Organisms possessing an organ forming from the fusing of the chorion and allantois that allows the embryo to obtain nutrients directly from the mother.

Organisms displaying extensive parental care.

Organisms possessing an allantois that encloses a space for the collection of metabolic waste from the developing embryo.

Organisms possessing a membrane bound  fluid filled cavity, which provides the embryo an aquatic environment within the mother, in which to develop.