178 
MR. F. M. BALFOUR ON THE DEVELOPMENT OF 
arise from the first as a continuous outgrowth of the spinal cord, from which a series 
of distal processes take their origin. I have, however, failed to demonstrate this point 
absolutely. The processes, which we may call the nerve-rudiments, are, as appears from 
the later stages, equal in number to the muscle-plates. 
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* 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 (Plate 16. fig. Da ,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-e mbryos. The cells forming the proximal portion of the rudi- 
ment have the same elongated form as the cells of the spinal cord, but the remaining 
cells are more circular. 
* Vid - 6 Balfour, “ Origin and History of Urino-genital Organs of Vertebrates,” Journal of Anatomy and 
Physiology, Oct. 1875. 
