In most mammals, embryonic development occurs within the mother's body, within a muscular organ, the uterus.
Except for the egg-laying mammals (platypus and echidna), all the others form the placenta, organ consisting of the vascularized internal wall of the uterus (endometrium) and structures derived from the embryonic trophoblast or trophoderm (in mammals, name given to the outermost lining chamber of the embryo). Food, oxygen, antibodies, and hormones pass from the maternal to the embryonic blood through the placenta, which in turn transfers the excreta and carbon dioxide to the mother.
In humans, the egg is of the oligolith type and the segmentation (cleavage) is total and equal, thus forming the morula phase. At this stage, the embryo enters the uterine cavity. Within this cavity comes the phase corresponding to the blastula, which in mammals is called the blastocyst. At this stage, the embryo is endowed with an outer layer of cells, the trophoblast, which surrounds an inner cell cluster, the internal cell mass. It is up to this cell mass the formation of the embryo's body, while the trophoblast It will be responsible for penetrating the embryo into the endometrium (the inner layer of the uterine wall), and for organizing the embryonic part of the placenta.
In the human embryo the trophoblast and the extra-embryonic mesoderm form the chorium. This double lining is responsible for the organization of chorionic villi, which invade the uterine endometrium; the blastocyst then deepens into this endometrium. As the invasion proceeds, the endometrium vessels and glands may be eroded by embryonic enzymes and maternal blood ends up gushing into the gaps that are forming. These gaps provide the embryo with initial nutrition and oxygen. However, maternal and embryonic blood do not mix. There is a barrier separating them, consisting of the wall of the villi.
As can be seen, the placenta is constructed with the participation of maternal and embryonic tissues. Contrary to what you might think, the placenta does not involve the embryo. This function is performed by the amnion (water bag), into which the embryo is immersed. This attachment is very developed in mammals. Chorium adheres to amnion and both bypass the amniotic cavity, filled with amniotic fluid.
In placental mammals, the yolk sac and allantoid are small in size and no longer perform the function performed in birds and reptiles. They contribute, however, to the formation of the umbilical cord, a kind of peduncle that connects the placenta to the embryo and is lined by the membrane of the amnion, which lines the regressed vitelline sac and allantoid. Inside the umbilical cord, two arteries carry blood from the embryo to the mother, while a vein carries blood in the opposite direction.
The Three Consequences of Fertilization
The first consequence of fertilization is the restoration of diploidy. The sperm is haploid and the egg too. Thus, the mixture of the chromosomal lots of both forms a diploid cell, the egg cell or zygote.
The second consequence is sex determination, a particularly important occurrence in mammals.
The third consequence of fertilization is that it triggers a series of events that will allow the development of the zygote in a future embryo. It is in the midst of this whole process, which has already been explained earlier, that twins formation, as we will see below.