The Origin of Vertebrates

by Walter Holbrook Gaskell

This is exactly what is found in the vertebrate, for the infundibular tube passes into the third ventricle of the brain, which forms, with the other ventricles of the brain, the large dilated cephalic portion of the so-called nerve tube, and at the junction of the medulla oblongata and spinal cord, this dilated anterior part passes into the small, straight, central canal of the spinal cord, which in the embryo terminates in the anus by way of the neurenteric canal. If the animal is regarded as not having been turned over, then the conclusion that the infundibulum was the original œsophagus leads immediately to the further conclusion that the ventricles of the vertebrate brain represent the original cephalic stomach, and the central canal of the spinal cord the straight intestine of the arthropod ancestor.

For the first time a logical, straightforward explanation is thus given of the peculiarities of the tube of the central nervous system, with its extraordinary termination in the anus in the embryo, its smallness in the spinal cord, its largeness in the brain region, and its offshoot to the ventral side of the brain as the infundibular channel. It is so clear that, if the infundibular tube be looked on as the old œsophagus, then its lining epithelium is the lining of that œsophagus; and the fact that this lining epithelium is continuous with that of the third ventricle, and so with the lining of the whole nerve-tube, must be taken into account and not entirely ignored as has hitherto been the case. If, then, we look at the central nervous system of the vertebrate in the light of the central nervous system of the arthropod without turning the animal over, we are led immediately to the conclusion that what has hitherto been called the vertebrate nervous system is in reality composed of two parts, viz. a nervous part comparable in all respects with that of the arthropod ancestor, which has grown over and included into itself, to a greater or less extent, a tubular part comparable in all respects with the alimentary canal of the aforesaid ancestor. If this conclusion is correct, it is entirely wrong to speak of the vertebrate central nervous system as being tubular, for the tube does not belong to the nervous system, but was originally a simple epithelial tube, such as characterizes the œsophagus, cephalic stomach, and straight intestine of the arthropod.

Here, then, is the crux of the position—either the so-called nervous tube of the vertebrate is composed of two separate factors, consisting of a true non-tubular nervous system and a non-nervous epithelial tube, these two elements having become closely connected together; or it is composed of one factor, an epithelial tube which constitutes the nervous system, its elements being all nervous elements.

If this latter hypothesis be accepted, then it is necessary to explain why parts of that tube, such as the roof of the fourth ventricle, the choroid plexuses of the various ventricles, which are parts of the original roof inserted into the ventricles, are not composed of nervous material, but form simple single-layered epithelial sheets, which by no possibility can be included among functional nervous structures. The upholders of this hypothesis can only explain the nature of these thin epithelial parts of the nervous tube in one of two ways; either the tube was originally formed of nervous material throughout, and for some reason parts of it have lost their nervous function and thinned down; or else these thin epithelial parts are on their way to become nervous material, are still in an embryonic condition, and are of the nature of epiblast-epithelium, from which the central nervous system originally arose.

The first explanation is said to be supported by embryology, for at first the nerve-tube is formed in a uniform manner, and then later, parts of the roof appear to thin out and so form the thin epithelial parts. If this were the right explanation, then it ought to be found that in the lowest vertebrates there is greater evidence of a uniformly nervous tube than in the higher members of the group: while conversely, if, on the contrary, as we descend the vertebrate phylum, it is found that more and more of the tube presents the appearance of a single layer of epithelium, and the nervous material is limited more and more to certain parts of that tube, then the evidence is strong that the tubular character of the central nervous system is not due to an original nervous tube, but to a non-nervous epithelial tube with which the original nervous system has become closely connected.

The comparison of the brain region of the different groups of vertebrates (Fig. [19]) is most instructive, for it demonstrates in the most conclusive manner how the roof of the nervous tube in that region loses more and more its nervous character, and takes on the appearance of a simple epithelial tube, as we descend lower and lower; until at last, in the brain of Ammocœtes, as represented in the figures, the whole of the brain-roof, from the region of the pineal eye to the commencement of the spinal cord, is composed of fold upon fold of a thin epithelial membrane forming an epithelial bag, which is constricted in only one place, where the fourth cranial nerve crosses over it.

Further, the brain of Ammocœtes (Fig. [20]) shows clearly not only that it is composed of two parts, an epithelial tube and a nervous system, but also that the nerve-masses are arranged in the same relative position with respect to this tube as are the nerve-masses in the invertebrate with respect to the cephalic stomach and œsophagus. This evidence is so striking, so conclusive, that it is impossible to resist the conclusion that the tube did not originate as part of the central nervous system, but was originally independent of the central nervous system, and has been invaded by it.

Fig. 19.—Comparison of Vertebrate Brains.

CB., cerebellum; PT., pituitary body; PN., pineal body; C. STR., corpus striatum; G.H.R., right ganglion habenulæ. I., olfactory; II., optic nerves.