THE CRANIAL NERVES OP SOYLLIUM. 99 



If fig. 3, showing the condition of the root of the seventh nerve at 

 stage k, be compared with fig. 1, showing the root of the fifth nerve 

 at a rather earlier stage, it will be seen at once that there is a very- 

 close resemblance between the two ; the sole point of difference being 

 that in fig. 1, though the nerve still retains its primary attachment, 

 the secondary has not yet been actually acquired. Balfour's figures 

 and description, already referred to, show that at a still earlier stage 

 the fifth nerve has exactly the same appearance and relations which 

 the seventh has in fig. 2 ; and it is mainly on this fact, coupled with 

 the close similarity between such specimens as those represented in 

 figs. 1 and 3, that we rely in support of the explanation we have 

 given above of the root of the fifth nerve. 



Inasmuch as figs. 1 and 2 are taken from the same embryo, it would 

 seem that the fifth nerve appears before the seventh, and is, during 

 the earlier phases of its formation, just one stage ahead of it in -develop- 

 ment. At a time (fig. 1) when the primary roots of the fifth have 

 already become widely separated by growth of the brain-roof, and the 

 secondary attachment (v /3) is on the point of being acquired, the two 

 seventh nerves (fig. 2) are still in contact with one another across the 

 top of the unexpanded brain-roof; and at stage k the seventh nerve 

 (fig. 3) is in exactly the same condition as the fifth is at the end of 

 stage i (fig. 1). 



Our observations appear, therefore, to prove conclusively that as 

 concerns the seventh nerve, while the change of position of the dorsal 

 or primary root (vn a) is due solely to rapid growth of the roof of the 

 brain, the lower or ventral root (vn /3) is a new and purely secondary 

 attachment. 



Whilst these results concerning the roots of the seventh are, we 

 believe, new as applied to Elasmobranchs, they are in perfect accord- 

 ance with the account previously given by one of us of the develop- 

 ment of the seventh nerve in the chick, in which the very same series 

 of changes — the separation of the primary roots by growth of the 

 brain-roof, and the acquiring of new or secondary roots — are shown to 

 occur in a precisely similar manner. 1 The close correspondence between 

 these two very different types of vertebrates is of much interest, partly 



1 Marshall, "Develop, of Cranial Nerves in Chick," 'Quart. Journ. Micr. Sc.,' Jan. 1878, 

 pp. 34 and 35. 



The prediction there made, that the secondary attachment of the nerves in Elasmobranchs 

 would prove on further investigation to be acquired in exactly the same manner as in the 

 chick, is now completely verifledi 



