chorion

Etymology

From Late Latin chorion, from Ancient Greek χόριον (khórion, “membrane surrounding the fetus, afterbirth”).

noun

1. (anatomy, biology) The protective and nutritive membrane in higher vertebrates that attaches the fetus to the uterus.
   2002, Rovert E. A. Stewart, Barbara E. Stewart, Female Reproductive Systems, entry in Encyclopedia of Marine Mammals, page 425, The allantois fuses with the chorion, forming a small round area, the allentochorion. This region becomes the placenta (Fig. 3).
   In this illustration, some license is taken, since the amnion is shown as still separate from the chorion; this is done to illustrate the process of apposition of the amniotic sac to the chorion and to accentuate the interposition of the yolk sac between the amnion and the chorion. 2006, Ona Marie Faye-Petersen, Debra S. Heller, Vijay V. Joshi, Handbook Of Placental Pathology, page 4
   The amnion and chorion begin to develop soon after fertilization and continue to grow until about 28 weeks′ gestation. 2007, Susan Tucker Blackburn, Maternal, Fetal, & Neonatal Physiology: A Clinical Perspective, page 106
2. (biology, entomology) The outer case of an insect egg.
   The chorion has a predominantly lamellar structure, the lamellae being two-dimensional networks of protein fibrils. 1994, Klaus Urich, Comparative Animal Biochemistry, page 392
   Even closely related species may have chorions differing in thickness. In the silkmoths, for example, the chorion of Bombyx is about 25 μm thick while that of Hyalophora is 55 μm thick. 1998, R. F. Chapman, The Insects: Structure and Function, page 326
   2008, John L. Capinera, Eggs of Insects, entry in John L. Capinera (editor), Encyclopedia of Entomology, page 1290, In addition to facilitating gas exchange and water conservation, the chorion must, in some cases, allow uptake of water or liquid nutrients from the environment.
3. (botany) The outer membrane of seeds of plants.