THE BRAIN. 259 



on the second clay (Fig. 112) : during the third and following 

 days it becomes much more clearly defined .(Figs. 114, 123, 

 IN); and about the eighth day (Fig. 116, IN), a pocket-like 

 diverticulum arises from its floor, which is directed backwards, 

 and becomes wedged in between the anterior end of the noto- 

 chord and the pituitary body. 



In front of the infimdibulum the floor of the thalamen- 

 cephalon becomes greatly thickened, in the later stages, by the 

 development of the optic chiasma (Fig. 116, n). 



The pituitary body, though not really a part of the brain, is 

 so intimately connected with this that it may conveniently be 

 described here. 



The pituitary body appears, towards the end of the second 

 day, as a pocket-like diverticulum of the anterior angle of the 

 stomatodasum, or mouth invagination (cf. Fig. 114, PT) ; it lies 

 wedged in between the anterior end of the mesenteron and the 

 floor of the infundibulum, and its blind extremity is in close 

 contact with the anterior end of the notochord. 



On the formation of the mouth perforation, which places the 

 stomatodasum in communication with the mesenteron, the 

 pituitary body (Figs. 114, 123, PT) persists as a diverticulum 

 from the roof of the mouth, with the same relations as before 

 to the infundibulum and to the notochord. 



During the succeeding days, while the face is being esta- 

 blished and the beak is growing forwards prominently, the 

 pituitary body, retaining its relations with the brain and the 

 notochord, becomes left further and further back in the roof of 

 the mouth. 



At the eighth day its position and relations are shown in 

 Fig. 116. The upper blind end, PT, has given off a number of 

 branching tubular diverticula, which together form a rounded 

 vascular mass, lying immediately below the infundibulum, IN, 

 and in the pituitary foramen at the base of the skull, between 

 the trabeculas cranii. The stalk of the pituitary body is still 

 present as a narrow tube, PT', which opens into the roof of the 

 mouth in the median plane, opposite the glottis, LT, and just 

 in front of the opening of the Eustachian tubes, ES. By the 

 twelfth day the stalk has become a solid rod of cells, and the 

 communication between the pituitary body and the mouth is 

 finally cut off. 



s 2 



