54 HEREDITY AND ENVIRONMENT 



cell masses in the formation of the gastrula, 

 alimentary canal, nervous system and other 

 organs. Indeed the entire process of develop- 

 ment, whether accompanied by visible move- 

 ments or not, may be regarded as a series of 

 automatic responses to stimuli. 



When the embryo becomes differentiated 

 to such an extent as to have specialized organs 

 for producing movement its capacity for mak- 

 ing responsive movements to stimuli becomes 

 much increased. If the responses of animals 

 and plants to stimuli are of such a sort that 

 the organism turns or moves toward or away 

 from a source of stimulus they are termed 

 tropisms; if the responses are very compli- 

 cated, one response calling forth another and 

 involving many reflexes, as is frequently the 

 case in animals, they are known as instincts. 

 In the embryo the rhythmic contractions of 

 heart, amnion and intestine are early manifes- 

 tations of reflex motions. These appear chiefly 

 in the involuntary muscles before nervous con- 

 nections are formed, the protoplasm of the 

 muscle cells probably responding directly to 

 the chemical stimulus of certain salts in the 



