The Limit Between Ectoderm and Entoderm. 51 
sistence of the preoral cavity (in various degrees) serves to mark 
the position of the preoral entoderm. It also explains the peculiar 
form taken by the hypophysis. The cleft meeting the cavity of the 
hypophysis suggests that the intact wall of the hypophysial invagina- 
tion constitutes, with the entoderm dorsal to the cleft, the dorsal 
wall or roof of the palzeostoma, while the absence of a ventral wall 
to the hypophysial invagination allows the hypophysis to open into 
Fic. 9. A. punctatum, after separation of hypophysis from ectoderm. 
Medial sagittal section. The pointed recess of the archenteron directed toward 
the letter m is the oral recess; the slight pit or concavity next to pr. en. is 
the palzeostomal recess. Hzemalum. 
the archenteron, the ventral wall or floor of the palseostoma being 
formed by entoderm alone. These relations are indicated im a 
diagram (Fig. 11). 
After the stage at which the partial connection between hypophysis 
and archenteron is seen, there is a further bending of the brain, the 
flexure becoming excessive in amphibians. The bending, together 
with the expansion of the forebrain, results in pushing the hypophysis 
