BRAIN OF THE AMERICAN ALLIGATOR II 



The most important change in the hypophysis itself is the complete 

 closure of the wide opening to the exterior, seen in figure 3. The 

 walls of the original opening are still distinct from each other, though 

 they are in close contact (fig. 4, h.s.), and form a sort of solid 

 stalk, the hypophysal stalk. The oral epithelium is still continuous 

 with the hypophysal stalk, but it is thickened for only a short dis- 

 tance on either side of it. 



The closure of the hypophysal opening has taken place in such a 

 way that the stalk is at right angles to the long axis of the 

 hypophysis proper. As seen in the present figure, the hypophysis is 

 a hollow body of irregular outline, with thick, dense walls. The 

 cavity consists of a central region (fig. 4b, h.), and three out- 

 growths, the largest, 0., extending back till it nearly reaches the 

 notochord, ch.; the second, o\, extending in the same direction from 

 the base of the hypophysal stalk ; and the third, o"., extending towards 

 the floor of the infundibulum, in. As the sections are followed 

 laterad these three outgrowths are found to extend for some dis- 

 tance on each side of the body of the hypophysis. The outgrowth 0'. 

 is the largest and remains unbranched ; it is hollow throughout. The 

 outgrowth 0". is also hollow and unbranched ; it is the shortest of 

 the three. The outgrowth 0. extends laterad for some distance, and 

 then divides into three nearly solid outgrowths which extend nearly 

 as far as the outgrowth 0'. 



Figure 4c represents a section of an embryo of approximately the 

 same size as the one under discussion. The plane of the section is 

 nearly that of the broken line in figure 4b. 



The section shows. one of the outgrowths, o., on each side of 

 the median cavity, h., the one on the left being cut tangentially so 

 that its cavity does not show. The larger outgrowth, 0'., is seen on 

 either side of the median cavity, just above the hypophysal stalk, h. s. 



The hypophysis at this stage, then, is a completely closed vesicle 

 of irregular shape, consisting of a central cavity with three main 

 diverticula on each side, and a solid stalk connecting it with the 

 oral epithelium. 



The pharyngeal wall, back of the hypophysis, exhibits several well- 

 marked wrinkles ; one of these, shown at p. s., in figure 4b, may 

 represent the pharyngeal sac, though it differs very little from any 

 other of the wrinkles. In this sac the epithelium, which is some- 

 what thickened, almost touches the mesoblast that surrounds the 

 notochord, ch. No connection whatever can be seen between this 

 invagination and the notochord. 



Figure 5, as was stated in connection with the paraphysis, rep- 

 resents a much later stage of development than the preceding. The 



