cho·ri·o·al·lan·to·is (kôr??-???-l?n?t?-?s)n. 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.cho?ri·o·al?lan·to?ic (-?-?l??n-t???k) adj.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.chorioallantois (?k?r????æl?n?t???s) n (Anatomy) the membrane of a fetus that is formed by the merging of the allantois and the chorionCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014cho?ri?o?al?lan?to?is (?k?r i o? ??læn to? ?s, -t??s, ?ko?r-) n. a vascular membrane surrounding the embryo in birds, reptiles, and certain mammals, formed by the fusion of the walls of the chorion and allantois. [1930?35] cho`ri?o?al`lan?to?ic (-?æl ?n?to? ?k) adj. Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.