The Development of the Mammalian Pituitary Body 



167 



epithelium, which in this situation is closely adherent to the wall of the 

 anterior cerebral vesicle. The posterior limb of the angle, also composed 

 of buccal epithelium, leaves the wall of the brain and bends downward to 

 form the anterior layer of the upper stump of the oral plate. At this stage 

 there is no invagination of the wall of the cerebral vesicle to form the 

 infundibulum, but its site is indicated by a definite depression. The 

 anterior end of the notochord doBs not touch the posterior limb of the 



d b c e 



Fig. 2.— Mesial sagittal section tlirougli part of head of a Gmiu. kitten. 



a, buccal invagination or Rathke's pouch : b, beginning of invagination of wall 

 of cerebral vesicle to form the infundibular process; c, blood - channel : 

 d, clump of cells derived from anterior end of fore-gut ; c, head of notochord. 



hypophysial angle, but is separated from it by a large blood-channel (c). 

 Behind the oral plate is a small dorsal invagination of the wall of the 

 fore-gut, which is the only indication of anything resembling Seessel's 

 pouch. Its wall is not thickened, and there is no evidence of any ento- 

 dermic origin for the pituitary in this specimen. 



In a 6-mm. cat embryo (fig. 2), the remains of the oral plate have 

 disappeared. The hypophysial angle has become a definite sac (a), Rathke's 

 pouch. This change appears to have been brought about by a bending 

 forwards of the upper stump of the oral plate and a proliferation of the 



