44 INBREEDING AND OUTBREEDING 



it can be shown that in these two and other similar in- 

 stances, the several young are the product of a single 

 fertilized egg which so develops as to form two or four 

 complete individuals. If sex was determined after fer- 

 tilization, one might expect a random sample of the two 

 sexes here, but this is not the case. 



Fio. 15. Identical quadruplets in the nine-banded armadillo. (After Newman from 

 Doncaster.) 



The chief support of this idea of sex determination, 

 however, comes from the microscope and the breeding 

 pen. In an ever increasing number of species, possibly 

 including man himself, it has been found that besides the 

 regular paired chromosomes, the autosomes, there is a 

 single chromosome or possibly a chromosome group com- 

 monly known as the x-chromosome, whose behavior in cell 

 division is somewhat different from the others, and whose 



