8i6 



DEVELOPMENT. 



[CH. LVIII. 



most of these is called the chorion ; at first this is covered with 

 villi containing blood-vessels ; the villi dip into the surrounding 

 decidua, but soon all of them atrophy and disappear, except 

 those that dip into the decidua serotina, where they become 

 greatly enlarged. 



The chorion is really formed by a fusion of two fetal mem- 



Fig. 630. 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 ; d v, decidua vera ; d >, 

 decidua .reflexa, with the sparser villi imbedded in its substance ; d s, decidua serotina, 

 involving the more developed chorionic villi of the commencing placenta. The foetus 

 is seen lying in the amniptic 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.) 



branes ; the false amnion and the allantois ; the allantois begins 

 as an outgrowth from the hind-gut ; the mesoblast which covers 

 it becomes developed into blood-vessels, and thus the false 

 amnion to which it becomes adherent is vascularised ; the main 

 vessels in the stalk of the allantois convey blood to and fro 

 between the foetus and the placenta. The placenta is formed 



