i8o Plant Genetics 



shown by the chromosome situation. Sex, therefore, 

 is predetermined by the kind of gamete that enters into 

 the sex fusion. 



This statement represents the chromosome theories 

 of sex determination, according to which it seems evident 

 that sex is absolutely predetermined. Outside conditions 

 can effect no change, and therefore sex caimot be con- 

 trolled. It results, therefore, with mathematical pre- 

 cision in 50 per cent males and 50 per cent females, just 

 as a Mendelian ratio comes out 3:1. 



Physiological theories. — In 1906 Richard Hert- 

 WEG (5) performed some sex determination experiments 

 with frogs. The eggs are laid free in the water before 

 fertihzation, so that they furnish unusually good material 

 for such experiments. Normally the eggs are fertilized 

 very soon after they are laid, with the result that the 

 progeny consists of approximately 50 per cent males and 

 50 per cent females. HERXwio took some of these 

 eggs and allowed them to overripen before fertilization 

 took place ; that is, he put aside some eggs as soon as they 

 were laid and allowed them to remain unfertilized for an 

 unusually long period. While these eggs were standing 

 in the water he found that they absorbed an unusual 

 amount of water, and the obvious conclusion was that 

 very ripe eggs show high water content. He then 

 allowed these overripe eggs to be fertilized, and the 

 resulting progeny were 100 per cent males. His con- 

 clusion was that sex was not determined by the chromo- 

 some equipment, but by the physiological conditions in 

 the egg, high water content resulting in males. 



This theory was confirmed in a striking way in 

 19 1 2 by Miss King (7), who performed the converse of 



