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.chorioallantoic (?k?r????æl?n?t???k) adj (Anatomy) relating to the chorioallantois Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014