xm] HYBRIDS 201 



has 56 chromosomes in the mitoses of the maturation divi- 

 sions, and, as nearly as can be counted, 112 in its diploid 

 nuclei; all are very small, but four (two in reduced nuclei) 

 somewhat larger than the rest. The mature germ-cells of 

 hirtarius have thus 14 (twelve large and two small); those 

 of zonaria have 56 (two of moderate size and 54 small). 

 The hybrid has in its spermatogonia, as nearly as it is pos- 

 sible to count, 70 chromosomes, of which 14 are noticeably- 

 larger than the remainder. This by itself would indicate 

 that the chromosomes of the two species retain their indivi- 

 duality in the hybrid, but the phenomena seen in the sper- 

 matocyte divisions confirm this conclusion in a very striking 

 way. As already described (p. 87), since there is only a 

 single set from each parent species, and the chromosom.es of 

 one species are not homologous with those of the other, 

 very few of the chromosomes are able to find mates in the 

 prophase of the maturation divisions, and in consequence 

 the spermatocyte equatorial plates have only a few chromo- 

 somes less than those of the spermatogonia, instead of half 

 the number. If the number and size of the chromosomes 

 depended simply on an inherent tendency of the species, 

 causing the chromatin to be aggregated into a constant 

 number of masses at mitosis, these facts are inexplicable, 

 but on the individuality hypothesis they are precisely what 

 would be expected. 



(4) The different physiological values of distinct chro- 

 mosomes. 



An argument based on the physiological function of the 

 chromosomes is necessarily speculative, since at present 

 their significance in this respect can only be inferred, or, 

 as some would say, guessed at. It seems clear, however, th^t 

 the idio-chromosomes have a definite function in connection 

 with sex-determination, and in the next chapter the possi- 



