A reinvestigation of some of the tissue movements involved in the formation of the neural tube and the eye/lens system of Triturus alpestris and Xenopus laevis.

Since the beginning of the century the generally accepted scheme of eye/lens development has been that proposed by Spemann (1901) and later confirmed by numerous workers. According to this scheme the two presumptive components of the definitive eye, the optic cup and the lens, are spatially separated at the flat neural plate stage. They later come into apposition as a result of tissue movements which occur during the formation of the neural tube; the optic vesicle then provides an inductive stimulus for the subsequent development of the presumptive lens tissue. Spemann's suggestions concerning the tissue movements involved in the early formation of the eye/lens system do not appear to have been fundamentally questioned until the publication of a number of papers by Chanturishvili (1943, 1949,1958,1959,1962), although the theory of lens induction has been modified by workers such as Liedke (1955), Jacobson (1963) and von Woellwarth (1962). Lopashov & Stroeva (1961) have reviewed the literature on eye/lens development and their paper should be consulted for further references. Chanturishvili denies the main tenet of the ' classical' scheme, namely the induction of a lens by the eyecup out of the ectoderm with which it comes into contact during the closure of the neural folds. Instead, evidence is presented (Chanturishvili, 1949,1958,1959) that the presumptive eyecup anlage is from the first continuous with the presumptive lens anlage within the confines of the flat neural plate and that later these two become separated by a split within the neural tissue itself. This split is subsequently invaded by mesoderm. Thus, according to Chanturishvili, both the optic vesicle and the lens are neural in origin. Chanturishvili further suggests that this neural lens is induced by the products of cytolysis of epidermal cells which are trapped within the newly formed lens vesicle and not by the optic vesicle as stated in the classical scheme. 1 Author's address: The Department of Microbiology, The University, Birmingham 15, U.K.