NOTES ON BLASMOBEANOH DEVELOPMENT. 257 



back continuously from the root of the glossopharyngeal to 

 the spinal cord, connecting together the posterior spinal roots 

 and the roots of the vagus and glossopharyngeal. It is not, 

 however, developed in front of the glossopharyngeal, the nerve 

 crest atrophying between the ninth and seventh, and between 

 the seventh and fifth nerves. 



My observations agree with this account except in one 

 point, and that relates to the nerve crest. In Scy Ilium and 

 Prist iurus the nerve crest is not a continuous structure, as 

 Wyhe and Kastschenko assert (Balfour and Marshall have no 

 observations on the cranial part of the nerve crest in Elasmo- 

 branchs). It is in three separate pieces. The first of these 

 is found in the anterior part of the brain ; the fifth nerve and 

 presumably the ophthalmicus profundus grow out from it. 

 The second is found a little further back, and gives origin to 

 the seventh and eighth nerves. The third piece occurs a little 

 further back, and reaches from the hind brain continuously 

 back the whole length of the spinal cord. The ninth and 

 tenth cranial nerves and the posterior roots of all the spinal 

 nerves grow out from it. It is this latter part of the nerve 

 crest which gives rise to the longitudinal commissure of 

 Balfour. 



There are three views as to the origin of the peripheral 

 nerves. 



1. According to Hensen^s^ view, the rudiments of the nerve- 

 fibres are present from the beginning as persistent remains of 

 the primitive connections between the incompletely separated 

 cells of the segmented ovum. 



3. Balfour^ regarded them as cellular outgrowths from the 

 central nervous system extending to the periphery. The 

 original continuity between the central and peripheral organs, 

 which must have existed, has, it was supposed, been lost in 

 ontogeny by rupture, and reacquired by means of these out- 

 growths. 



1 ' Virchow's Archiv,' vol. xxxi, 1864. 



^ 'Development of Elasmobranch Pishes,' Mem, Ed., p. 384, vol. i, 



