The Limit Between Ectoderm and Entoderm. 53 
against the brain, the concavity toward the entoderm (Fig. 9). It 
finally forms a somewhat ovoid mass and lies beneath the brain. 
Its further history does not concern us in this study, 
3. Coalescence of the Walls of the Foregut. 
It has already been indicated that the formation of the neural tube 
begins to compress the anterior part of the archenteron. This is 
Vie. 11. A diagram of the relations of the palzeeostoma and neostoma in A. 
punctatum. A camera drawing was made from the section from which Fig. 
7 was drawn and the cleft connecting the archenteron with the hypophysis 
was made wider, This is the only diagrammatic feature of the drawing. The 
palzeostoma is represented by the cleft hy- p. 
earried further by the formation of the mesodermic somites above 
and at the sides and of the heart below, and the bending down of 
the brain completes the influences which result in the complete oblit- 
eration of the cavity of the anterior part of the future mouth (Figs. 
12, 14). This condition is well known in amphibian embryos and 
requires little comment. ‘Two things should be noted. (a) The 
coalescence of the walls begins early and is followed by a relatively 
