In Reptiles, Birds, and Many Mammals
In reptiles, birds, and many mammals the amnion is developed in the following manner:
At the point of constriction where the primitive digestive tube of the embryo joins the yolk sac a reflection or folding upward of the somatopleure takes place.
This, the amniotic fold, first makes its appearance at the cephalic extremity, and subsequently at the caudal end and sides of the embryo, and gradually rising more and more, its different parts meet and fuse over the dorsal aspect of the embryo, and enclose a cavity, the amniotic cavity. This kind of amnion is known as pleuroamnion (formed by folding), as opposed to schyzoamnion (formed by delamination).
After the fusion of the edges of the amniotic fold, the two layers of the fold become completely separated, the inner forming the amnion, the outer the false amnion or serosa.
The space between the amnion and the serosa constitutes the extra-embryonic celom, and for a time communicates with the embryonic celom.
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