chorion
noun
cho·ri·on ˈkȯr-ē-ˌän 
: the highly vascular outer embryonic membrane of reptiles, birds, and mammals that in placental mammals is associated with the allantois in the formation of the placenta
Example Sentences
Recent Examples on the Web This is essentially a blood clot that can occur when the outermost membrane surrounding the embryo (the chorion) separates a little from the uterine wall, allowing some blood to pool in that space between the two. 
Amy Marturana, SELF, 28 Dec. 2018
Amy Marturana, SELF, 28 Dec. 2018 Word History
Etymology
New Latin, from Greek, afterbirth
First Known Use
1545, in the meaning defined above
Medical Definition
chorion
noun
cho·ri·on ˈkōr-ē-ˌän, ˈkȯr-
: the highly vascular outer embryonic membrane that is associated with the allantois in the formation of the placenta