Chorion

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.

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

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.

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.