190 HEREDITY AND SEX 



to mean that the eggs begin their development be- 

 fore there has been produced over their surface a 

 layer that in the mature egg seems to have an im- 

 portant influence in restraining sexual eggs from de- 

 velopment. 



This brings us at once to a consideration of what 

 keeps sexual eggs from developing until they are fer- 

 tilized. 



In recent years a great variety of methods has been 

 discovered by means of which sexual eggs can be made 

 to develop without fertilization. This process is 

 called artificial parthenogenesis. We owe especially 

 to Professor Jacques Loeb the most successful accom- 

 plishment of this important feat. The discovery in 

 his hands has led to very great advances in our 

 understanding of the developmental process. 



The chief importance of Loeb's work lies, in my 

 opinion, not only in the production of embryos with- 

 out fertilization (nature has long been conversant 

 with such methods) ; but in other directions as well. 



First, it has thrown light on the nature of the in- 

 hibitory process that holds back the sexual egg from 

 developing until the sperm enters. 



Second, the information gained in this way tells us 

 something of how the sperm itself may act on the egg 

 and start it on its course. 



Third, it opens up the opportunity of studying cer- 

 tain problems connected with the determination of sex 

 that can be gained in no other way. 



Let me attempt briefly to elaborate some of these 

 points. 



In many eggs, perhaps in all, a membrane is produced 



