408 REIGHARD—MAST. [Vov. XIX. 
above-mentioned evagination of the forebrain cavity and the 
exterior as neuropore (mediane Riechplatte and lobus olfac- 
torius impar of von Kupffer). 
Immediately in front of the neuropore the thickened por- 
tion of the inner layer of the ectoblast becomes still thicker, 
extending postero-ventrally in the form of the segment of a 
sphere. (Eig) hye) 
This is the fundament of the hypophysis. The ectoblast 
of the neuropore and that of the hypophysial fundament are 
thus seen to form parts of the same thickening of ectoblast. 
The nervous or inner layer of the ectoblast anterior to 
the fundament of the hypophysis as well as that on both 
sides of it is composed of a single tier of columnar cells. In 
the ectoblast which forms the fundament of the hypophysis, 
the columnar cells, judging from the arrangement of the 
nuclei, appear to have divided transversely, thus forming ap- 
proximately cubical cells. The cell walls could not be seen 
distinctly in the hypophysis in this stage, consequently it was 
impossible to determine the exact form of the cells and their 
arrangement. Some little distance anterior to the fundament 
of the hypophysis, there is a broad, low, thick-walled evagi- 
nation (Fig. 1, ad.) of the dorsal wall of the alimentary 
canal near the anterior end. This is the crescent as described 
in the preceding paper. Immediately in front of the crescent 
there is a smaller but deeper evagination (Fig. 1, m.) marking 
the position of the mouth opening. In the space between the 
ventral surface of the hypophysis the dorsal wall of the 
alimentary canal and the anterior wall of the forebrain there 
is a mass of rather closely packed mesenchyme cells (Fig. 
{ties 
A median longitudinal section (Fig. 2) of an embryo cor- 
responding in age to stage three (Fig. 6) of the preceding 
article, shows that the fundament of the hypophysis which 
formed a part of the inner layer of ectoblast in the preceding 
stage, is now connected with other ectoblast tissue only at 
its posterior or dorsal end where it is continuous with the 
neuropore. It has become a solid rod of cells slightly flat- 
