n. [ Gr. &unr_; new + E. genesis. ] (Biol.) The introduction during embryonic development of characters or structure not present in the earlier evolutionary history of the strain or species (as addition of the placenta in mammalian evolution); a modified evolution, in which nonprimitive characters make their appearance in consequence of a secondary adaptation of the embryo to the peculiar conditions of its environment; -- distinguished from palingenesis. [ Also spelled caenogenesis and formerly kenogenesis. ] [ WordNet 1.5 + 1913 Webster ]