STUDY OF CHROMOSOMES IN NOTONECTA 131 



spend with the two largest ones of the spermatocytes, and the 

 fifth being X; there are four small ones corresponding with the 

 small central one and the one just within the ring in the first 

 division (figs. 78 and 79). Although X and Y are sometimes 

 almost equal in the haploid groups, I have found no case of 

 equality in the diploid, even from the same individual. The 

 oogonial groups give the same number of chromosomes, 26, but 

 have six large ones instead of five, the sixth large one being the 

 second X chromosome (fig. 80) . 



h. Size relations of X and Y 



The X and Y chromosomes, as was mentioned above, differ 

 considerably in their relative sizes in different cells, and this is 

 true to a certain extent for different individuals. Although in 

 many individuals, the inequality of X and Y is considerable, in 

 others the inequality is less marked, and in some cells of these 

 individuals is not apparent at all. This relative inequality is, 

 however, not constant, for in those individuals where there is a 

 tendency to equality, certain cells are found where the inequality 

 is marked. In figures 68 to 74, there are shown the X Y pairs 

 from seven different individuals, the drawings being made as 

 carefully as possible and the extremes and means of inequality 

 being represented in each case. The inequality is well marked 

 in figure 68, fairly evident in figures 69 and 70, less so in the 

 others. In figures 73 and 74, X and Y are in some instances 

 apparently equal, though in one case in each individual, they 

 are markedly unequal. A variation in the size of these chromo- 

 somes has been found also by Wilson ('12), in Oncopeltus, and 

 the percentage of cases of equality and inequality was found to 

 vary widely in different individuals in this instance also. 



The variation in the relative size of X and Y is well shown 

 in sister polar views of the anaphase (f^gs. 75 to 77). Since these 

 groups go directly into the spermatids and spermatozoa, it is 

 evident that in certain cases (where X and Y are different in 

 size), these will fall into two classes, difTering in their chromatin 

 content. In those cases, however, where X and Y are equal 

 (fig. 77), there will be no visible difference. Such groups as 



