390 EDMUND B. WILSON 



regard to the following points. First, it is certain that each of these 

 chromosomes divides but once in the course of the maturation-process, 

 namely, in the first division; and this division is clearly longitudinal 

 and equational. The second 'division' of the XY-pair is obviously 

 not a division at all but only the disjunction of two separate chromo- 

 somes that have for a short time been in contact without loss of their 

 identity. This process is an evident and typical reduction-divi- 

 sion in the original sense. In these animals, therefore, it is quite 

 certain that the XT-pair undergoes a process of 'post-reduction' 

 (cf. Wilson, '05 c). It is a remarkable fact, proved by the studies 

 of Stevens, that in the Coleoptera and Diptera the -XT-pair fol- 

 lows the reverse order, as is also the case with the w-chromosomes 

 of the coreid Hemiptera. 13 



7. Comment on the sex-chromosomes in Oncopeltus 



The extremely close correspondence between Oncopeltus and 

 Lygaeus at every stage of the spermatogenesis leaves not the 

 least doubt of the identity of the sex-chromosomes of the two 

 forms. Apart from the size-differences of these chromosomes, 

 Lygaeus differs from Oncopeltus only in (1) the retention through- 

 out of a rod-like form by the JT-chromosome, (2) the earlier appear- 

 ance of the longitudinal split in both sex-chromosomes, (3) a 

 slightly more marked tendency for the sex-chromosomes to con- 

 jugate at the time of general synapsis. On the other hand, the 

 sex-chromosomes of the two forms agree in all the characteristic 

 peculiarities of these chromosomes shown in the Hemiptera gener- 

 ally, namely, (1) the retention of a compact and deeply staining 

 character from an early pre-synaptic period down to the spermato- 

 cyte-prophases, (2) their division as separate univalents in the 

 first spermatocyte-di vision, (3) their subsequent conjugation to 

 form a bivalent, which occupies a nearly central position in the 

 second spermatocyte metaphase-group, and (usually) divides in 

 advance of the other chromosomes. These facts -fully establish 



13 1 may point out that it is inadmissible to designate as 'm-chromosomes' any 

 pair of especially small chromosomes without respect to their other characteristics, 

 as has been done by several writers. The m-chromosomes of the Coreidae are not 

 always distinctly smaller than the other chromosomes, and they are characterized 

 by certain very definite peculiarities of behavior. Cf. Wilson, '05 c, '11 a. 



