Preformation theory

A term in biological development, it is opposed to the epigenetic theory. It refers to the idea that in individual development, the fertilized egg contains a reduced form of the organs of the adult, which develop as the embryo develops. With the development of microscopes from the latter half of the 19th century, various structures were observed in early embryos, which were previously thought to be structureless, and the preformation theory was strongly advocated. Major proponents of the preformation theory include Malpighi, Swammerdam, and Bonnet. Within the preformation theory, there was a conflict between the spermatogonial theory, which states that the adult offspring are nested one after another in the sperm, and the oogonial theory, which states that offspring are present in the egg. However, the discovery of the ovarian follicle and parthenogenesis gave the oogonial theory the upper hand. However, from the mid-18th century to the first half of the 19th century, developmental biology research made progress, and many observations and experiments were made in favor of the epigenetic theory, such as the discovery of germ layers, and the preformation theory gradually disappeared.

[Sadao Yasugi]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

生物発生学の用語で、後成説に対立する。個体発生において、受精卵中に成体の諸器官が縮小された形で備わっていて、それが発生とともに展開してくるという考え方をいう。19世紀後半からの顕微鏡の発達によって、それまで無構造であると考えられていた初期胚(はい)に種々の構造が認められるようになって、前成説は強く主張された。おもな前成説論者はマルピーギ、スワンメルダム、ボネなどである。前成説のなかでは、子孫の世代の成体まで精子中に次々に入れ子になって存在するという精原説と、卵子内にあるという卵原説が対立していたが、卵胞の発見や単為生殖の発見によって卵原説が有利になった。しかし18世紀中葉から19世紀前半にかけて、発生学の研究が進歩をとげ、胚葉の発見など後成説に有利な観察や実験が数多くなされて、前成説はしだいに消滅していった。

［八杉貞雄］

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