202 Basis of Sex Determination 



the X chromosome, but here it is only a difference in 

 size ; one-half of the spermatozoa having a large X chro- 

 mosome, the other half instead a smaller chromosome. 

 Calling this latter the Y chromosome, the sex deter- 

 mination in this form is as follows: leaving aside the 

 chromosomes which are equal in both egg and sperma- 

 tozoon we may say that there is one type of egg con- 

 taining one large X chromosome; there are two types 

 of spermatozoa in equal numbers, one possessing a large 

 X chromosome, the other possessing a small Y chromo- 

 some. Wilson showed by a study of the chromosomes in 

 males and females that when one of the spermatozoa 

 containing a large X chromosome enters the egg^ the 

 egg will develop into a female; while when one of the 

 spermatozoa containing a small Y chromosome enters 

 it will give rise to a male. Leaving aside the common 

 chromosomes of both sexes, a fertilized egg containing 

 XX gives rise to a female, while one containing XY 

 gives rise to a male. There is in this case as in that of 

 Protenor a preponderance of chromosome material in the 

 female, but this quantitative difference is not essential 

 for the determination of sex, since in some species the Y 

 chromosome may be as large as the X chromosome. 



The main fact is that the female cells have the 

 chromatin composition XX, the male cells the composi- 

 tion XY, where Y is apparently qualitatively different 

 and often, but not necessarily, smaller than X, or 

 entirely lacking. 



