70 CARL G. HARTMAN 
and represents the normal condition at this stage. The section 
in figure 1, plate 18, is the tenth of thirty-five sections through 
the embryonic area; that is, lies to one side of the midline. The 
definitive entoderm would seem from this section to clothe the 
entire inner surface of the area, but a study of the series dis- 
closes the fact that the entoderm has differentiated only in spots, 
chiefly at the periphery of the area. That these changes take 
place chiefly in the periphery first would appear also from 
ovum No. 339 (3) shown in figures 6A and 6 and in figure 2, 
plate 6. At HNT? is a group of cells which run through ten 
sections; they are primitive entodermal cells not yet differ- 
entiated. So also in figure 8 (egg No. 175’ (2) ) the entoderm 
has already advanced considerably toward the equator, although 
there are still some undifferentiated cells near the middle of the 
area. Similar undifferentiated cells are also seen in other 
vesicles, as at HNT?, figures 3 and 4. 
Litter No. 352 consists of small blastocysts which fall into 
the stage under discussion. The group of eggs was photo- 
graphed fresh in Ringer’s solution by transmitted light (fig. 1, 
pl. 9). The vesicles with their more or less opaque embryonic 
areas are very evident. In the largest specimens the ento- 
derm has entirely differentiated, except in one (fig. 14, pl. 
13; fig. 8, pl. 21) in which a large blastomere has retarded the 
spreading of the cells (compare fig. 2, pl. 9, and fig. 2, pl. 18). 
In these eggs the entoderm has also advanced some distance 
toward the equator of the blastocyst. 
m. Maximum attenuation of the blastocyst wall 
The trophoblastic ectoderm, it was seen above, begins its 
thinning and spreading process soon after the proliferation of 
entoderm begins (in 0.15-mm. ova) and reaches its maximum 
when the formation of new entodermal cells from entectoderm 
ceases, and the entoderm begins to line the lower hemisphere 
(0.50-mm. blastocysts). The spreading and attenuation, how- 
ever, affect the embryonic as well as the trophoblastic area and 
takes place rapidily. while the entoderm is migrating to the 
