Definition of "chorion" []

• The outer membrane enclosing the embryo in reptiles, birds, and mammals. In placental mammals it contributes to the development of the placenta. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "Covering the external face of the amnion and lining the inner face of the chorion is a double membrane, _the allantois_."
• "On the other hand, the villi on that part of the chorion which is in contact with the decidua placentalis increase greatly in size and complexity, and hence this part is named the chorion frondosum (Fig. 28)."
• "The inter-twin membrane has two layers (amnion-amnion) in monochorionic gestations, which appear thin, versus three layers (amnion-chorion-amnion) in dichorionic gestations."