chorion (ko're-on). 1

The external layer of the blastoderm; the outer enveloping of the membrane of the fecundated ovum, most characteristically developed in the human subject, being observed as soon as the ovum has entered the cavity of the uterus, and persisting through the whole period of gestation. A part of it takes a share in the development of the fetal portion of the placenta, and the remainder constitutes the outermost of the fetal envelopes, c. glabrum, c. lacve. See false c. c. pellucidum, c. primitivum. See primitive c. c. reticula tum, c. reticulosum. See shaggy c. c. secundarium. See secondary c. c. verum. See secondary c. false c. That portion of the c. which is left bare of villi after the formation of the placenta has begun in the remaining portion, primitive c. 1. The rudimentary c; a membranous product of the epithelial cells of the ovary and the oviduct deposited around the mature and fecundated ovum; a transformation of the vitelline membrane which becomes shaggy soon after its entrance into the uterus. 2. The discus pro liger us. secondary c. The true outer envelope of the embryo, by means of which the ovum is attached to the uterine wall, appearing (in man) in the second week of gestation and consisting of the single layer of basement epithelial cells which constitute the external layer of the blastoderm. shaggyc. A term applied to the c. during the period (two months in the human subject) that its entire outer surface (the vitelline membrane) is covered with villus outgrowths, smooth c. See false c. spongy c. The decidua. true c. The external layer of the allantois. vascular c. The secondary c. when it has united with the external surface of the allantois and become vascular. [Gr., chorion, skin, membrane.] Relating to the chorionic (ko-re-on'ik). under villus. chorion, c. villi. See ko'ri-oy d). choroi d, chorio id (ko'roy d, Resembling the chorion, pertaining to the c. coat of the eye; the middle coat of the posterior portion of the eyeball;chorion so called from its resemblance to the in vascularity. It is a dark-br own, highly vascular tunic lying between the sclera externally and the retina internally, and extending from the opening for the entrance of the optic nerve forward to the ciliary body, into which it is insensibly merged. It is connected with the sclera by a loose connective ti ue with large meshes more or le filled with pigment. [Gr., chorioeides, from chorion, skin, membrane, + eidos, resemblance.]