organisers: inducers of differentiation 189 



proliferative effect may, as we have seen above, be more or less 

 independent, the subsequent differentiation of lens-fibres appears 

 to be always dependent, usually on the eye-cup. But the action of 

 the latter in this case does not appear to be specific, for experiments 

 in which lens-rudiments are allowed to develop in proximity with 

 portions of brain or nose tissue show that the latter are also capable 

 of inducing the formation of lens fibres.^ 



Recent work on the American bull-frog, Rana cateshiana^ has 

 given additional results. This species shows an extreme of depen- 

 dent differentiation for the lens, rivalling or exceeding Rana tem- 

 poraria in this respect. Of greatest interest is the fact that here 

 the continued presence of the optic vesicle or eye is necessary for 

 the lens to achieve full differentiation and full size, even after it 

 has been initially determined. The lens-rudiment, however, once 

 determined, has a certain power of self-differentiation. After 

 determination but before visible differentiation, the lens-rudiment, 

 by itself, is only capable of producing lentoid structures without 

 differentiation of fibres. After visible thickening has occurred, 

 however, the rudiment left in situ after removal of the underlying 

 eye-cup will produce a true lens, but this is small and slightly 

 abnormal. There is thus a complementary action of inherent 

 potencies and external induction (see p. 264 for what may be a 

 similar effect with the avian gonad). Another interesting point is 

 that if the visible lens-rudiment at the same stage is separated from 

 the eye-cup and grafted heterotopically, it undergoes a certain 

 amount of regression and never reaches the same degree of dif- 

 ferentiation as if left in situ, though in both cases it is removed from 

 the inductive influence of the eye. Thus in this species, although 

 epidermis from any region can be made to form a lens, potencies 

 favourable to lens-differentiation are highest in the area of the 

 normal lens-field. 



The crystalline fibres of the lens in Amphibia are oriented in a 

 definite manner, normally converging to a sutural line which is 

 dorso-ventral on the outer surface, and antero-posterior on the 

 inner surface of the lens. It is to be noted that the plane of the ex- 

 ternal sutural line coincides with that plane of the eye-cup in which 

 the choroid fissure is situated, for this structure occupies the most 



^ Balinsky, 1930. ^ Pasquini, 1933. 



