INTERVILLOUS 



PaQ^\ 



CHORIONIC VILLUS 



Fig. 174. Primitive-streak stage in human development. The primitive streak 

 is shown in longitudinal section, and the head process extends forward under 

 the presumptive neural plate. The amnion encloses an amniotic cavity above the 

 blastoderm. Below it a flat sheet of endoderm is continuous with the yolk sac 

 and a small posterior diverticulum is the allantois. Mesoderm surrounds the yolk 

 sac and the amnion and extends around the inner surface of the chorion. The 

 early embryo, amnion, and yolk sac are thus suspended by a stalk of mesoderm, 

 the body stalk. The outermost extraembryonic membranes have differentiated 

 into a spongy layer, the syntrophoblast, and a compact layer, the chorion. The 

 latter becomes convoluted and together with the mesoderm forms chorionic 

 villi. Blood vessels from the embryo invade the mesoderm within the chorionic 

 villi, and blood sinuses filled with maternal blood develop in the synthrophoblast. 



269 



