532 Edmund B. Wilson. 



1. The paired microchromosomes or preferably "ra-chro- 

 mosomes," since forms may be found in which they are not smaller 

 than the others form an equal pair in the spermatogonia, and in 

 most of the forms thus far known are much smaller than the others. 

 These do not, ordinarily conjugate to form a bivalent in the 

 general synaptic period, and may (Alydus, Archimerus) or may 

 not (Anasa, "Chariesterus") condense early in the growth- 

 period to form two small separate chromosome-nucleoli which 

 can be distinguished in addition to the principal one (hetero- 

 tropic chromosome). They undergo a very late synapsis (in the 

 prophases of the first maturation division) to form a small sym- 

 metrical bivalent, typically central in position, that undergoes a 

 reduction-division in the first mitosis and an equation-division 

 in the second. Each spermatid nucleus therefore receives a single 

 ra-chromosome. They are always, as far as known, associated 

 with a heterotropic chromosome, and the number of spermato- 

 gonial chromosomes is odd (with the more than doubtful exception 

 of Syromastes). The first maturation-division shows a number of 

 chromosomes which when doubled is one more than the spermato- 

 gonial number (as in Orthoptera). Known to occur in Anasa, 

 "Chariesterus," Syromastes, Protenor, Alydus, Archimerus, Har- 

 mostes, (Edancala, and doubtless occur in many others. 



2. The idiochromosomes are typically unequal in size (Nezara 

 forms an exception) forming an unequal pair in the spermatogonia 

 (which accordingly show typically but one small chromosome); 

 they may conjugate to form a bivalent at the time of general 

 synapsis, or may remain separate, in either case condensing to 

 form a chromosome-nucleolus (or two separate unequal ones) 

 which persists throughout the greater part or the whole of the 

 growth-period. In either case they are in the Hemiptera always 

 separate univalents at the time of the first maturation-mitosis, 

 and separately undergo an equation-division in that mitosis. 

 This division accordingly shows one more than half the spermat- 

 ogonial number of separate chromatin-elements, the latter 

 number being in all cases an even one. At the end of the first 

 mitosis their products conjugate to form a bivalent dyad (thus 

 reducing the number of separate chromatin-elements to one-half 

 the spermatogonial number). This dyad, typically unsymmet- 

 rical, undergoes a reduction-division in the second mitosis, and 

 all of the spermatozoa receive the same number of chromosomes, 



