STUDIES ON CHROMOSOMES 359 



towards inequality in all individuals, so marked in some cases as 

 to characterize a large percentage of the cells, so slight in others 

 that it can not be distinguished by the eye in more than a small 

 percentage. A noteworthy fact remains to be mentioned. 

 Among my few smear-preparations of Oncopeltus is one slide, 

 showing great numbers of nuclei at nearly all stages, in which the 

 .ST-chromosomes are almost always equal in the growth-period 

 and earlier stages but invariably unequal in the prophases. I 

 distrust this evidence somewhat, for it is notorious that variations 

 of size are very readily produced in smears owing to different 

 degrees of flattening. Were this the only explanation, however, 

 we should expect to see the size-differences as great in the earlier 

 as in the later stages. If the result be trustworthy, it is interest- 

 ing as indicating the existence of some kind of material differ- 

 ence between X and Y that is expressed in a greater enlargement 

 of one of them at the period when both expand somewhat and 

 undergo longitudinal splitting. 



II. THE- GROWTH-PERIOD 



For the direct study of the actual process of synapsis and its 

 relation to the reduction-division, Oncopeltus and Lygaeus pre- 

 sent practical difficulties that I have thus far found insuperable; 

 hence no attempt will be made to describe synapsis in detail. 

 The transformations of the chromatin during the growth-period 

 will nevertheless be considered at some length, partly in order 

 to trace the complete history of the sex-chromosomes, partly 

 because of the interest of many features presented by the auto- 

 somes, and I will also describe certain facts observed in other 

 animals that may help to elucidate some of the problems here 

 encountered. 



1. Outline of the stages 



In Oncopeltus it is necessary to distinguish not less than twelve 

 well marked stages following the last spermatogonial division, as 

 follows : 



a. (Figs. 47 to 49.) The final spermatogonial telophases, in 

 which the anaphase-chromosomes break up into a confused net- 



