No. 6.] THE HYPOPHYSIS IN CHELONIA. 271 



small and its dorsal wall is doubled back on itself. The noto- 

 chord also at its anterior end near the point where it joins the 

 hypoblast has become very much twisted and curved, giving it 

 a knotted appearance in sagittal section. However, the parts 

 are easily recognized as having the same relation as those in 

 Stage A (Fig. i). In Stage B (Fig. 2), as before, the dorsal 

 wall of the fore-gut is the "secondary hypoblast" and the 

 ventral wall the "primary hypoblast," the division line between 

 the two being the point where the notochord joins the hypo- 

 blast. The pharyngeal membrane is still intact but breaks 

 up very soon after this stage. Directly in front of the place 

 where the mouth opening will appear the epiblast bends sharply 

 back on itself to follow the brain, forming a pocket in which 

 the cells are taller than in the neighboring parts of the epi- 

 blast. This is the beginning of the outpocketing which will 

 eventually form the hypophysis. The evaginating layer of 

 cells is clearly distinct from the hypoblast cells of the pre- 

 oral gut, as is well shown in PI. II, Fig. 3, which gives an 

 enlarged view of the hypophysial region of Fig. 2. Starting 

 from the condition of the stomodeal epiblast in the earlier 

 stage as shown in Fig. i, we see that an epiblastic groove 

 was originally formed at the point marked by the feathered 

 arrow (^) just in front of the future mouth by the forward 

 and downward bending of the fore-brain. It is at the bottom 

 and in the middle of this groove that the thickening and out- 

 pocketing of the cells start, and later form the epiblastic 

 pouch which becomes the oral portion of the hypophysis. 



Stage C. 



In an embryo but slightly older than the one last described 

 the pharyngeal membrane has already been broken. Fig. 4, 

 PI. Ill, represents a diagram of a median sagittal section 

 of such an embryo. On account of a lateral bend in the neck 

 region it was not possible to obtain a section which would show 

 the connection of the notochord and the preoral gut at the 

 same time with the mouth opening and the hypophysial evagi- 

 nation. The canal (H.C.) between the two premandibular 



