THE PLACENTA 353 



fetus after about the end of the third month. Through it the 

 vessels of the fetus and those of the mother are brought into most 

 intimate relations. 



It has been said that the villi of the chorion in one locality 

 become very highly developed. This is at the site of the reflec- 

 tion of the decidua serotina and is the chorion f rondo sum. The 

 union of these, with certain other developments, constitutes the 

 placenta. 



The decidua serotina becomes very spongy. It is filled with 

 sinuses, into which the enlarged villi of the chorion frondosum 

 project. The sinuses are filled with maternal blood, while the 

 capillaries of the villi contain fetal blood. There is no direct 

 connection between the vessels of mother and child, but the thin 

 lining of the villi and sinuses allows free interchange of materials 

 by osmosis. 



It seems that the interchange is under the influence of two 

 sets of cells, each disposed in a single layer one belonging to the 

 maternal and the other to the fetal part of the placenta. These 

 layers of cells are situated on either side of the membrane of the 

 villus. They seem to take out of the maternal blood materials 

 needed for the nutrition of the fetus, and out of the fetal blood 

 materials which require removal. The maternal blood per- 

 forms both alimentary and respiratory functions for the fetus. 



The placenta as a whole is discoid in shape. Its fetal surface 

 is concave and covered by the amnion. The mass has a diame- 

 ter of 4-5 in., and a thickness of half an inch. The villi receive 

 blood from the allantoic or umbilical arteries; it is returned by 

 the umbilical vein. 



At labor uterine contractions detach the placenta and the 

 decidua and expel them from the womb. The separation takes 

 place in the deeper part of the maternal placenta, or decidua 

 serotina, so that the mass discharged represents both the fetal 

 and maternal portions. The vessels entering the sinuses do 

 so obliquely; consequently uterine contractions ' at birth very 

 23 



