Received May 05, 1997
Accepted September 17, 1997

The authors describe in details the initial embryo development, over the first days before implantation. Initially, with 'in vivo" and "in vitro" fertilization, through cleavage stages, morula formation and blastocyst stage. Describe, in details, the polarization process reached by each blastomere with in the embryo,compaction of the blastomeres and the first cell differentiation in trophectoderm cells and inner cell mass cells and also its cell number in both groups.