Ohf THE DEVELOPMENT OF THE ELASMOBRACBfl FISHES. 3(13 



Bd. ix) has also found that the same is the case with the 

 Batrachians and Mammalia, I have little doubt it will be 

 found to be universally the ease amongst vertebrates. 



Probably the observations which lead to the supposition 

 that it was the throat which was pinched off to form the 

 pituitary body were made after the opening between the 

 mouth and throat was completed, when it would naturally 

 be impossible to tell whether the pinching off was from the 

 epiblast of the mouth involution or the hypoblast of the 

 throat. 



The Cranial Nerves. 



The cranial nerves in their early condition are so clearly 

 visible that I have thought it worth while giving a figure of 

 them, and calling attention to some points about their em- 

 bryonic peculiarities. 



From my figure (14) it will be seen that there is l)ehind 

 the auditory vesicle a nervous tract, from which four nerves 

 descend, and that each of these nerves is distributed to the 

 front portion of a visceral arch. When tlie next and last arch 

 (in this species) is developed, a branch from this nervous 

 mass will also pass down to it. That each of these is of an 

 equal morphological value can hardly be doubted. 



The nerve to the third arch becomes the glosso-pharyngeal 

 (fig. 14, g I), the nerves to the other arches become the 

 branchial branches of the vagus nerve (fig. 14, v g). Thus the 

 study of their development strongly supports Gegenbaur's 

 view of the nature of the vagus and glosso-pharyngeal, viz. 

 that the vagus is a compound nerve, each component part of 

 it which goes to aa arch being equivalent to one nerve, such 

 as the glosso-pharyngeal. 



Of the nerves in front of the auditory sac the posterior is 

 the seventh nerve (fig. 14, vii). Its mode of distribution to 

 the second arch leaves hardly a doubt that it is equivalent 

 to one such nerve as those distributed to the posterior arches. 

 Subsequently it acquires another branch, passing forwards 

 towards the arch in front. 



The most anterior nerve is the fifth (fig. 14, v), of which 

 two branches are at this stage developed. The natural inter- 

 pretation of its present condition is, that it is equivalent to 

 two nerves, but the absence of relation in its branches to any 

 visceral clefts renders it more difficult to determine the mor- 

 phology of the fifth nerve than of the other nerves. The 

 front branch of the two is the ophthalmic branch of the 

 adult, and the hind branch the inferior maxillary branch. 

 The latter branch subsequently gives off low down, -i.e. 



