120 THE THIRD DAY. [CHAP. 



lower vertebrate embryo (Elasmobranch) in Fig. 39, but is in all 

 important respects exactly similar in the chick. Very shortly after 

 the pituitary diverticulum becomes first established the boundary 

 wall between the stomodseum and the throat becomes perforated, 

 and the limits of the stomodaeum obliterated, so that the pituitary 

 diverticulum looks as if it had arisen from the hypoblast. During 

 the third day of incubation the front part of the notochord 

 becomes bent downward, and, ending in a somewhat enlarged 

 extremity, comes in contact with the termination of the pituitary 

 diverticulum. The mesoblast around increases and grows up, in 

 front of the notochord and behind the vesicle of the third 

 ventricle, to form the posterior clinoid process. The base of the 

 vesicle of the third ventricle at the same time grows downwards 

 towards the pituitary diverticulum, and forms what is known as the 

 infundibulum. On the fourth day the mesoblastic tissue around 

 the notochord increases in quantity, and the end of the notochord, 

 though still bent downwards, recedes a little from the termination 

 of the pituitary diverticulum, which is still a triangular space with 

 a wide opening into the alimentary canal. 



On the fifth day, the opening of the pituitary diverticulum 

 into the alimentary canal has become narrowed, and around the 

 whole diverticulum an investment of mesoblast-cells has appeared. 

 Behind it the clinoid process has become cartilaginous, while to 

 the sides and in front it is enclosed by the trabeculse. At this 

 stage, in fact, we have a diverticulum from the alimentary canal 

 passing through the base of skull to the infundibulum. 



On the seventh day the communication between the cavity 

 of the diverticttlum and that of the throat has become still 

 narrower. The diverticulum is all but converted into a vesicle, 

 and its epiblastic walls have commenced to send out into the 

 mesoblastic investment solid processes. The infundibulum now 

 appears as a narrow process from the base of the vesicle of the 

 third ventricle, which approaches, but does not unite with, the 

 pituitary vesicle. 



By the tenth day the opening of the pituitary vesicle into 

 the throat becomes almost obliterated, and the lumen of the 

 vesicle itself very much diminished. The body consists of 

 anastomosing cords of epiblast-cells, the mesoblast between 



