394 



Experimental Zoology 



Effects of Inbreeding 



It is a widespread belief that deterioration in strength or fer- 

 tihty or in both follows close inbreeding, and while there is some 

 evidence to the contrary in many species, still, in a few, the evi- 

 dence that we have at present indicates injurious effects. Such 

 weakening influence has been supposed to affect the determina- 

 tion of sex. Schultze has carried out a series of experiments 

 to test this view by closely inbreeding white mice. His results 

 show that while for a Umited number of births one or the other 

 sex may predominate, yet the general averages show approxi- 

 mately an equality. 



Size oj the Egg 



In three species belonging to widely different groups it has been 

 discovered that the size of the egg is correlated in some way with 

 sex. In Phylloxera the large eggs develop without fertilization 

 into females, and the small eggs into males. In Hydatina senta 

 the same rule holds. In Dinophilus apatris, also, there are large 

 and small eggs, both of which are supposed to be fertilized. 

 The large eggs produce females and the small ones males. One 

 of the most striking instances in which sex seemed to be deter- 

 mined by, or connected with, the size of the egg is that of the 

 silkworm. Joseph, in 187 1, and Mme. Brocadello, in 1896, 

 stated that the large eggs became females and the small ones 

 males. As the following table shows, the small eggs gave from 

 88 to 95 per cent of males and the large eggs 82 to 92 per cent 

 of females : — 



BROCADELLO'S TABLE FOR THE SILKWORM MOTH 



