II. THE PERIOD OF ORGAN DEVELOPMENT 



133 



Fig. 49. Independent differentiation of the lens in Rana esculenta. 

 (a) extirpation of the right eye primordium at the neural plate 

 stage; (d) cross section of the resulting larva, 14 days after the 

 operation; the right eye (left in this figure) is absent; nevertheless, 

 a lens (Z) has been formed, which, however, (b) is smaller than that 

 of (c) the normal left eye. After Spemann, 



Some details of this induction process may be mentioned. 

 First of all, it appears that there is a correlation between the 

 size of the induced lens and that of the inducing eye-vesicle. 

 The size of the latter can be reduced or increased experiment- 

 ally, by extirpation of material or fusion of two eye-primordia, 

 respectively. Such modified eye-vesicles always induce a lens 

 which approximately fits the eye-cup (Fig. 50). But this is 

 only true if eye-vesicle and lens ectoderm belong to the same 

 species, and not in the case of heteroplastic combinations. 

 Rotmann (1939) has shown that under the influence of the 

 (larger) eye-vesicle of Triton cristatus the ectoderm of T. 

 taeniatus forms a lens of the normal taeniatus size, and there- 

 fore too small in comparison with the eye. In the reverse 



