THE EARLY DEVELOPMENT OF THE MAMMAL 441 



II. Placenta syndesmochorialis. Uterine epithelium nearly or 

 wholly eroded. Chorionic epithelium in contact with the con- 

 nective tissue of the uterine mucosa. Ruminants. 



III. Placenta endotheliochorialis. Epithelium and connective tissue of 

 the uterine mucosa eroded. Chorionic epithelium in contact 

 with the endothelial walls of the uterine blood vessels. Carnivora. 



IV. Placenta hcemochorialis. Uterine epithelium, connective tissue, 

 and vascular endothelium eroded. Chorionic epithelium in 

 contact with maternal blood stream. Man. 



Variation in the extent of the erosion of the maternal tis- 

 sues determines to some extent also the character of the 

 nutritive , substances absorbed or resorbed by the chorionic 

 epithelium. The nutritive materials taken into the foetal 

 circulation are of two classes. (A) Food substances already 

 dissolved in the maternal blood. These, known as hcemotrophe, 

 may pass by diffusion or by active resorption directly into the 

 embryonic circulation. Obviously this process is easier, and 

 this type of nutritive substance more important, in those 

 placentae where the chorionic epithelium is more closely re- 

 lated with the maternal blood (Placenta endotheliochorialis and 

 PL hsemochorialis). (B) Products of the uterine mucous mem- 

 brane including the secretion of the uterine glands, the products 

 of the erosive or dissolving action of the trophoderm, and the 

 resultant extravasated blood. This is known as the ernbryo- 

 trophe or pabulum. Frequently these materials undergo a sort 

 of digestive process before their absorption into the embryonic 

 circulation. In forms whose chorionic (villous) epithelium is 

 less closely related to the maternal blood stream (PL epithelio- 

 chorialis, PL syndesmochorialis) the embryotrophe is the 

 more important source of nutrition. In the other types it 

 may be of great importance during the early 'period of devel- 

 opment, and gradually give place to the risemotrophe as the 

 relation between the villous and uterine blood streams becomes 

 more intimate. 



With these general facts regarding the variety of placental 

 arrangement in mind, we may describe the development and 

 structure of but a single type the human placenta (Placenta 

 discoidalis, hcemochorialis). (For a description of the placental 



