The placenta is formed from the chorion and allantois of the embryo and from the mother's endometrium. Its main function is to allow the exchange of substances between the foetus and the mother's body.So, option 'A' is correct.

Mammalian placenta originates from Allantois and chorion.

Placenta is a specialized organ that connects the developing fetus to the uterine wall of the mother. It facilitates the exchange of nutrients, gases, and waste products between the fetal and maternal circulations. The formation of placenta in mammals involves several embryonic membranes, including the allantois and the chorion.

Allantois:

The allantois is a sac-like structure that arises from the hindgut of the embryo. It initially serves as a repository for the waste products produced by the developing embryo. Later, it fuses with the chorion to form the chorioallantoic membrane, which surrounds the developing fetus.

Chorion:

The chorion is the outermost membrane that encloses the embryo and other embryonic membranes. It is formed by the fusion of the trophoblast and the extraembryonic mesoderm. The chorion plays a crucial role in the formation of the placenta by contributing to the formation of the fetal part of the placenta.

Formation of Placenta:

The formation of placenta involves the invasion of the trophoblast cells into the maternal endometrium. These cells differentiate into two types: syncytiotrophoblasts and cytotrophoblasts. The syncytiotrophoblasts form the outer layer of the placenta, while the cytotrophoblasts form the inner layer.

The chorioallantoic membrane, which is formed by the fusion of the allantois and the chorion, contributes to the formation of the fetal part of the placenta. The fetal blood vessels that develop within the chorioallantoic membrane penetrate the placental villi, which are the finger-like projections of the placenta that extend into the maternal blood sinuses.

The maternal blood sinuses are lined by the endometrial cells, which form the maternal part of the placenta. The exchange of nutrients, gases, and waste products between the fetal and maternal circulations occurs across the placental villi.

Conclusion:

In conclusion, the mammalian placenta originates from the fusion of the allantois and the chorion. The chorioallantoic membrane contributes to the formation of the fetal part of the placenta, while the endometrial cells form the maternal part. The placenta facilitates the exchange of nutrients, gases, and waste products between the fetal and maternal circulations, ensuring the proper growth and development of the fetus.