Paradigm shifts in neural induction.

SUMMARY. — The molecularization of developmental biology was originally seen as a challenge to the integrity of that discipline. However, important new insights from the analysis of gene expression soon transformed the field from one of experimental anatomy to one of developmental genetics. One of the main areas to be transformed from an anatomical to a molecular study was « primary embryonic induction ». The molecular analyses showed that some of the fundamental concepts concluded from the experimental embryological approach to primary embryonic induction were false. First, the neural fate of cells was not being induced. Rather, the epidermal fate was induced and the neural state was the default, uninduced, fate of ectodermal tissues. Second, primary embryonic induction was not something unique to vertebrates. Rather, the ventral neural cord of insects formed using the same mechanisms as the dorsal neural tube of vertebrates. Third, the brain formed in a matter distinctly different from that of the spinal cord. Despite these differences, there has been a clear and strong continuity between the experimental embryological tradition and the molecular genetic tradition, and these new results are seen by many contemporary developmental geneticists as strengthening, rather than destroying, the older