It may be pointed out, as must have been gathered from the description above, that the nerve-rudiments have at this stage but one point of attachment to the spinal cord, and that this one corresponds with the dorsal or posterior root of the adult nerve.
The rudiments are, in fact, those of the posterior root only.
The next or second stage in the formation of these structures to which I would call attention occurs at about the time when three to five visceral clefts are present. The disappearance from the notochord in the anterior extremity of the body of a special central area rich in protoplasm serves as an excellent guide to the commencement of this epoch.
Its investigation is beset with far greater difficulties than the previous one. This is owing partly to the fact that a number of connective-tissue cells, which are only with great difficulty to be distinguished from the cells which compose the spinal nerves, make their appearance around the latter, and partly to the fact that the attachment of the spinal nerves to the neural canal becomes much smaller, and therefore more difficult to study.
Fortunately, however, in _Torpedo_ these peculiar features are not present to nearly the same extent as in _Pristiurus_ and _Scyllium_.
The connective-tissue cells, though they appear earlier in _Torpedo_ than in the two other genera, are much less densely packed, and the large attachment of the nerves to the neural canal is retained for a longer period.
Under these circumstances I consider it better, before proceeding with this stage, to give a description of the occurrences in _Torpedo_, and after that to return to the history of the nerves in the genera _Pristiurus_ and _Scyllium_.
_The development of the Spinal Nerves in Torpedo._
The youngest _Torpedo_-embryo in which I have found traces of the spinal nerves belongs to the earliest part of what I called the second stage.
The segmental duct[49] is just appearing, but the cells of the notochord have not become completely vacuolated. The rudiments of the spinal nerves extend half of the way towards the ventral side of the spinal cord; they grow out in a most distinct manner from the dorsal surface of the spinal cord (Pl. 22, fig. D a, _pr_); but the nerve-rudiments of the two sides are no longer continuous with each other at the dorsal median line, as in the earlier _Pristiurus_-embryos. The cells forming the proximal portion of the rudiment have the same elongated form as the cells of the spinal cord, but the remaining cells are more circular.