DIFFERENTIATION, HEREDITY, SEX 293 



We may here suggest again the character of the evidence for 

 regarding the chromosomes of the germ nuclei of the very 

 greatest importance as factors controlling or directing the proc- 

 ess of development (i.e., the process of heredity), facts of such 

 constancy and universality that they must have some meaning. 



First of all comes the fact of the very high degree of morpho- 

 logical constancy of these organs throughout the tissues of the 

 species, not merely of the individual. This constancy, always 

 considering corresponding ages of course, concerns their number, 

 size, and form, and proves them to be specific organs in a real 

 sense. They are, with a few easily explained exceptions, pres- 

 ent in pairs of similar elements, whose history can be traced 

 back to their derivation in groups of unpaired elements in the 

 male and female gametes. During mitosis the distribution of 

 the chromosomes to the daughter cells is never a haphazard proc- 

 ess, but the whole process of mitosis appears to be an adapta- 

 tion toward securing their equal division, and the distribution 

 to the daughter nuclei of groups of similar morphological 

 composition. 



It is unnecessary to repeat here any of the evidence out- 

 lined in Chapter II for the idea of the genetic continuity 

 of the chromosomes from cell to cell. We saw there that it 

 is the chromatin granules which may be regarded as actually 

 morphologically continuous, and that these may seem to 

 become similarly associated every time that the chromosomes 

 visibly appear. What determines their constant association 

 in the reforming chromosomes of each cell generation is to be 

 answered only hypothetically if at all. But in spite of the 

 contrary belief held by some, the chromosomes may be regarded 

 as genetically continuous individual elements, although the 

 details of their composition may vary slightly from generation 

 to generation. And after all it may be that the most important 

 continuity is that of the chromatin granules, supposing them 

 to be qualitatively unlike and to play the role of specific de- 

 terminers. 



The validity of the chromosome hypothesis has always been 

 strongly indicated by the essential facts of syngamy, namely, 



