56 HEREDITY AND ENVIRONMENT 



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, 

 machine-like responses. Such tropisms or re- 

 flexes are seen in the movements of bacteria, 

 protozoa and many higher animals and plants 

 (Figs. 15-19) as well as in movements of 

 spermatozoa, the movements of the proto- 

 plasm in egg cells and embryonic cells, the 

 movements of cells and cell masses in the for- 

 mation of the gastrula, alimentary canal, ner- 

 vous system and other organs. Indeed the 

 entire process of development, whether accom- 

 panied by visible movements or not, may be re- 

 garded as a series of automatic responses to 

 stimuli. 



When the embryo becomes differentiated 



