in the endometrliiin, the layare 
trophcblast surface has divided ini 
Meanwhile, a sort cf priinltivs circula 
embedded blastocyst while mere 
■IP 
dS've^5sg 
err 
cturei 
tr. 
develop. The inner cell mass then s? 
les 
ies Itself hom the oveilvinc 
trophoblast. The resulting space is callsd 
layer of cells that forms its roof is called the s 
Another membrane called the chorloni 
S?v^ ( 
Dtic CE\nty an 
cion (Figure 7). 
sac develops from th( 
trophoblast and nearby tissue. Finally, outgrovvnhs of trophoblas 
aaiy 
the 
from the chorion project into the endometrium and are callsd prii 
chorionic villi, later giving rise to the placenta. Although 
blastocyst has become completely embedded in the endometrium 
and maternal blood bathes the chorionic villi, the maternal blood 
does not enter the blastocyst. Later, as the fetal circulation develops, 
the fetal and maternal blood systems still remain distinct and do not 
mingle. Nutrients, oxygen, and wastes diffuse in the appropriate 
direction across the placenta, but the two blood systems are 
individual and do not combine. 
PRE-PUBLICATION VERSION 
