organisers: inducers of differentiation 187 



in the induction of a lens.^ Thus at this stage, the lens of Rana 

 esciilenta is self-differentiating, but the eye-cup also possesses the 

 inductive power of forming a lens, so that there is here another 

 example of the principle of "double assurance." 



This state of affairs can be interpreted as follows. In Rana 

 esculenta, the lens is already determined irrevocably at a stage (early 

 tail-bud) when in Rana tenipovaria it is usually still plastic. We 

 may conjecture, therefore, that the determination of the lens occurs 

 precociously in Rana esciilenta. In this form, the lens is presumably 

 determined by the presumptive eye-rudiment while this is still 

 an invisibly determined region of the neural plate. 



We may also assume, however, that, just as with the presumptive 

 neural plate before gastrulation, there has been a preliminary labile 

 determination of the lens, so that the lens-forming potencies will 

 be more easily called forth at a certain spot, viz. the presumptive 

 lens region." When definitive determination occurs, we must as- 

 sume that some influence, presumably of a chemical nature, diffuses 

 from the eye-area, and affects the region of optimum lens-forming 

 potency. In a similar way (as will be seen in Chap, vii, p. 223) we 

 may note the limb is actually formed at a region of maximum limb- 

 forming potency, in a much more extensive potential limb-area. 



However, when the eye-rudiment of Rana esciilenta has become 

 converted into an optic cup, it still retains its lens-inducing power. 

 Indeed, it would seem that in some forms this power is retained 

 throughout life, for in many Urodeles it has been shown that the 

 adult eye can regenerate a new lens from its own margin if the lens 

 has been removed^ (see p. 237). It is interesting in this connexion 

 to note that the eye can resort to this method of lens-formation in 

 embryonic development and form a lens from its own margin if it 

 is deprived of contact with epidermis"^ (fig. 90). 



The apparent "double assurance" found in Rana esciilenta thus 

 apparently means {a) that there exists a region of optimum lens- 

 forming potency in the epidermis of the neurula, and {b) that the 

 power of the eye-cup to induce a lens persists after the lens has 

 differentiated, and after the remaining epidermis has been deter- 

 mined to form epidermis and has ceased to be capable of responding 



^ Filatow, 1925. - Spemann, 1912 b, ^ Colucci, 1891 ; G.Wolff, 1895. 

 * Spemann, 1905; Beckwith, 1927; Adelmann, 1928. 



