The chorioallantoic membrane is a vascular membrane found in eggs of some amniotes, such as birds and reptiles. It is formed by the fusion of the mesodermal layers of two developmental structures: the allantois and the chorion. Three different layers compose the chorioallantoic membrane; these are called the chorionic epithelium, the mesenchyme and the allantoic epithelium. Blood capillaries and sinuses are found between epithelial cells of the chorionic layer, allowing close contact (within 0.2 N<m) with air found in pores of the shell membrane of the egg. As a result, the chorioallantoic membrane allows exchange of gases, such as oxygen, to developing embryos. During embryonic development of birds, the chorioallantoic membrane also plays an essential role in bone formation by transporting calcium into the embryo from the eggshell. Chorioalloantoic membranes from developing chicken eggs are routinely used in biological and biomedical research to investigate development, angiogenesis, tumors, and to propagate and investigate viruses or helminths. [ http://en.wikipedia.org/wiki/Chorioallantoic_membrane ]