In 1952, Nieuwkoop et al. suggested a new hypothesis concerning the induction phenomenon determining the early development of the amphibian embryo. This hypothesis was based on cleverly planned experiments in which folds of competent epidermis were transplanted on to different regions of the neural plate of the neurula. According to this hypothesis, the invaginating archenteron roof is supposed first to activate the overlying ectoderm, enabling it to develop autonomously to archencephalon and its derivatives. This same archenteron roof is later thought to exert a second effect, which they called transformation. This second action is considered responsible for modifying the differentiation tendencies of the activated archencephalon so as to result in structures typical of more caudal regions of the nervous system. This process is regarded as a quantitative one, so that with increasing strength of transformation, the differentiation tendencies would be progressively more caudal.
Early development of the cranial sensory nervous system: from a common field to individual placodes