HEAD CAVITIES AND NERVES OF ELASMOBRANCHS. 79 
of development, the characteristic relation to the head cavities 
that is possessed by the fifth and seventh nerves, affords, perhaps, 
the strongest possible confirmation of the segmental value of the 
third nerve. Fig. 4 shows that this characteristic relation is 
acquired by the third nerve at any rate as early as stage K. 
I propose now to consider certain phases in the develop- 
ment of these three nerves, the seventh, fifth, and third pairs. 
My observations on the earlier stages are very fragmentary, and, 
therefore, I shall omit for the present all account of their deve- 
lopment previous to the latter part of stage 1. 
The Development of the Third (Oculomotor) Nerve—t! have 
not observed this nerve in specimens earlier than the commence- 
ment of stage K; but by this time it is already a nerve of consi- 
derable length, with its more important branches fully developed, 
so that it must certainly be present at a considerably earlier 
period. In the chick I have elsewhere shown! that the third 
nerve is one of the first nerves, if not the very first nerve, in the 
body to appear ; and my observations on Hlasmobranchs, though 
inconclusive on this point, yet show no reason why the same 
should not be the case in them. 
In embryos of stage x the third nerve is a conspicuous and 
easily recognisable object; it arises from the base of the mid 
brain, not far from the mid ventral line, the roots of the two 
nerves being only a small distance apart. Its root, which is 
expanded somewhat, and has a triangular shape, contains nume 
rous nerve-cells. From this ganglionic root (shown at stage L in 
fig. 14 (az) the nerve runs as a long slender stem almost 
horizontally backwards, then turns slightly outwards to reach 
the interval between the dorsal ends of the first and second head 
cavities, where it expands into a small ganglion (c. y., fig. 4). 
From this ganglion two main branches arise; of these, one 
(111 4, fig. 4) continues the course of the main stem of the nerve, 
and runs down between the first and second head cavities; the 
second branch (111 a, fig. 4) runs directly forwards from the 
ganglion, passing along the top of the first head cavity, then 
along the inner side of the eye, and finally terminates at the 
extreme anterior end of the head, just dorsad of the olfactory 
pit (off). 
The ganglion (ce. g.) receives also a short but very interesting 
branch (y. d., fig. 4) coming direct from the large ganglion at 
the root of the fifth nerve. This communicating branch between 
the third and fifth nerves is apparently that which is described 
and figured by Balfour as the rudiment of the ophthalmic branch 
1 Loc. cit., p. 27. 
