358 PITUITARY BODY. 



branching gives rise eventually become solid (fig. 255 pin). An infra- 

 pineal process is developed comparatively late^ and is not sharply 

 separated from the roof of the brain. 



No satisfactory suggestions have yet been offered as to the nature 

 of the pineal gland, unless the view of Gotte be regarded as such. It 

 appears to possess in all forms an epithelial structure, but, except at 

 the base of the stalk (infra-pineal process) in 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 not 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 stomodseum. 

 This fact has been demonstrated for Mammalia, Aves, Amphibia and 

 Elasmobranchii, and may be accepted as holding good for all the Crani- 

 ata\ 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 jDasses through both the fore-brain {fh) and the hind-brain. 

 Close to the base of the fore-brain are seen the mouth {m), and the 

 pituitary involution from this {])t). 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 (r v.c). 

 This figure alone sufiices 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 {pt) 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 is dilated 

 terminally; while the passage connecting it with the mouth becomes 

 narrower and narrower, and is finally reduced to a solid cord, which 

 in its turn disappears. 



Before the connection between the pituitary vesicle and the mouth 



^ 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 stomodiBum which I believe gives origin to it. Fuller details 

 are in any case required before we can admit so great a divergence from the normal 

 development as is indicated by Scott's statements. 



