4. J. B. Johnston. 
Or 
ereat elongation of the pharyngeal region. The necessity of this to 
make room for the branchial apparatus, heart and brain, is clear 
from Figs. 3, 9 and 10, and it is this chiefly which causes the for- 
ward projection of the head from the yolk and the straightening 
out of the embryo. (b) When the oral cavity appears it leads to the 
place which has been indicated in the early embryo as the future 
mouth opening. The history of the hypophysis and preoral ento- 
derm given above and the later history of the preoral entoderm make 
this clear. 
‘SS. 
Vic. 12. <A. punctatum, nearly median sagittal section of oropharyngeal 
region after the coalescence of the walls of the primitive mouth cavity. The 
yolk-laden entoderm is surrounded by the ingrowing dental ridges. Iron 
hematoxylin, fuchsin. 
4, Disappearance of Mouth Plate Hcetoderm. 
In Amblystoma no stomodzeum is formed. Instead, the ectoderm 
over the mouth plate area disappears, leaving the solid entoderm 
exposed on the free surface. Later the mouth cavity appears as a 
cleft in this entoderm. 
The well known arrangement of the cells of the ectoderm in two 
layers is clearly marked in the mouth plate area of Amblystoma in 
the neural plate stage. As the neural tube rolls up the ectoderm 
