From embryonic induction to cell lineages: revisiting old problems for modern study

A history of embryological studies of lens development and regeneration is sketched, paying special attention to the contribution of these studies to the conceptual aspect of embryology and later developmental biology. Emphasis is made on the fact that the interaction of different tissues during development, namely embryonic induction, was first discovered during studies of the lens and that the degree of the dependence of lens development on the inductive effect of the eye-cup varies among different species. Studies along the line of comparative-experimental embryology at the species level should be informative, in particular in combination with the recent trend of evo-devo studies. The processes of lens regeneration and in vitro transdifferentiation indicate the existence of multiple cell lineages with the potential of lens development in one animal. The occurrence of Wolffian lens regeneration in nature can be reconsidered from the new "eco-devo" viewpoint.