I20 COMPARATIVE ANATOMY 



The main facts concerning the development of a placenta by the chorio- 

 allantoic membrane have already been stated (see page 54). The 

 essential structures of the placenta are the highly vascular villi 

 produced by the chorio-allantoic membrane (Fig. 89). These villi may 

 be merely lodged in depressions in the uterine wall or they may pierce 

 more or less deeply into its tissues. Their terminal regions may lie 

 within a vascular connective-tissue layer of the uterus and the surface 

 of a villus may be in close contact with the very thin wall of a blood 

 vessel. These deeper penetrations of the villi are attained at the expense 

 of actual eroding or destruction of uterine tissue. In extreme cases there 

 is destruction of walls of uterine blood vessels and the extravasated blood 

 fills large spaces or sinuses in the uterine wall. The branched ends of villi 

 project into these sinuses so that the villous surfaces are directly bathed 



Fig. 90. — Fetus of Cat, removed from uterus without rupturing chorionic sac (C), 

 showing zonary distribution of placental villi. 



by maternal blood, an arrangement providing maximum efficiency in the 

 exchange of materials between fetal and maternal blood. In general 

 the extent of penetration of the villi and the degree of intimacy of their 

 relation to the maternal blood is an index of the degree of specialization 

 of the placenta. In the human placenta the villi are long, much branched, 

 and exposed directly to maternal blood which fills capacious sinuses in 

 the uterine wall. 



Mammals exhibit various types of placenta, depending on the dis- 

 tribution of villi in the chorionic surface. When the villi are uniformly 

 distributed over the chorion, as in the horse, pig and many other ungulates, 

 the placenta is called diffuse. In many ruminant ungulates, such as 

 cattle, the villi are localized in numerous patches or clusters of varying 

 sizes — the cotyledonary placenta. In carnivores the placenta usually 

 takes the form of a broad band or zone encircling the chorion at a position 

 about midway between head and tail of fetus — the zonary placenta 

 (Fig. 90). A discoidal placenta, in which villi are restricted to a single 

 relatively large area of the chorion, occurs in insectivores, bats, rodents 

 and higher primates including man. 



