218 Heredity and Environment 



II. DEVELOPMENTAL RESPONSES 



The character of developmental responses to stimuli depends 

 primarily upon (a) the nature of the organism and (b) the stage 

 of development at which the stimulus acts. Modifications are 

 more easily produced and are more profound during cell division 

 than during intervening periods and at early stages of develop- 

 ment than at later ones ; indeed conditions which have no serious 

 effect on an adult organism may greatly modify the development 

 of an embryo or germ cell. 



i. Modifications of Germ Cells before Fertilization. It has 

 been found by many investigators that development may be pro- 

 foundly changed by influences acting upon the germ cells before 

 fertilization. In general environmental changes acting during the 

 growth of eggs or spermatozoa and especially during their matura- 

 tion may produce marked changes in development though rarely 

 if ever in heredity. Tower maintained that unusual conditions 

 of temperature and humidity during the later stages of oogenesis 

 and spermatogenesis may lead to the production of new races in 

 the case of the potato beetle (Fig. 100) and MacDougal's experi- 

 ments on plants led him to the conclusion that chemical substances 

 may influence the ovules so as to change the hereditary character 

 of the plant. Other workers have failed to confirm these results 

 and it is doubtful whether these changes in hereditary constitu- 

 tion were caused by the changed environment. Bardeen and the 

 Hertwigs have shown that great monstrosities may be produced 

 if X-rays, radium or various chemical substances are allowed to 

 act on spermatozoa before fertilization, but there is no evidence 

 that these changes are inherited. 



Effects of Alcohol. Stockard subjected adult male and fe- 

 male guinea-pigs to the fumes of alcohol for some time before 

 breeding them and then studied the effects of this drug on their 

 offspring. He finds that the influence of alcohol on the sperma- 

 tozoa is as deleterious as when acting on the ova and that it pro- 

 duces sterility, or greatly reduced fertility, a great excess of still- 



