582 ADAM SEDGWIOK. 
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 Scyllium and 
Pristiurus 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. 
2. 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. 
2 «Development of Elasmobranch Fishes,’ Mem, Ed., p. 384, vol. i. 
