4o6 Experimental Zoology 



tain from this point of view the female elements. The conclu- 

 sion seems plausible, if we believe that the male and female ele- 

 ments as such become separated in the spermatozoa and eggs. 

 It seems to me, however, that a simpler hypothesis may be for- 

 mulated. The sex of the embryo is not laid down as such in 

 the egg or sperm, but may be determined later by the quantitative 

 relation resulting from the activity of the chromatin in the cells 

 of the embryo : not in the sense in which Richard Hertwig has 

 expressed this relation, viz. as depending on the size of the 

 nucleus, but rather as the result of the increase in the assimilative 

 function of cells containing more chromatin material. On the 

 other hand, if the number of chromosomes in the unfertilized 

 egg of the bee become doubled before segmentation, the fer- 

 tihzed and unfertihzed egg will both produce cells having the 

 same number of chromosomes, and it is difficult to see how a 

 purely quantitative difference exists. It remains, however, to 

 be shown what really takes place in the drone eggs. 



Since all intermediate stages have been found by Wilson and 

 by Stevens between cases where the accessory is single and where 

 it has a partner of unequal or of equal size, it may seem probable 

 that even when we cannot recognize it by its singleness or by 

 its size difference, it is still a sex determinant. This argument 

 would be vaHd if it could be established that unit-characters are 

 carried by individual chromosomes ; but if the influence of the 

 chromatin as a sex determinant is purely quantitative, the argu- 

 ment based on a continuous series loses its force ; for as soon 

 as the difference in size ceases, the quantitative factor disappears. 

 However this may be, the importance of the discovery of the 

 accessory in sex determination should not be minimized ; for 

 it is the one clear case in which an internal factor has been 

 found that is associated with sex production.^ 



LITERATURE, CHAPTER XXVI 



(See Chapter XXVII.) 



^ The case of the bee and the ant that have long been known furnish another 

 and perhaps a parallel case. 



