Historical evolution of preformistic versus neoformistic (epigenetic) thinking in embryology

In the classical embryology there exist two main concepts to explain the rising of a living being. At one hand there exists the theory of preformation i.e. all the parts of the future embryo would already exist preformed during the preembryonic or early embryonic period. On the other hand there is the epigenetic view in which it is propounded that all parts arise by neoformation from interacting (induction) previously existing, apparently simpler structures. We found evidence that initially a kind of preformation (or pre-existence), disposed in concentric circular layers, exists in the full grown oocyte and early germ. After normal development these circular layers will settle successively, from the centrum to the periphery, into the central nervous system, the notochord, somites, lateral plates (coelom) definitive endoderm. Under influence of mechanical and gravitational forces an early epigenetic development starts by unequal oblique uptake (unidirectional chaos) and segregation of ooplasmic determinants in the germ. So an epigenetic cascade of early general body plan formation occurs in the vertebrate embryo. Recently, we demonstrated by hemi-sectioning of avian unincubated blastoderms that both a kind of preformational (mosaicism) as epigenetic (regulation) behaviour depends on the spatial, ooplasmic distribution of Rauber's sickle material, homologous to the also sickle-shaped Wnt expressing gene region in ascidians. This clearly brings preformation and epigenesis much closer together since there is a common influencing ooplasmic factor. An ooplasmic continuity bridging the premature oocyte ooplasm to the embryonic primordial germ cells of the following generation is demonstrated. Both nuclear and ooplasmic continuity in the oocytes, present in the ovary of birds and mammals, is shown by radioactive premeiotic DNA labelling.