Yolk Sac Placenta

 Yolk Sac Placenta

        Yolk sac placenta is confined to small group of marsupials and is met with the very familiar marsupial the Dasyurus. In Dasyurus, the marsupial cat, the blastocyst is formed in usual pattern and is provided with a large yolk sac and a small allantois. As the development proceeds, the chorion is thickened by addition of more cells, thus forming together the branching syncytia. By the erosion of the uterine cells that hold the blatocyst, the blastodermic vesicle sinks into the uterine mucosa forming a shallow depression. 

        The yolk sac is in connection with the chorion. But now the yolk sac with the sinus terminals sinks into the uterine mucosa in the so-formed depression. At the same time, the contact is made more conspicuous with the formation of more folds of the chorion. This helps in increasing the chorionic surface to absorb the "uterine milk" secreted by the endometrium. In addition to this feeding method, the chorion is also associated with the maternal tissues are not torn from the uterine mucosa. The chorion is rolled off from the uterine wall so absorbed by the yolk vesicle and then after circulated in the embryo for the use in the vital activities of the latter.