392 - Multicellular Animals, Especially Man 



Fig. 21-15. Human embryo (7.1 mm long) enclosed in the embryonic mem- 

 branes. The yolk sac lies below, connected with the embryo by the umbilical 

 cord. The embryo almost completely fills the amniotic cavity, which is bounded 

 by the plainly visible amniotic membrane. The outermost membrane is the 

 chorion, from which the tufted chorionic villi (embryonic part of the placenta) 

 protrude. Gill slits appear on the side of the neck, and the mesodermal somites 

 show along the dorsal margin of the trunk. The limbs are present, but no 

 fingers or toes have yet been formed. (Courtesy of W. Chesterman, Oxford 

 University.) 



which penetrate deeply into the uterine wall 

 (Fig. 21-15). This region, where the tissues of 

 the chorion and uterus are intermingled, is 

 the placenta, which permits exchanges to 

 occur between the blood of the fetus and 

 that of the mother. The chorionic villi are 

 richly supplied with blood capillaries, de- 

 rived from the umbilical artery. This large 

 artery extends out from the embryo along 

 the stalk of the allantois, and the chorionic 



capillaries are drained by the umbilical vein, 

 which returns to the embryo parallel to the 

 umbilical artery (Fig. 21-16). The uterine 

 wall, in the region of the chorionic villi, is 

 also highly vascularized, and this tissue con- 

 stitutes the maternal part of the placenta. 

 The central part of the placenta has the 

 form of a circular disc, embedded in the 

 uterine wall at the site of implantation (Fig. 

 21-16), but some placental tissue tends to 



