chorion

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

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

from The Century Dictionary.

• noun In anatomy, the outermost fetal envelop; the external membrane which invests the embryo, forming in the higher vertebrates the outer layer of the bag of waters, and contributing to the formation of the placenta.
• noun By analogy — The membrana putaminis or egg-pod of those eggs which have calcareous shells.
• noun The external investment of the ovum of an insect, derived from the epithelial layer of the oviduct.
• noun Same as corium.
• noun In botany, the pulpy matter which fills the young ovule and is absorbed during development.

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

• noun The outer membrane which invests the fetus in the womb; also, the similar membrane investing many ova at certain stages of development.
• noun The true skin, or cutis.
• noun (Bot.) The outer membrane of seeds of plants.

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

• noun anatomy, biology The protective and nutritive membrane in higher vertebrates that attaches the fetus to the uterus.
• noun biology, entomology The outer case of an insect egg.
• noun botany The outer membrane of seeds of plants.

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

• noun the outermost membranous sac enclosing the embryo in higher vertebrates (reptiles, birds and mammals)