PRIMITIVE SEGMENTATION OF THE VERTEBRATE BRAIN. 467 
characteristic radial arrangement of cells, which toward the 
crest become more numerous, more compacted, and larger ; 
while beyond the brain itself the proliferated cells of the first 
nerve extend down on either side to the already well-formed 
olfactory pits. The first neuromere finally loses itself poste- 
riorly in a large but gradual inward convexity of the brain 
wall, which in sections at a lower level may be seen to occlude 
the lumen, while toward the dorsal surface they fall rapidly 
away from the lateral walls (figs. 5, 6, 7, and 8, c. s.). These 
are the corpora striata. Immediately behind these the brain- 
walls sweep outward again to form the well-marked second or 
optic neuromere. Here there is no evidence of nerve origin ; 
but tracing this region through successive sections from lower 
to higher levels, it may be seen that the optic diverticula are 
thrust out immediately dorsal to this segment on either side, 
and merge insensibly into it below (figs. 5, 6, 7, and 8). This 
position occupied by the optic diverticula relative to the second 
neuromere is strangely homologous to that held by nerves of 
acknowledged segmental character to their respective neuro- 
meres, and seems to me to point to the conclusion that the 
second pair of nerves, before their great specialisation, may 
have been perfectly comparable to the other cranial nerves, and 
were probably in connection with the second neuromere, and 
are, therefore, deserving of a place in the list of segmental 
nerves. In speaking of the primitive nerve relations of Ammo- 
cetes, Kupffer (16, 4) notes that of the lens of the eye to 
the ganglionic chain, and remarks that as the auditory organ 
is related to the principal ganglia, so the eye appears to belong 
to the epibranchial series. Behind the optic is seen the third 
neuromere of the fore-brain, in connection with which here, 
as in the cod, there seems to be no nerve. For this the early 
stage of the material may perhaps be held to account, at the 
same time that it affords proof of the fact that the first nerve 
is much in advance of the others in development. Behind the 
third neuromere there appears to be a small portion of brain 
wall, which is either unsegmented or a part of an additional 
segment ; for as the series is traced dorsalwards the fore-brain 
