OBSERVATIONS ON THE CRANIAL NERVES OF SCYLLIUM. 49] 
posterior, muscular, and deep branch is the ramus mandibularis 
internus v. profundus of the same authors. Both these branches 
have already acquired, by stage n, not only the characteristic 
distribution of the adult nerves, but nearly all the minor branches 
as well. 
To recapitulate ; we find that by stage nN the seventh nerve 
has acquired all the important branches of the adult nerve, the 
main trunks and many of the branches being fully developed at 
a much earlier period—stage L. ‘The seventh nerve at stage N 
has two roots, a dorsal or primary, and a ventral or secondary. 
From the dorsal root (v11 a) arise two branches: (1) the ophthal- 
mic (vila) and (2) the buccal (vm d@), both of which appear to 
be purely sensory nerves. The connecting branch (J. ¢’.) to the 
fifth nerve, though it appears in longitudinal section (fig. 11) to 
be a distinct branch, in transverse sections (fig. 6) seems to be 
only the deeper portion of the buccal nerve. From the ventral 
root arises the main or hyoidean branch (vite), from which the 
branch vir runs forward over the top of the spiracle, dividing, 
almost immediately, into the palatine (ya) and ‘spiracular (sp) 
nerves, whilst the hyoidean itself divides distally into the sensory 
ramus mandibutaris externus (V ¢, 1) and the motor ramus mandi- 
dularis internus (Vc, 2). 
The Sixth (Abducens) Nerve.—Concerning the development 
of the sixth nerve our observations simply confirm the account 
already given by one of us.! 
The whole length of the nerve is shown in longitudinal and 
vertical section in fig. 13 (v1), where it is seen arising from the 
base of the brain by a number of small non-ganglionic roots 
which unite to form a slender nerve; this nerve runs forwards a 
short distance, then turns downwards, pierces the investing 
mass (7. v.), and ends in the posterior extremity of the rectus 
externus muscle (1. é.). . 
Fig. 7 shows the sixth nerve in transverse section at the same 
stage (N): on the left side of the figure the termination of the 
nerve in the rectus externus is seen; while on the right side, 
which is taken from a more posterior section, one of the roots 
of origin is seen. This figure shows that the roots of the 
sixth are considerably nearer the mid ventral line than are the 
secondary roots of the seventh nerves, and also that the sixth 
and seventh nerves are quite independent of one another. At 
stage N the sixth nerve appears to be altogether behind the 
seventh, but in stage o it is situated rather further forwards, 
so that the same section may pass through the roots of both 
nerves. 
! Marshall, loc. cit., pp. S9—93. 
VOL. XXI.—NEW SER. KK 
