402 THE HEREDITARY FACTORS AND DIFFERENTIATION 



Another example is provided by the exposure of the eggs of the 

 frog or of certain fish to the action of weak toxic substances. In 

 such cases (already noted in Chap, ix, p. 348) the animals develop 

 with one median cyclopic eye instead of the normal pair.^ Since it 



Fig. 194 

 Absence of cohesion in the blastomeres of sea-urchin eggs in calcium-free sea- 

 water, a-c, Successive stages in one egg. «, 2-cell stage. &, 4-cell stage, c, i6-cell 

 stage. The cell-membrane has become radially striated, and the cells fail to 

 remain united, d. Disintegration of a blastula into its component cells when placed 

 in the same medium. (After Herbst, from Jenkinson, Experimental Embryology , 

 Oxford, 1909.) 



it known from palaeontological evidence that fish have possessed 

 paired eyes since the Silurian dpoch, these experimental results are 

 an illuminating example of the fact that hereditary factors, however 

 long the time during which they have been transmitted to successive 

 generations, can only produce their normal effects by interacting 



^ Stockard, 1910; Bellamy, 1919. 



