THE CRANIAL NERVES OP SCYLLIUM. 115 



ending abruptly in the external epiblast, and giving off no branches 

 ivhatever. At stage l it is in very close relation with 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 intemus, and the 

 obliquus 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 N.c. and N. c'. as per- 

 sistent 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 N. we are in 'much more 

 doubt; its apparent connection with the olfactory nerve at l, if 

 confirmed, would tell in favour of its being regarded as a similar com- 

 missure between the third and olfactory nerves, and would greatly 

 support views previously advanced by one of us concerning the mor- 

 phological value of the olfactory nerve. 2 On the other hand, the 

 extension forwards of the nerve N. 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, connection is the connecting nerve 

 (N.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 Scyllium this nerve retains the relations to other nerves 

 which it has clearly acquired by stage n ; it is described in the adult 

 by Schwalbe 3 as " dieser scheinbare Zweig des Oculomotorius." 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 



1 For these commissures in Elasmobranchs, vide Balfour, op. cit., pp. 158—160, and 

 PI. XI, fig. 18, and PI. XIV, fig. 15&. In the chick, Marshall, ' Quart. Journ. Mic. Sc.', Jan., 

 1878, PI. Ill, figs. 27 and 28. 



2 Marshall, ' Quart. Journ. Mic. Sc.,' vol. xix, pp. 300 seq. 



3 Schwalbe, ' Das Ganglion Oculomotorius,' p. 16. 



