80 CARL G. HARTMAN 
above the surface of the ovum like a blister, a condition probably 
due to its greater density and resistance to shrinkage as com- 
pared with the trophoblastic area. 
The growing contrast between the two regions of the egg, 
which is now as clear-cut in sections as in whole mounts, is due 
to the increasing difference in the structure, as well as to the 
number of the formative cells. These are taller, much more 
crowded, and contain a denser and more granular cytoplasm, 
and this contrast in the types of cells is a constant character, 
no matter what the fixation, and the differences that exist 
among the specimens are those of degree only. ‘These points 
are evident from an inspection of figures 1A, 2A, and 4 to 7, 
plate 21, which were drawn as nearly as possible in imitation of 
the tone of the specimens. 
While there is great variability in the thickness of the em- 
bryonic areas in 1-mm. blastocysts, it is true that in most cases 
the area has become considerably thickened as the vesicle has 
grown in volume and as the area- has increased in diameter 
(pl. 21). The cells have become mostly tall cubical to columnar 
and in the embryonic area are now nearly or quite as much 
crowded together as in older stages (compare fig. 12, pl. 19, and 
fig ply 22)k 
The embryonic ectoderm is arranged strictly in a single 
layer, never stratified or pseudostratified. The nuclei are 
practically on a level throughout, and this is one of the points 
of contrast with the blastoderm of other mammals. Mitotic 
spindles usually stand with their axes parallel to the surface of 
the egg (fig. 7, pl. 21). Frequently the cell that is in mitosis 
juts out above the level of the ectodermal layer (fig. 5, pl. 21). 
as in the blastocysts of the rabbit and other mammals, and such 
cells almost always stain less deeply, a fact that applies both to 
sections and to surface views. 
The trophoblastic area has also developed more mass and 
thickness, proportionally quite as much as the embryonic area 
(figs. 6 and 7A, pl. 21). Typically it is about 8 to 10 » in thick- 
ness. It is usually uniform in structure at all points and fits 
closely to the albumen (fig. 6), except when artificially separated 
