CH. LTX.] 



THE AMNION AND CHOBION 



841 



The amniotic fluid consists of water containing small quantities of 

 albumin, urea, and salts. It is an exudation from the foatal and the 

 maternal blood, and the urea in it comes from the foetal urine which is 

 passed into the amniotic cavity in the later part of pregnancy. 



The function of the fluid appears to be purely mechanical. It 

 supports the embryo on all side^s, and protects it from blows and other 

 injuries to the abdomen of the mother, and from sudden irregular 

 contractions of the abdominal walls. 



Chorion. The chorion is that portion of the surface of the ovum 



FIG. 637. Diagram of a longitudinal section of an ovum, showing later stage of formation, amnion 

 and foetal part of placenta than that shown in fig. 636. 



1. Amnion cavity almost completely 3. Allantoic diverticulum from cloaca. 7. Yolk sac. 



closed in. 4. Epiblast of chorion ) o affmlmir 8. Ccelom. 



2. Placental villi of chorion. 5. Somatic mesoblast / J " 10. Pericardium. 



which does not enter into the formation of the embryo or amnion, 

 and after the separation of the amnion, it forms the whole of the outer 

 surface of the ovum, completely surrounding the embryo, the amnion, 

 and the allantois. 



At a very early period its surface is set with fine processes, the 

 chorionic villi, which at first consist of epiblastic cells, alone, but very 

 soon they acquire cores of somatic mesoblast, which becomes vascu- 

 larised by the allantoic vessels which rapidly extend throughout 

 the whole of the chorionic mesoblast. 



