MALE GERM CELLS IN NOTONECTA i t 



('12). In order that the hnkage may take place, however, in 

 N. insiilata, we must assume that a particular chromosome of 

 one pair always associates with a particular one of the other pair 

 and never with its mate. When these two chromosomes are 

 permanently associated, as is probably the case in N. irrorata, one 

 chromosome might serve as the basis of the linked characters. 

 3. The XY-pair. The observations on Notonecta add nothing 

 new to the main facts in regard to the XF-chromosomes. The 

 difference in size between the two components is much more 

 marked in N. undulata than in the other two species; similar differ- 

 ences between related species have been found in Nezara, Euchis- 

 tus and other Hemiptera. The only departure from the usual 

 behavior of the A"}"-pair is in the failure of the two components to 

 conjugate. Usually in the Hemiptera the components come to- 

 gether in the prophase of the second division, in contrast to all the 

 other chromosomes which have paired before the first division. 

 In Notonecta, however, in all three species this pairing frequently 

 does not take place, and the two components of the XF-pair lie 

 side by side in the metaphase of the second division and pass to 

 opposite poles. A similar condition has been seen by Mont- 

 gomery ('10) in Euchistus, but here it is apparently very excep- 

 tional for he found it only in one case out of 672. As to the time 

 of conjugation of chromosome pairs, there is a graded series. The 

 autosomes conjugate in the general synaptic period; the ?^i-chro- 

 mosomes undergo a late synapsis in the prophases of the first 

 division; the XF-chromosomes in most Hemiptera do not finally 

 conjugate till the end of the first division; the XF-chromosomes 

 of Notonecta frequently do not ever form a definitive dyad. It 

 is of interest, from the point of view of the mechanics of division, 

 to find that a linear arrangement of the components of a chromo- 

 some pair is not necessary for their distribution to the opposite 

 poles of the spindle. 



