Fig. 1

Early embryology. Day 0: The zygote results from the fusion of the spermatozoon and the oocyte II; Week 1: Cell division without volume expansion takes place during the 1st week forming a structure which resembles a mulberry—the morula. It will originate not only the embryo but also the membranes, placenta, and associated structures. Intense cell division and reorganization will then transform the morula into a single-layered hollow sphere—the blastula; Week 3: Left: Some blastomers move to a more interior location, forming a three-layered central disk during a process called gastrulation; right: craniocaudal polarity becomes apparent in the gastrula stage [14]