It is found in the allantoic liquid of the cow, and in the urine of sucking calves.
As a rule there is no allantoic placenta forming the means of communication between the blood of the parent and the foetus, and when such a structure does occur its development is incomplete.
More remarkable still is the development of a small allantoic placenta.
Near affinity; and while more nearly related to the Marsupialia (q.v.), in which an imperfect allantoic placenta is sometimes developed, it is broadly distinguished therefrom by the invariable presence of a functional placenta by the aid of which the foetus is nourished throughout the greater portion of intra-uterine life.