354 REPRODUCTION 



Of course there is at first a considerable cavity left be- 

 tween the reflex and the vera, but as the embryo increases in 

 size the space becomes smaller and is obliterated by the end 

 of the fifth month. After this time both vera and reflexa 

 undergo retrograde changes due to pressure and become 

 closely attached to the chorion. They are discharged with 

 the membranes at birth. 



Placenta. The placenta is the organ of nutrition for the 

 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 

 reflection of the decidua serotina and is the chorion fron- 

 dosum. The union of these, with certain other develop- 

 ments, 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 pla- 

 centa. 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 fe- 

 tus, and out of the fetal blood materials which require re- 

 moval. The maternal blood performs both alimentary and 

 respiratory functions for the fetus. 



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

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

 diameter 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. 



