Embryonic development begins with the zygote, which will be considered an embryo from day 2. At this time, the embryo contains 4 cells and day after day they will multiply successively. Once day 5-6 of embryonic development is reached, the embryo will be called a blastocyst.
The blastocyst contains two clearly differentiated structures. On the one hand, there is the inner cell mass responsible for the formation of tissues and organs.
The other structure that forms part of the blastocyst is the trophoblast, from which the placenta and the umbilical cord will derive.