chorioallantois

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The highly vascular fetal membrane that consists of the fused chorion and allantois, found adjacent to the eggshell in reptiles and birds and constituting the placenta in most mammals.

from the GNU version of the Collaborative International Dictionary of English.

• noun a very vascular fetal membrane composed of the fused chorion and adjacent wall of the allantois.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun anatomy The vascular extraembryonic membrane, formed by the fusion of the allantois and the chorion, that makes up the placenta in most mammals; the chorioallantoic membrane

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the very vascular fetal membrane composed of the fused chorion and adjacent wall of the allantois