INFLUENCE OF ENVIRONMENT 321 



offspring, though this is by no means proved; 

 while Hertwig concludes that the great pre- 

 valence of the drug habit may seriously affect 

 the germ cells and their subsequent develop- 

 ment. Forel has for many years maintained 

 that one of the most serious causes of human 

 malformations and degenerations is to be 

 found in the effect of alcohol on the germ cells, 

 especially at the time of conception. 



2. Fertilization Stages. — Environmental 

 changes acting during fertilization may cause 

 more than one spermatozoon to enter the egg 

 or may injure the egg or sperm; in either 

 case the resulting development is abnormal. 

 Where two or more spermatozoa enter the 

 egg the nuclear divisions are usually abnormal, 

 as Boveri has shown in the case of the sea 

 urchin; the distribution of chromosomes to 

 different cleavage cells is unequal and such 

 cells do not undergo typical development, 

 while the embryo or larva produced is not 

 capable of continued life. In cases where 

 an egg is fertilized by a spermatozoon belong- 

 ing to a different phylum or class (heterogen- 

 eous fertilization) the foreign sperm, after 



