52 HEREDITY AND ENVIRONMENT 



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 fertihzed 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 

 sensitivity of the oosperm, and thus general 

 sensitivity, which is a property of all proto- 

 plasm, becomes differential sensitivity and 

 special senses in the process of embryonic dif- 

 ferentiation. Such sensitivity is the basis of 

 all psychic processes; sensations are the ele- 

 ments of the mind. 



2. Tropisms, Reflexes, Instincts. — All the 

 responses of germ cells, and of the simplest 

 organisms, to stimuli are in the nature of trop- 

 isms or reflexes, that is, relatively simple, 

 automatic responses. Such tropisms or re- 



