Influence of Environment 221 



Hoppe believes that a single drunken debauch may so injure 

 the germ cells of man as to produce abnormal and defective off- 

 spring, though this is by no means proved; while Hertwig con- 

 cludes that the great prevalence of the drug habit may seriously 

 affect the germ cells and their subsequent development. 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 con- 

 ception. 



2. Modifications During Fertilization Stages. Environmental 

 changes acting during fertilization may cause more than one sper- 

 matozoon 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 belonging to 

 a different phylum or class (heterogeneous fertilization) the 

 foreign sperm, after stimulating the egg to begin development, 

 may itself die or remain inactive, in which case the hereditary 

 traits which develop are those of the mother only. In many ani- 

 mals unfertilized eggs may be stimulated to begin development by 

 a great variety ,of changes in the medium, all such cases being 

 known as ."artificial parthenogenesis." 



3. Modifications of Development after Fertilisation. Envi- 

 ronmental changes, acting upon the oosperm after fertilization, or 

 upon the embryo, may produce an almost infinite variety of ab- 

 normal types of development, but so far as known none of these 

 modifications becomes hereditary. It seems probable that changes 

 in hereditary constitution take place in the main before fertiliza- 

 tion and especially during the maturation divisions and all changes 

 that affect germ cells must of course occur in the "germ track." 



