IV. 2 INTERACTIONS OF THE PARTS 277 



of the cornea provided that the optic cup could touch the ecto- 

 derm, and if this same condition is fulfilled, the cornea will 

 be differentiated in the ectoderm that grows in after the layer 

 that originally lay over the eye had been destroyed, but if the 

 cup and lens are taken away no new cornea is formed even 

 after four weeks. The size of the cornea is in proportion to the 

 area of contact. Finally, if, at a later stage still, the optic cup 

 and lens are taken away, the already formed cornea degenerates. 

 Should the results of these extremely interesting investiga- 

 tions be confirmed, the development of some parts at least of the 

 vertebrate eye would be processes of ' dependent differentiation ' l , 

 dependent, that is, on causes outside themselves, on stimuli 

 exerted by other parts, though not, of course, wholly dependent, 

 since the structureof the reacting, as well as that of the stimulating, 

 organ is contributory to the quality of the effect. How far such 

 an action of the parts upon one another is of general occurrence 

 as a factor in differentiation future research alone will show. It 

 seems probable, however, that it will in any case be found to be 

 limited to the first moments in the formation of the organs 2 , for 

 we know from other evidence that as development proceeds the 

 system which was ' equipotential ', to use Driesch's phrase, 

 becomes ' inequipotential ', that the parts lose their totipotence, 

 that they gain independence and the power of self-differentiation, 

 that the value of the correlations between them diminishes. This 

 is what Minot has called the law of genetic restriction. But 

 in early stages there are correlations between the organs, the 

 differentiation of one does depend upon that of another, and it is 

 this mutual stimulation of the parts that figures so prominently 

 in the theory of development worked out by Driesch. This 

 theory we shall have to examine in the following chapter. 



1 Although the expression ' dependent differentiation ' is due to Roux, 

 and although Roux expressed the opinion that certain developmental 

 processes the beginning of parthenogenetic development, the deter- 

 mination of the position of the grey crescent and first furrow by the 

 entrance-point of the spermatozoon, the 'post-generation 1 of the dead 

 blastomere by the living one in the Frog's egg, for instance were of the 

 nature of responses to stimuli, still the role assigned by the ' Mosaik- 

 Theorie' to the influences of the parts upon one another in differentiation 

 was subordinate, and restricted to the later stages of development. 



2 Except, presumably, in cases of ' composition ' where elements of 

 different origin unite in the formation of a single organ. 



