i86 organisers: inducers of differentiation 



Bufo, Triton, and the chicks agree with Rana temporaria in the 

 conditions of formation of the lens (see fig. 21, p. 55). In the 

 chick, the interesting observation has been made that the optic 

 vesicle, as well as the optic cup, is capable of inducing lens-forma- 

 tion.^ This means that the degree of histological differentiation of 

 the eye is immaterial for the inductive effect. 



In Rana esculenta, however, removal of the eye-rudiment, even 

 at the early neural fold stage, does not prevent the formation of a 

 lens, which latter structure is therefore self-differentiating in this 

 species at a stage even earlier than that at which it is dependent- 

 differentiating m Rana temporaria.^ The lens, however, is sometimes 

 subnormal in size. Bomhinator pachypus is intermediate between 

 the two species of Rana in this respect, for after removal of the 

 eye-cup a small lens-like structure develops. This occasionally 

 happens in Rana temporaria^ (figs. 88, 89). 



Although this experiment shows that the lens of Rana esculenta 

 is self-differentiating, it gives no information concerning the power 

 of the eye-cup of this species to induce the formation of a lens by 

 dependent differentiation. This can be tested by grafting foreign 

 epidermis of the same species, from various regions of the body, 

 over the eye-cup. The resuhs obtained differ according to the age 

 of the epidermis used. At the late tail-bud stage in Rana temporaria 

 and in Hyla arborea, epidermis from any region is capable of forming 

 a lens when in contact with an eye-cup, while in Bomhinator, lens- 

 forming potencies are restricted to the epidermis of the head. In 

 Rana esculenta at the late tail-bud stage, no epidermis other than 

 that of the presumptive lens region can be made to form a lens,^ 

 though at the early tail-bud stage, epidermis from any other regions 

 can do so.^ 



The inducing power of the eye-cup of Rana esculenta may be 

 further tested by grafting over it some epidermis from another 

 species, in which the lens is normally dependent in its differentia- 

 tion, such as Bufo vulgaris, and such experiments invariably result 



1 Danchakoff, 1924. - Hoadley, 1926 b. 



"^ It appears that the lens in Rana esculeiita is not invariably self-differentiating, 

 especially at low temperatures. Further experiments on the modifiability of lens 

 induction and lens differentiation in different species are much to be desired. See 

 von Ubisch, 1924. 



4 von Ubisch, 1927. ^ Spemann, 1912B. ^ von Ubisch, 1927. 



