allantois: the vascular fetal membrane that lies below the chorion and develops from the hindgut in many embryonic higher vertebrates (reptiles, birds and mammals)
fetal membrane: any membrane that functions for the protection or nourishment or respiration or excretion of a developing fetus
thymol: a colorless crystalline solid used in perfume or preserving biological specimens or in embalming or medically as a fungicide or antiseptic