The Limit Between Eetoderm and Entoderm. 49 
no actual cavity, but the arrangement and pigmentation of the cells 
suggest a cavity, and the wall next to the entoderm is thin and devoid 
of nuclei. By this time the ectoderm of the mouth plate, caudal to 
the hypophysis, consists of only a single layer of flat cells (see 
Fie. 6. A. punctatum, in a stage slightly advanced over that in Fig. 5. 
Medial sagittal section of hypophysis. Borax carmine. 
Fic. 7. A. punctatum, at the height of the hypophysial invagination before 
separation from ectoderm. Note the concavity of hypophysis and the absence 
of ectoderm from the mouth plate. Heemalum. 
below). As the hypophysis pushes in, its thin posterior or ventral 
wall is devoid of cells, and soon the cavity of the hypophysial invagi- 
nation is bounded on that side by entoderm (Figs. 6 and 7). The 
hypophysis appears as a hook-shaped membrane (sagittal section) 
