54 HEREDITY AND ENVIRONMENT 



alkali and alcohol, they gather in certain 

 chemical substances and not in others, they 

 collect in great numbers around fertilizable 

 egg cells, etc. 



The movements of fertilized egg cells, 

 cleavage cells, and early embryonic cells are 

 usually limited to flowing movements within 

 the individual cells. These movements, which 

 are of a complicated nature, are of the greatest 

 significance in the differentiation of the egg 

 into the embryo ; they are caused chiefly by in- 

 ternal stimuli and by non-localized external 

 ones. Modifications of the external stimuli 

 often lead to modifications of these intra- 

 cellular movements and to abnormal types of 

 cleavage and development — in short, these 

 movements show that the fertilized egg is dif- 

 ferentially sensitive. 



In the further course of development 

 particular portions of the embryo become es- 

 pecially sensitive to some kinds of stimuli, 

 while other portions become sensitive to oth- 

 ers. In this way the different sense organs, 

 each especially sensitive to one particular 

 kind of stimulus, arise from the generalized 



