File:Ewart1897 02.jpg

Fig. 2. Represents a young opossum and its foetal appendages

The wall of the yolk sac (y.s.) is vascular as far as the circular blood vessel (s.t., sinus terminalis). In the area a, b, c, the yolk sac blends with the outer embryonic sac. Through this area the cells (trophoblastic) of the outer sac are modified so as to assist in taking up nourishment— the uterine milk — from the uterus, and in fixing the embryo during its uterine development. The allantois (all.) never reaches the outer sac. It is vascular, and serves only as a breathing organ.

After Osborn and Selenka.

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Reference

Ewart, J.C. A Critical Period in the Development of the Horse. London: Adam and Charles Black (1897).