The Concept of Evolutional Embryonization/Disembryonization of Ontogeneses

The article attempts to analyze and generalize into a single system of views the variety of hypotheses about the evolutionary significance of embryonization and disembryonization of ontogeneses, as well as to evaluate the contribution of these hypotheses to the understanding of the phylogeny of living organisms. In the course of such an analysis, it is demonstrated that the initial (primary) embryonic development was not an adaptation to certain living conditions, but just one of the ways of constructing a multicellular body from the oogamete through its palintomic or syntomic divisions. Secondary embryonization repeatedly occurs on the basis of ancestral ontogeneses, in which the fragmentation of the egg (or spore) leads to the appearance of an independent, but underdeveloped stage, very different from the adult organism; as a result of embryonization, such stages are partly or totally hidden under the egg and/or embryonic shells. Of all the variants of embryonization, only secondary external embryonization (under the shells of the egg, spore, seed, or fruit) in most studied cases gives the impression of a direct evolutional response of a taxon to changing environmental conditions. Complete embryonization of underdeveloped stages of ontogeny (cryptometabolie) is a relatively rare phenomenon and appears to be disadvantageous from a biological diversity point of view. The reverse process—secondary disembryonization and premature (in comparison with ancestral groups) completion of embryogenesis—is even less common and never leads to a complete abandonment of the embryonic mode of development and a return to the archaic protonemal or siphonoseptal modes of multicellularity formation.