142 DISTRIBUTION OF THE CHROMOSOMES 



foreign cytoplasm. In the maturation a few of the 

 chromosomes seem at times to unite in pairs, but 

 most of them fail to do so, so that while the number 

 of the chromosomes at the first maturation division 

 is slightly less than the full number it is much more 

 than half of that number. Different types of 

 hybrids behave slightly differently in respect to the 

 extent to which union in pairs takes place. The 

 failure to unite indicates that in normal maturation 

 homologous chromosomes mate with each other, 

 for here there are few^ or no chromosomes that are 

 strictly homologous and yet there is just as much 

 opportunity as in normal maturation for non- 

 homologous chromosomes from the same parent to 

 unite. 



When the first spermatocyte division takes place 

 in the hybrid, all the unmated chromosomes divide, 

 but the few chromosomes that are mated pre- 

 sumably separate. Consequently each of the daugh- 

 ter cells has the double number of chromosomes 

 (a set from each parent species), except for the few 

 chromosomes that had been united in pairs. At 

 the second maturation division the chromosomes 

 again divide, so that the spermatozoa too should 

 receive nearly the double number of chromosomes, 

 one set from one species, the other set from the other 

 species. 



If, then, the factors are contained in the chromo- 

 somes, we should expect that, except for any factors 

 in the few chromosomes that mate and separate, the 

 hybrid would transmit to all its offspring the same 



