168 
Monitqrihg Stem Cell Eismimmm 
Extra- 
embroynic 
Coelom 
Commatmg Stalk 
S&Gond&rw 
Yolk Sao 
Trophoblagt 
\Mx\rm ffefK? 
C'PO'sanit: 
cavsty 
j? 
Jf 
Primary 
Chorionic 
villus 
Amnion 
Prochordal 
plate 
Secondary 
/oik sac 
Figure 7A: Sections of completely implanted blastocysts at the end of the second 
week, illustrating how the secondary yolk sac forms. The presence of primary 
chorionic villi on the wall of the chorionic sac is characteristic of blastocysts at the end 
of the second week. A primitive uteroplacental circulation is now present. (Reprinted 
from Moore, Essentials of Human Embryology, 1988, Fig. 2-2, p. 13, with permission 
from Elsevier.] 
PRE-PUBLICATION VERSION 
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