Stages of embryonic development. This image shows the development of a mouse embryo from the moment of fertilization.
The first image (far left) shows a secondary oocyte that has completed meiosis - characterized by the prescence of a polar body (small red cell). Once the egg (ovum) has been fertilized it divides, doubling each time: 2-cell, 4-cell, 8-cell, 16-cell-stage. After multiple divisions the cells compact to form a morula and then start to differentiate forming a blastocyst (far right image).
From the blastocyst stage the cells will go on to form one of two cell fates: they will either become part of the trophectoderm (white cells) that develop into the placenta and supporting cells, alternatively cells of the inner cell mass (large red cells) will go on to form the body of the embryo.