172 DEVELOPMENT OF THE SPINAL NERVES 



to the fact that the cells forming the proximal portion of the 

 outgrowth are seen in every transverse section at this stage, 

 and therefore exist the whole way along, whereas the distal 

 portion is seen only in every third or fourth section, accord- 

 ing to the thickness of the sections. It may be concluded 

 from this that there appears a continuous outgrowth from the 

 spinal canal, from which discontinuous processes grow out. 



In specimens of a very much later period (PI. 23, fig. L) 

 the proximal portions of the outgrowth are unquestionably 

 continuous with each other, though their actual junctions with 

 the spinal cord are very limited in extent. The fact of this 

 continuity at a later period is strongly in favour of the view 

 that the posterior branches of the spinal nerves 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 struc- 

 tures to which I would call attention occurs at about the time 

 when three to five visceral clefts are present. The disappear- 

 ance 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 dif- 

 ficult to study. 



Fortunately, however, in Torpedo these peculiar features 



