THE DETERMINATION OF SEX 87 



demonstrated that delayed fertilization of frogs' eggs produces 

 an excess of males. Unfortunately it is not clear how this 

 result is brought about. An American lady, Miss King, has 

 made extensive investigations 42 upon the influence of external 

 conditions on the determination of sex in toads' eggs. Nutri- 

 tion and temperature are apparently without effect, but if the 

 eggs lose water then more females develop. Even if we should 

 pass in review the entire literature upon the determination of 

 sex through external conditions we should not get much 

 further than we could from the examples I have presented to 

 you. We are safe in saying that external conditions are prob- 

 ably not of great importance, and at the most are merely fav- 

 orable or unfavorable for the development of one sex or the 

 other. The essential conditions must be sought in the cells 

 themselves, and this view has had brilliant confirmation 

 through recent researches. 



It is very pleasant for me as exchange professor to have 

 the privilege of reporting a series of American investiga- 

 tions which are of the highest value because they have pro- 

 cured for us entirely new views of the determination of sex. 

 Only recently have similar investigations been entered upon 

 in Europe. The new doctrine arose from the observation of 

 the developmental processes which lead to the formation of 

 the male elements in certain insects. The founder of the doc- 

 trine is Professor C. E. McClung, 43 who, after serving many 

 years at the University of Kansas, became last autumn 

 Professor of Zoology at the University of Pennsylvania in 

 Philadelphia. His first memoir upon the spermatogenesis of 

 insects appeared in the year 1900, and contains the results of 

 his investigations on the process in the Acrididse. McClung's 

 most important discovery was that one chromosome during 

 the evolution of the sexual elements behaves quite differently 



