106 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 



The Pituitary Body. 



All my sections seem to prove that it is a portion of the 

 epiblastic involution to form the mouth which is pinched off to 

 form the pituitary body, and not a portion of the hypoblast of 

 the throat. Since Gotte (A rc/iiv. fiir Micr. Anat. 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 case 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 embry- 

 onic peculiarities. 



From my figure (14) it will be seen that there is behind 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 the 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 bran- 

 chial 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 an 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 



