272 ZELENY. [VOL. II. 



head cavities is, however, shown, and the small mass of cells 

 connected with it and directed toward the hypoblast of the 

 fore-gut is the strand which connects the head cavities with the 

 preoral gut. The preoral gut itself is not shown in this figure, 

 the cavity (F.G.') being a part of the fore-gut which has assumed 

 a position anterior to the mouth because of the bending of the 

 alimentary canal, which takes place at the same time with the 

 cephalic flexure. Except for the break in the pharyngeal mem- 

 brane the relation of the hypophysis to the epiblast is the same 

 as in Stage B. The hypophysial outpocketing is here, as before, 

 on the epiblastic side of the mouth opening and is undoubtedly 

 made up entirely of epiblast cells. 



Stage D. 



In Fig. 5, PL III, we have a median sagittal section of a some- 

 what later stage. The points/ and/' show the position of the 

 ends of the broken pharyngeal membrane, and the dotted line 

 (P.M.} between them represents the former position of the now 

 ruptured membrane. The hypophysial pouch (Hyp.} is shown 

 very distinctly on the epiblastic side of the membrane. It has, 

 however, been pushed back from its original position with 

 relation to the point /' by the rapid growth of the fore-brain. 

 The anterior end of the notochord (N.} is still more curved 

 and wrinkled than in the last stage, but it retains its connec- 

 tion with the anterior wall of the preoral gut (P.G.) by means 

 of a string of cells. In this string of cells we see the section 

 of the canal (H.C.) which connects the two anterior or pre- 

 mandibular head cavities. The hypophysial evagination from 

 the very beginning is in. close contact with the wall of the 

 infundibulum, but the two layers of cells always remain clearly 

 distinct. The epiblast cells of the hypophysis also remain 

 clearly distinct from the hypoblast cells of the preoral gut. 



Stage E. 



Fig. 6, PL IV, is a diagram of a section of Chelydra ser- 

 pentina. It shows a continuation of the same process of 

 enlargement of the fore-brain and the consequent pushing 



