OBSERVATIONS ON THE CRANIAL NERVES OF SCYLLIUM, 495 
end of the head, ending abruptly in the external epiblast, and 
giving off no branches whatever. At stage L it is in very close 
relation to the olfactory nerve, and in some specimens seems to 
be connected with it, though we cannot speak with certainty on 
this point. 
As soon as the eye muscles are established they have very 
definite relations to this nerve; the rectus superior and internus, 
and the od/iquus superior lying above it, and the remaining three 
muscles below it (fig. 11). It also passes through the substance 
of the sclerotic, as noticed in a former portion of this paper. 
Concerning the morphological significance of these nerves, 
bearing in mind their very early appearance and the total absence 
of evidence for regarding them as branches of either of the 
nerves they serve to connect, we are disposed to view the nerves 
NV.c. and JN. c.’ as persistent portions of the neural ridge between 
the outgrowths to form the third, fifth, and seventh nerves, and 
as being, therefore, homologous with the primitive commissure 
connecting the posterior roots of the spinal nerves together.1_ As 
to the nerve VV. we are much more in doubt; its apparent con- 
nection with the olfactory nerve at L, if confirmed, would tell in 
favour of its being regarded as a similar commissure between 
the third and olfactory nerves, and would greatly support views 
previously advanced by one of us concerning the morphological 
value of the olfactory nerve.” On the other hand, the extension 
forwards of the nerve WV. beyond the olfactory nerve to the 
extreme anterior end of the head must, for the present, be 
regarded as almost conclusive against its commissural nature. 
In this case it can only be a branch of the third nerve, for the 
only other nerve with which it is in direct, or indirect, connec- 
tion is the connecting nerve (V.c.) between the third and fifth, 
which, if it does not belong to the third, there is at any rate no 
reason for referring to the fifth. 
In the adult Scy/Ziwm this nerve retains the relation to other . 
nerves which it has clearly acquired by stage N; it is described 
in the adult by Schwalbe? as “ dieser scheinbare Zweig des Oculo- 
motorius.” In Mustelus Schwalbe describes it as a branch of 
the fifth. We much regret that we have had no opportunity of 
studying the development of this nerve in Afustelus ; should it 
prove to arise as in Scy//iwm, then it must definitely be regarded 
as a branch ofthe third. 
As we have already pointed out, the nerves V. c. and WV, toge- 
‘ For these commissures in Elasmobranchs, vide Balfour, op. cit., pp. 158 
—160, and Pl. XI, fig. 18, and Pl. XLV, fig. 154. In the chick, Marshall, 
‘Quart. Journ. Mier. Sci.,’ Jan., 1878, Pl. LI, figs. 27 and 28, 
? Marshall, ‘Quart. Journ. Mier. Sci,’ vol. xix, pp. 300 seq. 
3 Schwalbe, ‘ Das Ganglion Oculomotorius,’ p. 16. 
