The Origin of Vertebrates

by Walter Holbrook Gaskell

2. Of these centres the caudalmost have alone remained large and vigorous, constituting the nucleus masticatorius, which in the fish is divided into an anterior and posterior group, thus indicating a double rather than a single nucleus; while the foremost ones have dwindled away until they are represented only by the cells of the descending root, the muscles of these segments being still represented by possibly the tensor veli palati and the other muscles innervated from these cells.

3. The headmost of these cells takes up actually a position dorso-lateral to the central canal, so that the groups on each side nearly come together in the mid-dorsal line; a very unique and extraordinary position for a motor cell-group, but not improbable when we recall to mind Brauer's assertion as to the shifting of the foremost prosomatic ganglion-cells of the scorpion from the ventral to the dorsal side of the alimentary canal.

On the sensory side the evidence is also suggestive, the question here being not so much the distribution of the sensory nerves as the number of ganglia belonging to each of the cranial nerves.

With respect to this question, morphologists have come to the conclusion that there is a marked difference between spinal and cranial nerves, in that whereas the posterior root-ganglia of the spinal nerves arise from the central nervous system itself, i.e. from the neural crest, the ganglia of the cranial nerves arise partly from the neural crest, partly from the proliferation of cells on the surface of the animal; and because of the situation of these proliferating epidermal patches over the gill-clefts in the case of the vagus and glossopharyngeal nerves, they have been called by Froriep and Beard branchial sense-organs. Beard divides the cranial ganglia into two sets, one connected with the neural ridges, called the neural ganglia, and the other connected with the surface-cells, which he calls the lateral ganglia. This second set corresponds to Kupffer's epibranchial ganglia. Now it is clear that in the case of the vagus nerve, where, as is well shown in Ammocœtes, the nerve is not a single segmental nerve, but is in reality made up of a number of nerves going to separate branchial segments, the indication of such segments is not given by the main vagus ganglion or neural ganglion, but by the series of lateral ganglia. So also it is argued in the case of the trigeminal, that if in addition to the ganglion-cells arising from the neural crest separate ganglion-masses are found in the course of development, in connection with proliferating patches of the surface (plakodes, Kupffer calls them), then such isolated lateral ganglia are indications of separate segments, just as in the case of the vagus, even though the separate segments do not show themselves in the adult. So far the argument appears to me just, but the further conclusion that the presence of such plakodes shows the previous existence of branchial sense-organs, and, therefore, that such ganglia are epibranchial ganglia, indicating the position of a lost gill-slit, is not justified by the premises. If, as I suppose, the trigeminal nerve supplied a series of non-branchial appendages serially homologous with the branchial appendages supplied by the vagus, then it is highly probable that the trigeminal should behave with respect to its sensory ganglia similarly to the vagus nerve, without having anything to do with branchiæ.

Such plakodal ganglia, then, may give valuable indication of non-branchial segments as well as of branchial segments. The researches of Kupffer on the formation of the trigeminal ganglia in Ammocœtes are the chief attempt to find out from the side of the sensory ganglia the number of segments originally belonging to the trigeminal. The nature and result of these researches is described in my previous paper (Journal of Anatomy and Physiology, vol. xxxiv.), and it will suffice here to state that he himself concludes that the trigeminal originally supplied five at least, probably six, segments. As I have stated there, the evidence as given by him seems to me to indicate even as many as seven segments.

In the full-grown Ammocœtes, as is well known, there are two distinct ganglia belonging to the trigeminal, the one the ganglion of the ramus ophthalmicus, the other the main ganglion.

According to Kupffer the larval Ammocœtes possesses three sets of ganglia, not two, for between the foremost and hindmost ganglion he describes a nerve (x., Fig. [113]), with four epibranchial ganglia, which do not persist as separate ganglia, but either disappear or are absorbed into the two main ganglia (Fig. [113]). This discovery of Kupffer's is very suggestive, for, as already stated, a transformation takes place when the Ammocœtes is 5 mm. long, so that the arrangement of the parts before that period is distinctly more indicative of the ancestral arrangement than any later one.

If we use the name plakodal ganglia to represent that part of these ganglia which was originally connected with the skin, then Kupffer's researches assert that in the larval Ammocœtes there were seven such plakodal ganglia, one in front belonging to the foremost trigeminal ganglion, two behind, parts of the hindmost ganglion, and four in between, which do not exist later as separate ganglia.

Fig. 113.—Ganglia of the Cranial Nerves of an Ammocœtes, 4 mm. in length, projected on to the Median Plane. (After Kupffer.)