As we have explained throughout the article, there are several key stages during the growth of the embryo until its implantation. First, immediately after fertilization, we find the zygote (a single cell that results from the union of the genetic endowment of the egg with that of the sperm).
The embryo is then divided into 2 cells, then into 4, then into 8... and so on until the so-called morula is formed (day 4). This is a grouping of about 32 cells that begin to compact and, therefore, we do not clearly appreciate their limits.
This conglomerate of cells will compact completely and give rise to the blastocyst, in which we distinguish three clear parts: the cells of the trophectoderm or trophoblast, the cells of the inner-cell mass and the blastocoel or central cavity.