Allantoic Placenta || Advanced 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.

Advanced allantoic placenta

        Advanced allantoic placenta may be conveniently studied under two main divisions, i.e., the deciduate placenta and the indeciduate placenta. The deciduate placenta is developed in human beings, and other primates such as the insectivores, chiropterans, hedgehogs, shrews, mice and rabbits. While the indeciduate placenta is present in remaining mammals.

        In deciduate placenta the union between chorion and the uterine mucosa or the endometrium is much more intimate. In this case, the superficial layer of the uterus grows extensively over the blastodermic vesicle and is pushed away from the uterine wall. This structure is now called decidua. The decidua is so overgrown that it envelops the entire embryo with its foetal membranes. The chorionic villi which are richly branched arise from the decidua like root system of a tree. If these villi are detached, they tear some part of decidua. This type of placenta is present in man. While the indeciduate placenta is more simpler than that described earlier. In this case, the chorion is simple one with wrinkles and projections like villi. These villi establish a contact with uterine mucosa, which is not very well developed but rather loose. Thus at the time of birth, maternal tissues are not torn from the uterine mucosa. The chorion is rolled off from the uterine wall so that the villi are pulled out of the uterine mucosa or endometrium in the same way as the are pulled out of a glove.