The umbilical cord is a set of blood vessels that connect the fetus with the placenta. In the fetus, one extremity of the cord inserts into the center of the abdominal wall (the later scar of this insertion is the umbilicus).
The function of the umbilical cord is to allow the transport of substances, nutrients, gases, and residuals, between the fetus and the mother's body.
The placenta has endocrine function since it secretes the hormones progesterone and estrogens that maintain the endometrium (internal covering of the uterus) and prevent menses during pregnancy. The placenta also secretes other important hormones for pregnancy regulation.
Under normal conditions, there is no passage of cells across the placenta during gestation. The placenta has a smooth mucosa separating the richly vascularized region in contact with the mother's endometrium from the umbilical cord in contact with the fetal blood. This barrier is known as placental barrier. Although permeable to some substances (selective permeability), the placental barrier forbids the passage of cells.
Image Diversity: umbilical cord placental barrier
From the mother to the fetus the main transferred substances through the placenta are water, oxygen, nutrients, and antibodies. From the fetus to the mother, metabolic wastes including urea (nitrogen waste), and carbon dioxide are transferred.
True placenta is present in placental mammals.
The placenta is formed from the chorion of the embryo and from the mother's endometrium. Its main function is to allow the exchange of substances between the fetus and the mother's body.
Image Diversity: placenta placental mammals
The chorioallantois membrane is formed by juxtaposition of some regions of the chorion and the allantois. Since it is porous, the chorioallantois membrane allows the passage of gases between the embryo and the exterior thus making aerobic cellular respiration possible.
The amnion is also an adaptation to dry land since one of its functions is to prevent desiccation of the embryo.
Amnion is the membrane that covers the embryo. Chorion is the membrane that covers the amnion, the yolk sac, and the allantois. The space delimited by the chorion and the amnion is called amniotic cavity and it is filled with aminiotic fluid. The amniotic cavity has the functions of preventing desiccation of the embryo and of protecting it against mechanical shocks.
Image Diversity: amnion chorion
The allantois is an adaptation to dry land because in embryos of oviparous terrestrial beings, like reptiles and birds, the metabolic residuals cannot be immediately excreted to the aquatic surrounds (as fishes and amphibian larvae do). It was necessary then the appearing of a structure capable of storing the embryonic excretes until hatching.
The allantois is the extra embryonic membrane whose function is to store excretes of the embryo.
In placental mammals, the allantois is present but it does not exert that function since the embryonic wastes are collected by the mother's body through the placenta.
Image Diversity: allantois