IN ELASMOBRANCH FISHES. 173 



are not present to nearly the same extent as in Pristiurus and 

 Scy Ilium. 



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 pro- 

 ceeding 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 Scy I limn. 



TJie 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 1 is just appearing, but the cells of the 

 notochord have not become completely vacuolated. The rudi- 

 ments 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 

 (PI. 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 Pristmrus-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 (;;//) 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 distin- 

 guishing 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 (PI. 22, 



1 Vide Balfour, " Origin and History of Urinogenital Organs of Vertebrates," 

 Journal of Anatomy and Physiology, Oct. 1875. ' [This edition, No. vn.] 



