78 CARL G. HARTMAN 
b. The trophoblastic area 
The cells of this area are very attenuated in the late unilami- 
nar stage, but as they multiply they also thicken and increase 
their volume. The thickening is often most pronounced at the 
vegetative pole where numerous mitoses may occur (figs. 5 
and 7, pl. 18). This is a matter of some interest, for in larger 
blastocysts this region may continue to be more thickened than 
the remainder of the trophoblastic area; or certain ‘blisters’ may 
occur there, such as will be described under the 1 mm. blastocyst 
(O; figs: 2.and:3, pl. 21). 
In all stages the cytoplasm of the trophoblastic cells is very 
diffuse and loosely reticular, but in the early bilaminar stage 
especially it tends to break down into strands and ret’ culum, 
leaving the cell membrane collapsed and wrinkled. Only 
around the nuclei is there a denser mass of well-fixed cytoplasm 
and the nuclei themselves maintain their form and structure as 
perfectly as in the embryonic area (fig. 2, pl. 20). Normally, 
the trophoblastic layer follows the curvature of the albumen 
layer to which it remains closely applied, as is apparent from a 
study of the living eggs and photographs of them and as the 
best preparations show (fig. 4, pl. 1; fig. 17, pl. 13). Frequently, 
however, the vesicle collapses more or less in this area, leaving 
as an artifact a space between the vesicle and the albumen 
(fig. 4, pl. 10; ART, fig. 1, pl. 19). At this stage the albumen 
seems to be most dense nearest the shell membrane and often 
very loosely layered near the vesicle, which would account for 
the more frequent breaking down of the albumen at this point 
in the specimens. 
c. The entoderm 
Soon after completely lining the blastocyst wall the entoderm 
is everywhere the same, passing over the junctional line without 
change. This condition remains so throughout the bilaminar 
stage, except for certain modified cells to be mentioned in con- 
nection with older blastocysts. At first the entoderm is ex- 
tremely delicate so that it appears in section to be discon- 
