Any cell, if separated, can develop into a full organism. This is the stage of multiplication. At the moment of fertilization, the cell membrane becomes depolarized. This process creates a gradient which later controls the differentiation of cells. The top side of the blastula eventually develops into the ectoderm and the bottom side develops into the endoderm.
Further development involves more complicated processes such as gastrulation.
More information on embryogenesis can be found at the Frog Embryology web page . A similar process of multiplication and differentiation occurred in the evolution of segmentation in invertebrates. Further evolution of segmented worms led to the specialization of segments. Arthropods originated from worms by further specialization of segments. The head developed from several segments that are hard to distinguish.