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.
From the summit of the muscle-plates (mp) an outgrowth of connective tissue has made its appearance (c), which eventually fills up the space between the dorsal surface of the cord and the external epiblast. There is not the slightest difficulty in distinguishing the connective-tissue cells from the nerve-rudiment. I believe that in this embryo the origin of the nerves from the neural canal was a continuous one, though naturally the peripheral ends of the nerve-rudiments were separate from each other.
The most interesting feature of the stage is the commencing formation of the anterior roots. Each of these arises (Pl. 22, fig. D a, ar) as a small but distinct outgrowth from the epiblast of the spinal cord, near the ventral corner of which it appears as a conical projection. Even from the very first it has an indistinct form of termination and a fibrous appearance, while the protoplasm of which it is composed becomes very attenuated towards its termination.