492 PATTERNS AND PROBLEMS OF DEVELOPMENT 



erated tissue (Schotte, 1940). Unless Nile blue has spread from cells of 

 the implant to those of the host and host cells are involved in the regenera- 

 tion, it appears that the regenerating flank ectoderm from a hatched larva 

 is capable of developing lens under inductive action of the eye. Potency 

 for lens formation, apparently lost in the course of development, may re- 

 appear in cells undergoing reconstitution. This seems to be a case of de- 

 differentiation and indicates clearly the relative character of so-called 

 "determination." 



THE QUESTION OF SPECIFICITY IN AMPfflBIAN LENS INDUCTION 



In amphibian lens induction, as in neural induction, there is no tissue-, 

 organ-, or species-specificity. Nonlentogenous epidermis of Bujo vulgaris 

 transplanted over the optic cup of R. esculenta develops lens. Hetero- 

 plastic transplantations between Triton species and between Triton and 

 axolotl also give positive results (Mangold, 1929c; 1931a, p. 277). In 

 heteroplastic transplantations between Triton species with lenses of dif- 

 ferent size the induced heteroplastic lens maintains, in general, donor 

 size, though it may sometimes be smaller in consequence of inhibiting con- 

 ditions. The optic cup undergoes more or less adjustment to lens size, 

 apparently through influence of the lens on its growth (Rotmann, 1939). 



Various tissues, living or dead, can induce lens — for example, boiled 

 posterior neural plate, entodermal cells killed by alcohol, fresh liver, and 

 boiled heart of salamander (Holtfreter, 1934a, h). In Triturns pyrrhogastcr 

 nasal primordia, otic vesicle, brain, heart, liver, neural plate, dorsal ar- 

 chenteric wall, ectoderm, mesoderm, and entoderm of head region im- 

 planted in place of optic primordium induce lens in the presumptive lens 

 epithelium (Okada and Mikami, 1937). The optic cup can induce other 

 organs than lens. Implanted below the skin of regenerating tails of large 

 frog tadpoles, it may induce lens, also olfactory parts, otic vesicle, and 

 mouth cavity (Schotte, 1937). In this case the optic cup apparently in- 

 duces something resembling the head region in the regenerating tissue; 

 doubtless the high level of activity and the slight degree of differentiation 

 in the regenerating tissue are concerned in the result. 



A chemical substance produced by the optic cup or other tissue has 

 commonly been supposed to be the lens-inductor, but the positive results 

 with transplantations between species and genera and lens induction by 

 various tissues, living and dead, raise the same questions as regards speci- 

 ficity of the inducing factor that arose concerning neural induction. To 

 what extent activity-level of epidermis or attainment of a certain degree 



