Epigenesis means to grow upon, as is opposed to preformation.
One of the most important issues in the premodern biology of the 18th century was the struggle between preformationist and epigenetic theories of development. The preformationist view was that the adult organism was contained, already formed in miniature, in the sperm, and that development was the growth and solidification of this miniature being. Preformationists assumed that the germs of all living beings were preformed and had been since the Creation. Preformationism sought to maintain and secure--against the irritation posed by the complexity of organic phenomena--the claim for a thorough and rational determination of the material world.
The Deistic theory of preformism assumed that God had determined the organizational forms of all organisms at the creation of the world. After that, the mechanical laws of matter did the rest. Thus preformism could be a mechanistic theory in which the divine watchmaker conceives a plan of the object which guides its production. (This is the Aristotelian causa formalis ) According to Descartes, "if one knew in detail all the parts of the seed of a particular species of animal, for instance, Man, one could deduce from that alone for reasons entirely mathematical and certain, the whole figure and conformation of each of its parts."
For preformationists, the egg, or rather the germ, supposedly housed a homunculus, a tiny version of the adult, each part of which expanded into the corresponding part of the adult. The main theories of how germs were stored were panspermism, in which the germs were disseminated everywhere (advocated by Claude Perrault and Charles Bonnet) and the emboitement or encasement of homunculi in the sexual organs, either in the female --ovism -- (Malebranche) or in the male -- animalculism. (Leibniz) Charles Bonnet described this encapsulation as mise en abime.