DEVELOPMENT. 191 



oxygen, and in turn removes the carbonic acid and excrementitious 

 material which the foetus must lose ; that is, the placental circula- 

 tion supplies the place taken in after-life by the alimentary and the 

 respiratory tracts. 



Does the entire placenta leave the uterus immediately after the 

 birth of the child ? 



No. The foetal part is almost all, excepting some of the capil- 

 lary tufts which are torn off, discharged in the after-birth ; but the 

 decidua is not entirely disposed of in this way, the portions remain- 

 ing being in part absorbed and in part found in the lochia which 

 occur for a few days after the birth. 



What is the appearance of the placenta ? 



It appears as a thick, cake-like disk of vascular tissue. Its ma- 

 ternal and foetal portions are so intermingled that they cannot be 

 separated. In size it covers about one-third of the uterine wall. 



At what period of gestation is the placenta formed ? 



At about the third month of pregnancy. Before that time the 

 chorion is covered by the decidua reflexa and nourishes the embryo, 

 but as the placenta becomes more developed other parts of the 

 chorion atrophy. 



From what tissues is the umbilical cord formed ? 



From the vascular allantois, which 



carries the arteries and vein. It has, FIG. 63. 



however, an external coating of the 

 amnion and the shrivelled umbilical 

 vesicle and its duct (Fig. 63). How 

 this occurs will readily be seen by 

 reference to the accompanying cut. 



Why do the maternal vessels not 

 bleed excessively after the pla- 

 centa is torn from its implan- 

 tation ? 

 There is, of course, a loss of the 



Vklnnrl nrmtainorl in +Vio nfovino oin Human Embrvo and its Envelopes at 



the End of the Third Month, show- 

 USeS, but the general balance of the ing the enlargement of the amnion. 



circulation is not disturbed at child- 

 birth. The reason for this is the oblique entrance of the placental 



