid down originally as an epithelial tube in order to provide for the further development of the nervous system by the conversion of more and more of that tube into nervous matter. Yet this latter proposition has been seriously put forward by professed believers in evolution and in natural selection.

This tissue bears no resemblance whatever to any form of connective tissue, either fatty or otherwise. By every test this tissue tells as plainly as possible that it is a vestige of some former organ, presumably glandular, which existed in that position; that it is not there as packing material because the brain happened to be too small for its case, but that, on the contrary, the brain is too small for its case, because the case, when it was formed, included this organ as well as the brain; in other words, this tissue is there because it is the remnant of the great glandular mass which so closely surrounds the brain and alimentary canal in animals such as Limulus. In my paper in the Quarterly Journal of Microscopical Science, in which I was comparing the tube of the vertebrate nervous system with the alimentary canal of the invertebrate, I spoke of this tissue as being the remnant of the invertebrate liver. At the same time the whole point of my argument was that the glandular material surrounding the brain of Limulus was made up of two glands—liver and generative gland—so that this tissue might be the remnant of either one or the other, or both. All I desired, at that time, was to point out the glandular appearance of this so-called packing tissue, which surrounded the brain-region of Ammocœtes, in connection with the fact that the brain and alimentary canal of Limulus were closely surrounded with a glandular mass composed partly of liver, partly of the generative gland. At present, I think these large cells found round the brain in Ammocœtes are much more likely to be the remnant of the generative gland than of the liver; the size of the cells and their arrangement recalls Owen's picture of the generative gland in Limulus, and seeing how important all generative glands are in their capacity of internal secreting glands, apart entirely from the extrusion of the ripe generative products, and how unimportant is an hepato-pancreas when the alimentary canal is closed, it is much more likely that of the two glands the former would persist longer than the latter. It may be that all that is left of the old hepato-pancreas consists of the pigment so markedly found in between these cells, especially at the place where the old liver-duct reaches the surface of the brain; just as the only remnant of the two pineal eyes in the higher vertebrates is the remains of the pigment, known as brain-sand, which still exists in the pineal gland of even the highest vertebrate. This, however, is a mere speculation of no importance. What is important is the recognition of this tissue round the brain as the remnant of the glandular mass round the brain of animals such as Limulus. Still further confirmation of the truth of this comparison will be given when the origin of the auditory organ comes up for discussion.

I conclude, therefore, from the evidence of Ammocœtes, that the generative glands in the ancestral form were situated largely in the cephalic region, and suggest that the course and direction of the ciliated pseudo-branchial grooves on each side indicate the direction of the original opercular ducts by which the generative products were conveyed to the uterine chamber, i.e. to the chamber of the thyroid gland, and thence to the common genital and respiratory cavity, and so to the exterior.

It is easy to picture the sequence of events. First, the generative glands, chiefly confined to the cephalic region, communicating with the exterior by separate ducts on the inner surface of the operculum as in Limulus. Then, in connection with the viviparous habit, these two oviducts fused together to form a single chamber, covered by the operculum, which opened out to the exterior by a single opening as in Scorpio: or, in forms such as Eurypterus, in which the operculum had amalgamated with the first branchial appendage and possessed a long, tongue-like ventral projection, the amalgamated ducts formed a long uterine chamber which opened internally into the genital chamber—a chamber which, as in Thelyphonus, was common with that of the two gill-chambers, while at the same time the genital ducts from the cephalic generative material opened into two uterine horns which arose from the anterior part of the uterus, as in Thelyphonus.