34 



HUMAN ANATOMY. 



they form a respirative and nutritive apparatus. The intimacy between the uterine 

 mucous membrane and the chorionic tufts presents all degrees of association, from 

 simple apposition, as seen in the sow, where the feebly developed and almost uni- 

 formly distributed vascular projections are received within corresponding depressions 

 in the richly vascular uterine tissue, to the firm and complex attachment found in the 

 highly developed human placenta. 



Fig. 4t. 



Villi of extraplacental chorion 

 Gut-tube 



Ectoblast 



Amniotic meso- 

 blast 



_^Space between am- 

 nion and chorion 



Subchorionic 

 space 



Vitelline sac 



Mesoblast 



Entoblast 



Allantois 



— —Allantoic blood- 

 vessels 

 Allantoic sac 



Villi of placental 

 chorion 



Maternal blood-spaces 



Decidua placentalis 



:horion. 



Diagram showing villous chorion, differentiation of placental area, and vascularization of c 



In contrast with the chorion of those animals in which the nutritive relations 

 between the maternal tissues and the embryo are uniformly distributed are the local 

 specializations seen in the chorion of those types in which a placental area is de- 

 veloped. The animals in which the latter condition obtains are known as placentalia^ 

 of which three subgroups are recognized depending upon the multiple (cotyledons), 



Fig. 42. 



"^ 



D 



'''^. 



Diagrams illustrating the various types of development of the chorion. A, uniformly developed villi (hog, 

 horse) ; B, multiple placentae or cotyledons (cow, sheep) ; C, zonular placenta (cat, dog) ; D, discoidal placenta 

 (monkey, man). A-B comprise non-deciduate ; C-£>. deciduate mammals. 



zonular, or discoidal form of the placenta, man and the apes representing the highest 

 specialization of the last division. In its general plan of development, therefore, the 

 placenta is formed of a foetal and a viaternal portion, the former consisting of the 

 vascular villi which are unusually well developed within a particular portion of the 

 chorion, and the latter of the opposed uterine lining which becomes higMy special- 

 ized throughout a corresponding area and more or less intimately united with the 



