EMBRYONIC INDUCTORS AND ORGANIZERS 491 



optic cup by lens formation is less limited in those species in which the 

 presumptive lens region is not already determined, and vice versa. Von 

 Ubisch, on the other hand, maintains that all amphibian species are es- 

 sentially alike at corresponding developmental stages, as regards lens de- 

 termination/" It seems that final conclusions concerning this point must 

 await further investigation. Inhibiting effects of the operative procedures 

 on transplanted epidermis or optic primordium or on the host tissues, 

 rate of healing, and various other factors may differ with different ma- 

 terial and in different experiments. Since reactivity of the epidermis to 

 the inductor decreases and becomes regionally more limited as develop- 

 ment progresses, slight difference in stages used by different experimenters 

 may be responsible for some of the differences in results. Inductive in- 

 tensity of optic cup may differ in different species; in some it may be 

 sufficient to induce lens formation only in more anterior levels, in others, 

 from any level. Epidermal reactivity also appears to differ regionally, and 

 the anteroposterior differential may be greater in some species than in 

 others. It is generally agreed that the optic cup influences the lens after 

 the original inducing action. The lens primordium, or the lens after fiber 

 development has begun, may develop somewhat further after removal of 

 the optic cup, when placed in the blastocoel of a gastrula or transplanted 

 to other regions; but sooner or later its structural pattern is lost and it 

 degenerates.^' 



Recent experiments have placed the whole question of lens potency 

 and its restriction in a new light. Optic primordia implanted in regenerat- 

 ing tissue of tadpole or adult urodele tails can induce lens from the re- 

 generating cells. Regenerating leg tissue of urodele or regenerating tail 

 tissue of urodele or anuran tadpole transplanted to the lensless eye of an 

 adult urodele or a large anuran tadpole can give rise to a lens."*^ 



In other experiments a Nile blue stained ectoderm from the side of a 

 larva is transplanted to the eye of a larva from which all corneal and 

 orbital epidermis has been removed. Later implanted epidermis is removed 

 from over the eye, permitting regeneration from the Nile blue stained 

 edges of the implant, and lenses or lentoids may develop from the regen- 



"t" For a discussion of this point see O. Mangold, 1931a, pp. 288-89. Spemann, 1936, pp. 

 24-57, and 1938, chap, iii, discusses lens induction at length with numerous figures and refer- 

 ences. 



•" Le Cron, 1907, Amhlystoma; Fischel, 1917, Salamandra; Filatow, 1925c, Triton; Kriiger, 

 1930, Triton. 



-•^ Schotte, 1937, 1938; Schotte and Hummel, 1939. 



