chorion

alkion suonikalvo, sikiön suonikalvo puhekieltä 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, http://books.google.com.au/books?id=RsEKkDNF5f4C&pg=PA425&dq=%22chorion%22%7C%22chorions%22+-intitle:%22%22+-inauthor:%22%22&hl=en&sa=X&ei=yJ2kUMbFJsvjmAXI4ICYDw&redir_esc=yv=onepage&q=%22chorion%22%7C%22chorions%22%20-intitle%3A%22%22%20-inauthor%3A%22%22&f=false 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.

The amnion and chorion begin to develop soon after fertilization and continue to grow until about 28 weeks′ gestation.

The chorion has a predominantly lamellar structure, the lamellae being two-dimensional networks of protein fibrils.

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.

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.