i88 



organisers: inducers of differentiation 



to induction. There is thus an overlap in time between these two 

 phases of differentiation ordinarily spoken of as dependent differ- 

 entiation and self-differentiation. There seems to be little reason 

 to doubt, however, that both the methods concerned in '* double 

 assurance " are ultimately referable to one and the same causative 

 agent: in this case presumably situated in the rudiment of the 

 eye-cup and in the fully formed cup which later arises from it. 



Fig. 90 



Lens-formation from the margin of the optic cup in ontogenetic development. 

 The presumptive eye-rudiment of an embryo of Triton was grafted into the side 

 of the body of another embryo, and developed by self-differentiation, deep 

 beneath the epidermis. Under these circumstances it has given rise to a lens from 

 the margin of its own cup, in themanner characteristic of regeneration experi- 

 ments. Br. portion of grafted brain tissue ; I.e. wall of intestine ; L. lens ; S. epi- 

 dermis of ventral side. (From Adelmann, Arch. Entwmech. cxiii, 1928.) 



The divergent results obtained with different species are ap- 

 parently to be accounted for by differences in the rates at which 

 the two processes, of capacity of the eye-cup to induce and of the 

 epidermis to differentiate, occur. 



The proliferation of cells from the epidermis is not, however, 

 the only process involved in lens-formation: the cells require to 

 become converted into the characteristic lens-fibres. While the 



