94 STRUCTURE OF THE VERTEBRATES 



vertebrates. The chorion of meroblastic embryos lies in contact 

 with a porous shell, oxygen being absorbed by the capillaries 

 of the allantois from diffused air, and food from the yolk gran- 

 ules. The essentials of nutrition are supplied the placental em- 

 bryo by the development of a placenta. 



Hair-like villi develop on the chorion. These processes secrete 

 an enzyme which eats into the maternal tissue to form numer- 

 ous cavities which eventually fuse to form a relatively large 

 sinus in the thickened wall of the uterus. When the allantois 

 reaches the chorion the growth of the villi is accelerated in this 

 region and the allantoic tissue grows into the villi. Sj^nchro- 

 nously the villi of the other parts of the chorion degenerate. As 

 the allantoic villi burrow into the maternal sinus numerous veins 

 and arteries are cut, so that the sinus fills with maternal blood. 

 The placenta is the mass of tissue formed by the chorion and 

 allantois, with its villi and blood vessels. In the rodents and 

 most primates the placenta is discoidal in shape, and is con- 

 nected with the embryo by the body stalk. 



Blood vessels pass downward from the embryo to the pla- 

 centa and there break into capillaries and enter the villi. By 

 osmosis through the membranes of the placenta waste products 

 are given out into the maternal blood sinus, and oxygen and 

 food are taken up from the blood entering the sinus from the 

 cut arteries. The blood is constantly being drained from the 

 sinus by the veins with the result that the supply is always 

 fresh. 



The connection between the placenta and the body of the 

 embryo is the elongated body stalk, known as the umbilical 

 cord. After birth the cord and placenta slough off and are lost. 

 The point of attachment of the embryo remains as the umbi- 

 licus or navel. 



The sub-class Placentalia received its name from the placenta, 

 which is typical of the group. The structure is not found in any 

 other mammal, with the exception of a primitive homologue 

 developed by one marsupial (Perameles). There are several 

 types of placentae, apparently equally efficient. (1) The discoid 

 placenta is found in the rodents, the higiier primates, and in 

 other orders; (2) the diffuse placenta, spread evenly over the 

 chorion, is found in the horse, pig, and lemurs; (3) a cotyle- 



