The subclass Prototheria contains the egg-laying mammals, which are the most ancestral forms in the class Mammalia.

In a cotyledonary placenta, the villi are found in groups or patches, while the rest of the chorion surface is smooth. In sheep and deer, cotyledonary placenta is found.

In such case, not only there is a loss of maternal tissue but also of the fetal portion of the placenta, both of which are absorbed in the situ by maternal leucocytes.

Chorion is the outermost of the two fetal membranes (the amnion is the innermost) that surround the embryo. The chorion develops villi. Yolk sac placenta develops both from yolk sac and chorion.

Here the placenta consists of two disc-shaped villous areas.

Hippopotamuses and pigs have an epitheliochorial placenta. Here a layer of fetal tissue is merely pressed close against the uterus wall.

The placenta (also known as afterbirth) is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gaseous exchange.

Placentitis is the infection of placenta by bacteria that migrate along the reproductive tract.

Relaxin is a hormone produced by the ovary and the placenta with important effects in the female reproductive system and during pregnancy.

Implantation is the very early stage of pregnancy at which the embryo adheres to the wall of uterus.

Here the chorionic sac penetrates into the substance of the uterine lining.

The umbilical cord develops from and contains remnants of the yolk sac and allantois.

It is absent in syndesmochorial, epitheliochorial and endotheliochorial type of placenta.

These exhibit placentation. Placentation refers to the formation, type and structure, or arrangement of placentas.

It is not true as the placenta is impermeable to bacteria and macromolecules. Therefore, it reduces the chances of appearance of maternal ill health on the fetus.