CH. LIX.] 



THE PLACENTA 



889 



In some mammals in which the connection between the chorion 

 and the decidua is less intimate than in the human subject, the 

 glands persist to a greater or less extent, and secrete a fluid called 

 uterine milk, which is absorbed by the chorion. 



The portion of the decidua which undergoes the greatest change is 

 the decidua basalis. In it a number of large blood spaces are formed, 

 and these are separated into masses or cotyledons by fibrous strands. 



Fio. 575. Diagrammatic view of a vertical transverse section of the uterus at the seventh or eighth 

 week of pregnancy, c, c, c 7 , Cavity of uterus, which becomes the cavity of the decidua, opening at 

 c, c, the cornua, into the Fallopian tubes, and at cf into the cavity of the cervix, which is closed by 

 a plug of mucus ; dv, decidua vera ; dr, decidua reflexa, with the sparser villi imbedded in its 

 substance; ds, decidua basalis or serotina, involving the more developed chorionic villi of the 

 commencing placenta. The foetus is seen lying in the amniotic sac ; passing up from the umbilicus 

 is seen the umbilical cord and its vessels passing to their distribution in the villi of the chorion ; 

 also the pedicle of the yolk-sac, which lies in the cavity between the amnion and chorion. (Allen 

 Thomson.) 



The cotyledons are penetrated by chorionic villi, and it is this con- 

 junction of chorionic villi and decidua basalis which produces the 

 placenta. The blood-vessels of the chorionic villi are formed by the 

 mesoblastic covering of the allantois, another foetal outgrowth. Its 

 origin from the hind -gut is shown in fig. 5*72. 



The placenta is the organ of foetal nutrition and excretion, and at 

 full term it is seven or eight inches across and weighs nearly a pound. 

 Its blood sinuses are filled with maternal blood, which is carried to 



