630 



MAMMALIAN EMBRYOLOGY 



chorion and villi. This means that there are now two layers of epi- 

 thelium covering the mesodermal core of all the villi, and that it is in 

 these villi that the chorionic circulation of the embryo is established. 



The blood vessels of the uterus 'open into the little blood-lacunae, 

 which is another way of saying that the syncytial trophoderm which 

 covers the villi is bathed in maternal blood. This is where the nourish- 

 ment of the embryo takes place. The maternal blood itself does not pass 

 into the developing embryo. 



Fig. 361. 

 Diagram to show relationship of mammalian embryo and maternal membranes. 



At first the villi cover the entire surface of the chorion, but in man, 

 after a few weeks, the villi located away from the point of attachment 

 begin to degenerate and finally leave that portion smooth. This smooth 

 region is called the chorion laeve, while the attached portion which re- 

 tains the villi is known as the chorion frondosum (Fig. 362). It is the 

 chorion frondosum, together with the decidua basalis, which constitutes 

 the placenta. And it is the chorion frondosum to which the embryo is 

 attached by the body-stalk which later comes to be called the umbilical- 

 stalk or umbilical cord. 



