THE MECHANISM OF REPRODUCTION 43 



behavior of particular chromosomes of which we have not 

 hitherto spoken (Fig. 14). 



The evidence in favor of this view of the determination 

 of sex at the time of fertilization through the chromosome 

 complex is from several very different sources. 



First, there is the phenomenon of multiple births 



P rot en or cf 



Fio. 14. Diagram showing the distribution of the sex chromosome in Protenor. (After 

 Morgan.) 



among mammals. In general, animals in which this is 

 the rule, bear both males and females, through all of the 

 individuals must have been under the same environmental 

 conditions. There are multiple births, however, in which 

 the young are invariably of the same sex. Such is the 

 case with those remarkably similar human twins known as 

 identical twins. Such is the case with the four young in 

 each litter of the nine-banded armadillo (Fig. 15). Now 



