brain of the american alligator 9 



The Hypophysis 



The hypophysis is first seen in the alHgator in embryos of about 

 7 mm. length. In this animal, at any rate, there is no reason for 

 doubt as to whether it be derived from the ectoderm or the entoderm. 

 As may be seen in figure i, h., the hypophysis originates as an in- 

 vagination of ectoderm close under the floor of the fore-brain. The 

 superficial ectoderm gradually thickens as it passes under the fore- 

 brain, and in the region of the invagination it is several times as 

 thick as in other regions of the body. Followed towards the pharynx 

 it gradually thins out again as it becomes continuous with the 

 entoderm of the fore-gut. 



Gaupp (17) says that in lizards and perhaps in all reptiles the 

 hypophysis has a three-fold beginning : A large, round middle part, 

 and two lateral parts, pushed in from the epithelium of the mouth 

 cavity. In the alligator this three-fold origin cannot be made out. 

 The conical invagination seen in figure i gradually diminishes in 

 depth and width as it is followed laterad, until it disappears, but no 

 lateral pouches are seen. 



The general ectoderm of the body is composed of a single layer of 

 cuboidal or even flattened cells, while the walls of the hypophysal 

 invagination are made up of a single layer of long, narrow, columnar 

 cells with distinct nuclei. 



Just caudad to the hypophysis is seen in figure i an indefinite and 

 smaller invagination of the thickened epithelium, p.s.; this may 

 represent what has been called, in other forms, the pharyngeal sac. 

 Bawden (2) says that in the duck the pharyngeal sac appears earlier 

 than the hypophysis, reaches its maximum development in five days, 

 and soon disappears. It lies between the hypoblast and the noto- 

 chord, with the latter of which it is connected. 



In the present figure no connection between the pharyngeal sac 

 and the notochord, ch., is to be seen, unless it be represented by the 

 slight condensation of the mesoblast that extends from the tip of 

 the notochord to the inner surface of the sac. 



Figures 2 and 2a represent the condition of the hypophysis in an 

 embryo of about 10 mm. length. The general topography of the 

 head has been described in speaking of the paraphysis of this stage. 



Figure 2 is laterad to the exact median plane. It shows the main 

 invagination of the hypophysis at the end of the reference line, h., 

 caudad to which are several wrinkles of the thickened epithelium. 

 Between the invagination just mentioned and the floor of the in- 

 fundibulum, in., are two small, oval, compact masses of cells which 

 are lateral branches of the main invagination or stalk, as it may be 



