NERVOUS SYSTEM OF THE VERTEBRATA. 435 



Mammalia, in the wall of which there are nerve-fibres, no 

 nervous structures are present in it in the adult state. 



The pituitary body. Although the pituitary body is nut 

 properly a nervous structure, yet from its intimate connection 

 with the brain it will be convenient to describe its development 

 here. The pituitary body is in fact an organ derived from the 

 epiblast of the stomodaeum. This fact has been demonstrated 

 for Mammalia, Aves, Amphibia and Elasmobranchii, and may 

 be accepted as holding good for all the Craniata 1 . The epiblast 

 in the angle formed by the cranial flexure becomes involuted to 

 form the cavity of the mouth. This cavity is bordered on its 

 posterior surface by the front wall of the alimentary tract, and 

 on its anterior by the base of the fore-brain. Its uppermost end 

 does not at first become markedly constricted off from the 

 remainder, but is nevertheless the rudiment of the pituitary 

 body. 



Fig. 256 represents a transverse section through the head of 

 an Elasmobranch embryo, in which, owing to the cranial flexure, 

 the fore part of the head is cut longitudinally and horizontally, 

 and the section passes through both the fore-brain (fb) and the 

 hind-brain. Close to the base of the fore-brain are seen the 

 mouth (?;z), and the pituitary involution from this (//). In 

 contact with the pituitary involution is the blind anterior 

 termination of the throat (al) which a little way back opens to 

 the exterior by the first visceral cleft (i. v.c.}. This figure alone 

 suffices to demonstrate the correctness of the above account of 

 the pituitary body; but its truth is still further confirmed by 

 fig. 252; in which the mouth involution (pi) is in contact with, 

 but still separated from, the front end of the alimentary tract. 

 Very shortly after the septum between the mouth and throat 

 becomes pierced, and the two are placed in communication, the 

 pituitary involution becomes very partially constricted off from 

 the mouth involution, though still in direct communication with 

 it. In later stages the pituitary involution becomes longer and 



1 Scott states that in the larva of Petromyzon the pituitary body is derived from 

 the walls of the nasal pit; Quart. J. of Micr. Science, Vol. xxi. p. 750. I have not 

 myself completely followed its development in Petromyzon, but I have observed a 

 slight diverticulum of the stomodaeum which I believe gives origin to it. Fuller 

 details arc in any case required before we can admit so great a divergence from the 

 normal development as is indicated by Scott's statements. 



282 



