The unfertilized egg is divided into two broad hemispheres, giving it an intrinsic asymmetry. The site of sperm entry defines the ventral side of the embryo and leads to rotation of the cortical cytoplasm. Fertilization leads to rapid cell divisions and formation of the early blastula (not shown), a hollow ball of 32 cells called blastomeres. Signals from the vegetal pole induce the formation of mesodermal cells in the marginal zone (blue) separating the vegetal and animal caps in the midblastula stage. During gastrulation, mesodermal cells fold into the embryo. Signals from the invaginating mesoderm induce the development of both the underlying endoderm and neural tissue from the overlying ectoderm. The anterioposterior axis is determined by the mesoderm; cells that invaginate first induce anterior structures. Subsequent interactions between different cell populations play an important role in organogenesis. [Adapted from E. M. De Robertis et al., Sci. Am. 263(1):46.]