78 PROFESSOR A, MILNES MARSHALL, 
The main trunk of the seventh or facial nerve lies immediately 
behind and in very close contact with the third or hyoidean 
cavity ; this is shown in figs. 5, 8, and 9 (vir). Similarly the 
maiu trunk of the fifth or trigeminal nerve lies wedged in 
between the second and third, the mandibular and hyoidean 
cavities, as is clearly shown in figs 5, 9, and 10 (vy). 
The relations of these two nerves—the seventh and the fifth— 
to the hyoidean and mandibular cavities are very definite, and 
are very early acquired; they are fully established before the 
close of stage 1, and have been already fully described by Bal- 
four. They establish the fact that if the head cavities are to 
be taken as indicating head segments, about which there can be 
but little doubt, then these nerves must also be spoken of as 
segmental nerves. 
I now turn to a point of very considerable interest, which has 
not, I believe, been noticed hitherto. Wedged in between the first 
and second, premandibular and mandibular cavities, and occupy- 
ing a position precisely analogous to that held by the fifth and 
seventh nerves one or two segments further back respectively, 
is a ganglion, shown in figs. 4, 10, and 11 (¢.g.). From this 
ganglion a nerve can be traced forwards, figs. 13 and 14 (1m), 
to the base of the mid brain. This nerve is conclusively shown 
by its origin from the base of the mid brain, by its course, and 
by its distribution, at a slightly later stage, to certain of the eye 
muscles, to be the third or oculomotor nerve; while the gan- 
glionic expansion on it, between the first and second head cavi- 
ties, I believe I shall be able to prove, in a later section of this 
paper, to be the ciliary ganglion. 
I am not aware that the third nerve has been described by 
previous observers in Hlasmobranch embryos, but if the figures 
T here give of it be compared with the figures and descriptions I 
have previously given of the development of the same nerve in 
the chick,? it will be seen that the resemblance between the two 
forms is of a very striking and conclusive character. 
If the relations of the fifth and seventh nerves to the second 
and third head cavities demonstrate the segmental value of 
these nerves, then the relation which I have just pointed out the 
third nerve bears to the first and second head cavities must also 
be held to demonstrate the segmental value of this nerve. 
Thave previously attempted, in my paper on the nerves of the 
chick, referred to above, to prove that the third nerve is a 
segmental nerve strictly equivalent to the fifth, the seventh, or 
any other of the segmental cranial nerves. ‘The discovery that 
the third nerve has, in Elasmobranchs, from a very early period 
1 Op. cit., p. 197, seg. 
2 This Journal, January, 1878, p. 23, seg, and Plates IZ and III. 
