The Limit Between FEetoderm and Entoderm. 55 
becomes somewhat thinner, but not as thin as that covering the 
ventral surface of the yolk mass. Following this the inner cells of 
ectoderm disappear from the mouth plate. In either transverse or 
sagittal sections the entoderm is seen to be covered by the thin flat 
cells of the outer layer of ectoderm, while the inner layer of more 
cuboidal cells stops abruptly around the borders of the mouth plate 
(Figs. 5, 6, 13). Anteriorly the mouth plate is bounded by the 
hypophysis. I have not been able thus far to see in detail how this 
Fic. 13. Detail drawing of the mouth plate and dental ridges in the section 
drawn in Fig. 12. Here it is seen that the outer layer of ectoderm is partly 
broken down but still covers the entoderm. 
condition comes about. Whether the inner cells of ectoderm migrate 
apart and draw away from the mouth area or whether some of these 
cells disintegrate in situ is not wholly clear. I have seen no signs 
of disintegrating cells, and there is to be seen a heaping up or thick- 
ening of the inner layer of ectoderm around the mouth plate. I am 
strongly inclined to think that the inner cells draw away from the 
mouth plate in preparation for the formation of the dental ridges 
to be described below. Certain it is that by the time the hypophysis 
begins to be actively invaginated (Fig. 6) the ectoderm of the mouth 
