Since the enunciation by Harvey of the aphorism Omne vivum ex ovo in the seventeenth century, the statement has frequently been made that every animal begins its individual existence as an egg. While this is not strictly true, since no eggs occur in the life history of many one-celled animals (PRO TOZOA), and a large number of multicellular animals (Metazoa) are known to develop from buds or by fission, still the majority of animals arise from a single cell the egg (Fig. 4, A). In most cases this egg, or female sex-cell, is unable to develop in nature unless it is penetrated by a spermatozoon or male sex cell (Fig. 4, B). The single cell resulting from the fusion of an egg and a spermatozoon is known as a zygote. One of the most remarkable of all phenomena is the development of a large, complex organism from a minute, and apparently simple, zygote.
According to the older scientists, a miniature of the adult individual was present in the egg, and development consisted in the growth and expansion of rudiments already preformed. This belief could not continue to exist after Caspar Wolff's brilliant researches proved that adult structures arise grad ually from apparently undifferentiated material ; that is, development is epigenetic. Epigenesis, however, does not explain development ; it simply maintains that it occurs.
During the years since the theory of epigenesis was proposed a new theory of preformation has entered into our conception of development, a theory which we may designate as predetermination. We know from our microscopical studies that the germ cells possess a certain amount of organization, and that the zygote contains certain structures contributed by the egg and other structures brought into the egg by the spermatozoon. Hence, to a certain extent, development is predetermined, since the initial structure of the zygote determines the characteristics of the individual that arises from it. On the other hand, development is also epigenetic, and our modern conception includes certain features of each theory.