Allantoic Placenta || Primitive allantoic placenta

 Allantoic Placenta

Allantoic placenta is met with a number of different groups of mammals but in most marsupials a primitive type of this placenta is present and so primitive allantoic placenta may be described under a separate heading.

Primitive allantoic placenta

        This type of placenta is simpler placenta developed from allantois and leads to the complex placenta of higher mammals. In Bandicoot (Perameles) and the Wallaby (Hamaturus) such placenta is present.

        As the developmental study reveals, in this case too a yolk sac develops into a large structure provided with a network of blood vessels and a sinus terminalis. The allantois also grows comparatively large and establishes a contact with the chorion. The chorion is attached to the uterine mucosa on that side where it is in connection with the allantois. The uterine mucosa encloses the blastodermic vesicle which is provided with the number of blood vessels. The blastocyst loses its chorion more or less in this region, thus enabling the allantoic vascular net to establish a contact with the maternal blood. For instance, it may be recalled that maternal blood and the foetal blood are never in direct contact. The exchange of gases, food supply and removal of nitrogenous wastes are made through membranes.

         The exchange will be accelerated if there are fewer membranes. And so, this primitive allantoic placenta that allantois applied to uterine wall here we find it is simpler, flat and with many wrinkles or folds. Moreover, the trophoblast of this region has also disappeared and thus an easy communication between allantoic blood vessels and maternal blood vessels is established. However, the blastodermic vesicle still depends greatly on the "uterine milk" secreted by uterine wall for its food supply. Some food, no doubt, is also absorbed in the region where there is contact between maternal and allantoic blood vessels. The absorbed food is passed into the embryo by the allantoic vascular circulation. Although the blastocyst is not firmly held in the pit, yet it is always there but it may be dislodged easily.

         This shows that true allantoic placenta may be developed by firm adhesion of the blastocyst to the uterine wall and the foundation of which has already been laid here in primitive allantoic placenta.