NOTES ON SOME EMBRYOS OP CHLAMYDOSELACHUS ANGUINEUS GARM. 99 



0. 



Cut 2. 



Four transverse sections passing through the region of Rathke's and Seessel's pouches, 

 of the embryo of 32 mm. There are 47 sections between A & B, 19 between B & C, and 17 bs- 

 tween C & D, each equal to 10 



their external openings can be readily recognized from outside. In sections it is 

 seen that the epiblast surrounding the openings is thickened. There is as yet 

 no cloacal opening, but its position is marked by a distinct prominence, where 

 the wall of the alimentary canal and the skin are in close contact. In the ven- 

 tral view of the head a pit can be seen in the median line directly behind the 

 depression separating the two halves of the upper jaw (fig. 5). This is Rìthke's 

 pouch, or the pituitary involution, which is closely connected with the infundi- 

 bulum. In cut 2 I have reproduced some of the transverse sections passing 

 through this region. In A both the pituitary involution and the infundibulum are 

 to be seen, the former extending for 36 sections (each=10 p.). About 0.405 

 mm. behind the posterior border of Rathke's pouch there is another involution, 



