492 PROF, MILNES-MARSHALL AND W. B, SPENCER. 
We have not yet detected the sixth nerve in embryos younger 
than stage L: concerning the morphological value of this nerve 
we adhere to the opinion already expressed that it is to be 
viewed as bearing the same relation to the seventh that the 
anterior root of a spinal nerve does to its posterior root. 
The Eighth (Auditory) Nerve.—This nerve also we can 
dispose of briefly: at stage K it appears as a large ganglionic 
posterior branch of the seventh nerve, given off immediately 
beyond the root of origin. It is from the first connected with 
the ventral or secondary root (vii 3). The condition at stage 
L is shown in fig. 10 (vit1). At stage N (fig. 11) its root, though 
still intimately connected with that of the facial, shows a very 
evident line of separation from it; the ganglionic character of 
the auditory nerve placing it in marked contrast with the non- 
ganglionic root of the facial. This distinction between the two 
roots becomes more marked in the later stages. 
General Considerations.—Several questions of a more general 
character arise out of the facts we have recorded above, and 
we propose to conclude the present paper with a brief notice of 
the more important of them. The problems in connection with 
the roots of origin of the nerves have been already sufficiently 
discussed, so that we turn at once to the consideration of their 
branches, concerning which the most important points are the 
determinations of the equivalence of the branches of the different 
nerves to one another. 
We commence with the ophthalmic branches of the fifth and 
seventh nerves, the branches named v a and vita in our figures. 
These two nerves, whose courses and relations are well shown in 
figs. 11 and 12, accompany one another very closely along 
their whole length; they appear to be both sensory nerves, their 
branches being distributed exclusively to the skin of the top 
and front of the head, and more especially to the mucous canals 
of these parts. Of the two nerves the branch of the fifth nerve 
(v a) is the smaller and the more ventrally placed of the two: 
though the smaller, its branches are, especially in the earlier 
part of its course, more numerous than those of the seventh. 
The two branches in their course through the orbit Ze dorsad of 
all the other contents of the orbit. They are at first quite dis- 
tinct from one another (figs. 11 and 12) and lie close beneath 
the external epiblast (fig. 5, vi a); the branch of the seventh 
being the more superficial of the two. In the later stages of 
development, as in the adult, the two nerves lie in very close 
contact with one another (fig. 16, va and vir a), the branch of 
the seventh lying immediately dorsad of the branch of the fifth ; 
