TEE CELLULAR BASIS OF HEREDITY 1 33 



plain the greater number of males than of females. In certain insects 

 it is known that females only develop from fertilized eggs, and in one 

 of these cases, viz., Phylloxera, Morgan has discovered that this is due 

 to the fact that all the male-producing spermatozoa degenerate and that 

 only female-producing spermatozoa become functional. Possibly experi- 

 mental alterations of the sex ratio, such as Hertwig, King and others 

 have brought about, may be explained in a similar way. At least the 

 chromosomal theory of sex determination is so well established in so 

 many cases and has been found to be true in so many instances where 

 at first it seemed impossible of application, that it ought not be aban- 

 doned until unmistakable evidence can be adduced against it. For the 

 present at least we are justified in concluding that sex is irrevocably 

 determined at the time of fertilization. 



{To be concluded.) 



