259 
4. The Nerves. 
The fusion of the lateral edges of the neural plate extends 
anteriorly to the extremity of the primitive alimentary @anal (see 
fig. 5, n. cl.). Just beyond the end of the canal a depression marks 
the Anlage of the optic stalk. Fig. 8 II, shows this depression 
at a later stage when the anterior alimentary canal has given rise 
to mesoderm cells, and when the most anterior edges of the neural 
plate have completely fused, having a more posterior opening 
of the neural canal to the exterior through the neuropore, beyond 
which the edges of the plate are again closed. The anterior limit of 
the fusion of the edges of the neural plate is, therefore, not the neu- 
ropore, but the Anlage of the optic stalk. van WIJHE affirms this 
to be also the case in Scyllium (loc. cit.). The relation of the 
rudiment of the optic stalk to the line in which the neural plate closes 
may be most surely traced in series of horizontal sections at stages 
intermediate between those represented in figs. 5 and 8. The optic 
nerve which later develops in the optic stalk, is therefore primarily 
a dorsal structure, and morphologically the first or most anterior of 
the cranial nerves. 
The neural ridge extends forwards to the lateral margin of the 
neuropore. From it cells are early proliferated in a wide sheet for- 
ming the continuosus trigeminal and trochlear rudiment (fig. 13, IV and V). 
Anterior to this sheet of nerve cells, and separated from it by the 
vesicle of the midbrain, another band of cells is proliferated from the 
neural crest, in the region of the posterior margin of the neuropore. 
These cells extend down the walls of the brain and end in the Anlage 
of the ciliary ganglion, at which point the cells of the anterior and 
smaller neural outgrowth meet and fuse with those of the trigeminal 
and trochlear rudiment. Soon the posterior sheet of cells divides and 
one recognizes the dorsal roots, or primary attachments of trigeminus 
and trochlearis (Fig. 12 V and IV). The further development of the troch- 
learis, the loss of its primary attachment to the brain, the acquisition of a 
secondary one by the migration of ganglion cells towards the brain and 
the outgrowth of fibres, probably motor, from the brain, have been dis- 
cussed in my former paper (loc. cit.), I need here only say that in 
Acanthias the development of the trochlearis in all essential respects 
so completely corresponds to that of the trigeminus and facialis, that 
like them it must be considered to combine primarily those dorsal 
and ventral elements which have separate roots in the nerves of the 
trunk. It can, therefore, not be regarded as the ventral root of another 
segmental nerve, but must be ascribed an independent segmental value. 
