310 HEREDITY AND ENVIRONMENT 



tial embryos have been formed. In general 

 monstrous and defective forms of development 

 are due to alterations of the normal environ- 

 mental stimuli rather than to defective he- 

 reditary constitution. 



II. Developmental Responses 

 The character of developmental responses 

 to stimuli depends primarily upon (a) the na- 

 ture of the organism and (b) the stage of 

 development at which the stimulus acts. Modi- 

 fications are more easily produced and are 

 more profound during cell division than during 

 intervening periods and at early stages of de- 

 velopment than at later ones. 



1. Modifications of Germ Cells before Ferti- 

 lization. — It has been found by many investi- 

 gators that development may be profoundly 

 changed by influences acting upon the germ 

 cells before fertilization. In general environ- 

 mental changes acting during the growth of 

 eggs or spermatozoa and especially during 

 their maturation may produce marked changes 

 in development and even in heredity. Tower 

 has found that unusual conditions of temper- 

 ature and humidity during the later stages of 



