REPRODUCTIOX. 



473 



The chorion (Fig. 227), or false amnion, is formed simultaneously with the 

 true amnion, and like it from somatopleure. It is a thickened vascular mem- 

 brane, completely surrounding the amnion with the contained embryo. Be- 

 tween it and the amnion there is at first a considerable space, traversed by the 

 umbilical cord and filled with the chorionic fluid (which is probably of the 

 same general nature as the amniotic fluid). But later this space is obliterated 



Decidua serotina. 

 Chorion frondosum. 



—Mucous plug within cervical canal. 



Fig. 227.— Diagram of the human uterus nt the seventh or eighth week of pregnancy modified from 

 Allen Thompson). The fetal villi are shown growing into the Binuses of the decidua serotina and the 

 decidua reflexa; in the latter they are becoming atrophied. They are marked by the black fetal vessels, 

 whiehcanbe traced backward along the umbilical conl to the embryo. The placenta comprises the 

 decidua serotina and the chorion frondosum. 



by the enlargement of the amnion. Externally the chorion presents, at first, 

 a shaggy appearance due to the existence of very numerous columnar pro- 

 cesses, called vUU, extending outward in all directions and joining by their 

 tips the decidua serotina and the decidva reflexa. Later the villi are aborted 

 except in the region of the serotina, where they become more prominent and 

 constitute an important part of the placenta. The blood-vessels of the chorion 



