STUDIES ON CHROMOSOMES 357 



The later stages of this division have been so often described 

 as to call for no further account. The final result is that the sper- 

 matid-nuclei receive the haploid number of chromosomes — seven 

 in Lygaeus, eight in Oncopeltus — half the nuclei in each case 

 receiving A" and half Y. The facts seen so clearly in both these 

 species remove every possible doubt that the X- and Y-chromo- 

 so77ies ivhich thus enter the spermatid-nuclei are the same individual 

 chromosomes that conjugate at the end of the first division and persist 

 throughout the interkinesis to disjoin in the course of the second, 

 division. It is of course possible that some exchange of material 

 may take place between them during the brief period of their 

 association. Of this, however, there is no evidence; and it is 

 certain that their individual boundaries are not lost to view, and 

 that not even an apparent fusion takes place at this period or 

 any earlier one. 



5. The size-relations of the sex-chromosomes in Oncopeltus 



In my first examination of this species my attention was given 

 mainly to some excellent preparations from two individual males 

 (designated by the numbers 711 and 712) in which the X- and Y- 

 chromosomes appear equal in a large majority of the nuclei. 

 The facts in Nezara hilaris (Wilson, '11a) led me to extend the 

 examination to other individuals of Oncopeltus, when to my sur- 

 prise one individual was found (later two others) in which a slight 

 but evident inequahty was obvious in a large percentage of the 

 cells at all stages. Upon reexamination of the entire series 

 the interesting discovery was made that in every individual cases 

 could be found of both equality and inequality, the ratio between 

 them varying widely in different individuals. In the extreme 

 cases this is perfectly apparent to the eye, so that individuals of 

 predominantly equal or unequal type may readily be distinguished 

 even by casual inspection. In other cases one is often in doubt 

 until large numbers of the nuclei have been tabulated. As exam- 

 ples of the extreme types I give below the results of a study of 

 two individuals (nos. 712 and 760) representing the best material 

 as to fixation and staining. In these a comparison of the X- and 



