THE DETERMINATION OF SEX 277 



4. P<XLYEMBRYONY 



Closely allied to the chromosomal basis of sex are 

 the facts of polyembryony, for when more individuals 

 than one are formed from a single ovum they are inva- 

 riably of the same sex. Classical examples are para- 

 sitic hymenoptera, principally of the families Procto- 

 trypidoe and Chalcididce, in which thousands of indi- 

 viduals often result from a single egg. Other examples 

 are the quadruplets formed in the nine-banded arma- 

 dillo, Tatusia, and identical or monochorial twins in 

 man and other mammals. 



In the case of mammals the type of sperm, either 

 with or without the ^-chromosome, is undoubtedly the 

 deciding factor in sex determination, for the reason 

 that when all of the chromosomes of the zygote divide 

 normally the sex of the resulting individuals must be 

 the same. In other ways also they will be genetically 

 identical. 



In hymenoptera sex depends entirely upon whether 

 fertilization or parthenogenesis takes place. A fer- 

 tilized egg will result in females and an unfertilized one 

 in males, a supposition based upon direct cytological 

 observation. The facts of polyembryony thus offer 

 strong substantiation to the idea of chromosomal de- 

 termination of sex. 



5. SEX-LINKED INHERITANCE 



The association of Mendelian characters with par- 

 ticular chromosomes is nowhere better shown than in 



