3.3 Summary of Section 3
For the first week after fertilization, the conceptus (early embryo) foats freely in the female reproductive tract, obtaining some of its nutrients from the fuid in which it is bathed.
The fertilized egg begins a series of divisions to give 2, then 4, then 8 undifferentiated cells. There is no net cell growth, so each generation of cells is smaller than the last.
The 8- to 16-cell division is different, and important. Each cell divides asymmetrically, to yield one large, outside cell and one small, inside cell.
The two populations of cells are different from each other, and cannot substitute for each other in development. They are differentially adhesive, and express different subsets of genes.
The large, outside cells give rise to the trophoblast, which will make the placenta. The small, inside cells make the inner cell mass, which will form the embryo and associated membranes.