838 



DEVELOPMENT 



[CH. LIX. 



and the lining of the uterine cavity after parturition. The inter- 

 mediate parts of the glands in the decidua vera and the decidua 

 basalis become very much enlarged, and form a stratum of the decidua 

 called the spongy layer, and ultimately this layer is converted into a 

 series of clefts, and it is along the line of these clefts that the 

 decidua is separated at birth. 



In some mammals in which the connection between the chorion 

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



FIG. 634. Diagrammatic view of a vertical transverse section of the uterus at the seventh or eighth 

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

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

 a plug of mucus ; dv, decidua vera ; dr, decidua reflexa, with the sparser villi embedded 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.) 



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, formerly called the decidua serotina. In it a 

 number of large blood spaces are formed, and these are separated into 

 masses or cotyledons by fibrous strands. The cotyledons are penetrated 

 by chorionic villi, and it is this conjunction of chorionic villi and 



