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A fertilized egg undergoes cleavage to become multicellular, during which differentiation takes place among cells and various tissues and organs are formed. This process is called embryonic development.

Concerning the mechanism of embryonic development, there were once two different ideas. One is called preformation, according to which the egg or sperm contains a completely formed miniature embryo that simply grows during its development. Another is called epigenesis, in which the form of the embryo emerges gradually from a relatively formless egg.

In 1881, the German embryologist Wilhelm Roux killed one of the blastomeres of frog embryo at the two-cell stage with a hot needle, and later obtained a half-embryo with one side of the body formed. As seed in this experiment, eggs in which the fate of parts of the embryo is already determined are called mosaic eggs. On the other hand, in 1891 another German embryologist, Hans Driesch, vertically split a sea urchin embryo at the eight-cell stage, which resulted in two normal larvae, as seen in this experiment, eggs in which parts of the embryo have the potential to form a complete individual are called regulative eggs.

At present, these two ideas are thought not to be incompatible. The difference between the two types of egg is attributable to differences in time when the fate of the part of the embryo is determined.