PREFORMATION AND EPIGENESIS
THE problem of the transmission of form in animal and plant species from generation to generation, and consequently of the evolution of the embryo, has puzzled observers throughout the ages. In the fourth century B.C. Aristotle had already contrived experiments relating to the development of the hen's egg. Until the discovery of the sex elements, however, such experiments could not be said to have been of a scientific nature, that is to say until 1667 when Leeuwenhoek discovered spermatozoa.
Spallanzini in 1754 and Prevost and Dumas some time later in 1824 showed the necessity of having spermatozoa in the fertilization process, since on filtering the sperm secretion of a frog prior to mixing it with the egg-cells, the latter failed to develop. Not until 1875 did Hermann Fol and Oscar Hertwig, working independently of each other, describe the penetration of sperm into the egg-cell and the formation of the zygote nucleus, the egg, by combination of nuclear chromatin from such sperm with that of the egg-cell nucleus.
The male and female sex cells must possess, therefore, organogenetic properties, since their progressive differentiation and increase gives place to the evolution of the embryo. The most direct theory could not help but be the assumption that in such cells there must be material or potential representations of every part of the organism. Hartsoeker in 1694, for instance, gave a sketch of a large-headed mannikin dwelling in the sperm chromatin, growing up to become the foetus and feeding on the egg substance at the beginning and then on sustenance directly supplied by the mother. Other writers of the eighteenth century held that a microscopic ready-formed being was to be found in the egg-cell whilst the sperm must be some sort of parasite absolutely superfluous as far as reproduction went.
The theory of preformation has had its partisans in all ages: Swammerdam, Bonnet, Haller, Malpighi and Buffon in earlier times and Baer, Remak, Kölliker, Haeckel and others in more recent centuries, and still closer to our own times Wilhelm Roux with his