No. 6.] THE HYPOPHYSIS IX CHELOXIA. 269 



Stage C. Slightly older than Stage B. The pharyngeal 

 membrane has been broken. 



Stage D. The downward growth and enlargement of the 

 fore-brain have pushed the hypophysis back from its primary 

 relation to the broken ends of the pharyngeal membrane. 



Stage E. The hypophysis has been shifted backward so 

 as to assume a position relatively far back in the pharyn- 

 geal cavity. 



Stage F. The hypophysis has become differentiated into 

 a terminal, broad, sac-like part and a narrower connecting 

 stalk. 



Stage A. 



In an embryo with five or six mesoblastic somites and in 

 which the medullary folds have not yet united above to enclose 

 a medullary canal the hypophysial evagination has not begun to 

 form. A median sagittal section of such an embryo, however, 

 shows the relation of the parts surrounding the point at which 

 the hypophysis will appear at a later stage. Fig. i, PI. I, rep- 

 resents a diagrammatic median sagittal section of an embryo 

 of Chrysemys marginata 2.5 mm. in length and in which there 

 are five distinct mesoblastic somites with indistinct traces of a 

 sixth. The figure is a combination of the six sections nearest 

 to the median line. Although the medullary groove is still 

 open above, the medullary folds in the head region have grown 

 to a considerable height, as represented by the dotted line 

 D. At the same time the whole anterior region has been 

 folded and bent downward. The fold of the blastoderm which 

 comes up over the head as a result is the proamnion (Pa.), and 

 consists of epiblast and hypoblast. At the bottom of the head 

 fold the hypoblast has traveled back much farther than the 

 epiblast, leaving a space in which the cardiac mesoblast (Cm.) 

 develops. As we trace this hypoblast (End.\) forward along 

 the floor of the fore-gut (F.G.) we find that it is bent ven- 

 trally so as to come into contact with the epiblast. This point 

 of contact, represented by the double-headed arrow ( \ ), is the 

 region at which the mouth will be formed. In front of this 

 we may recognize a short, wide, preoral gut (Pr.G.). This is a 



