STUDIES ON CHROMOSOMES ' 351, 



are often somewhat unequal in later stages; but this, too, is incon- 

 stant. Oncopeltus is in fact, therefore, a form in which the sexual 

 differences of the chromosome-groups are too slight or too elusive 

 to be distinguished by the eye. It is, however, perfectly certain 

 that an XF-pair is present, the members of which show all the 

 characteristic peculiarities of behavior that characterize these 

 chromosomes in other forms. 



^. The first spermatocyte-division 



The maturation-divisions are shown with remarkable clearness 

 in Oncopeltus and Lygaeus — indeed, either of them might be 

 taken as a model of those Hemiptera in which a simple XF-pair 

 is present. The following account applies primarily to Oncopel- 

 tus, Lygaeus being described only by way of comparison. 



In the first division appear nine separate chromosomes in a 

 grouping of remarkable constancy. Seven of the nine bivalents 

 are grouped in an irregular ring, near the center of which lie the 

 univalent X- and F-chromosomes, side by side but not in contact 

 (figs. 8, 9, 11, photo. 3). The constancy of this grouping appears 

 from the following data. Two hundred clear polar views, taken 

 at random, did not show a single case of more than nine chromo- 

 somes; plus variations of number in this division (such as are 

 occasionally seen in many species) must therefore be very rare — 

 indeed, I have never seen such a case.^ Of the two hundred cases, 

 one hundred and seventy-two showed the grouping just described. 

 In the remaining twenty-eight the deviations were unimportant; 

 the ring may show a gap at one side (figs. 10, 12), one of the biva- 

 lents may lie inside it (fig. 10), or (rarely) one or both the sex- 

 chromosomes may lie in the ring (fig. 13). In only two cases did 

 the sex-chromosomes not lie side by side; in these, they were 

 separated by one bivalent (fig. 13). 



The size-relations alone sufficiently indicate that the two small 

 central chromosomes are the univalent sex-chromosomes (a fact 



^ Apparent minus deviations are of course common, but are disregarded because 

 evidently due in most (all?) cases to the fact that one or more chromosomes lie 

 outside the plane of section. 



