caenogenesis

introduction during embryonic development of characters or structure not present in the earlier evolutionary history of the strain or species (such as the addition of the placenta in mammalian evolution)

胚発生の過程で、その系統や種の初期の進化史には存在しなかった形質や構造（哺乳類の進化における胎盤の付加など）が導入されること