Studies on Chromosomes. 535 



the Hemiptera that possess a heterotropic chromosome. Of 

 these accounts (in cases positively known to have such a chro- 

 mosome) there are but four, namely, Henking's original account 

 of Pyrrochoris ('90), Paulmier's ('99), and Montgomery's ('01, 

 '04) accounts of Anasa, Montgomery's of Alydus pilosulus ('01) 

 and Gross's more recent one of Syromastes ('04). Henking states 

 that he counted but four cases, one of which seemed to show 

 twenty-three, the other three twenty-four, and it is evident both 

 from the figures and from the frank statement of this able observer, 

 that he adopted the latter number more on account of theoretical 

 considerations than as a result of any adequate study of the facts. 

 I have shown the counts of Paulmier and Montgomery to be 

 erroneous in the case of Anasa, and also that of Montgomery in 

 the case of Alydus pilosulus. There remains therefore the single 

 case of Syromastes; but perhaps, in view of the results 

 I have reached in other forms, I may be allowed the pre- 

 diction that a reexamination of this one will lead to a similar 

 conclusion. 



If this expectation is verified every ground will be removed for 

 considering the heterotropic chromosome as a bivalent body; 

 and I think that until definite evidence to the contrary is forth- 

 coming we are bound to take this chromosome at its face-value, 

 so to speak, as univalent. This conclusion involves a series of 

 other conclusions and possibilities of which I shall here undertake 

 to indicate only the more important. 



1. As was indicated by McClung ('02, p. 71), if the "accessory" 

 be univalent, its behavior in the maturation-mitoses at once falls 

 into line with that of the other spermatogonial chromosomes; for 

 each of these, too, undergoes but one division in the course of the 

 two maturation-mitoses. One of these divisions (the reduction 

 division) merely separates the univalent chromosomes that have 

 previously paired in synapsis (as is so convincingly shown in case 

 of the idiochromosomes or the m-chromosomes); and only the 

 fact that the "accessory" has no mate with which to pair renders 

 its behavior in one of the divisions apparently different from that 

 of the ones that do pair. 



2. The objections that I myself urged to the suggestion made 

 in the first of these studies regarding the origin of the heterotropic 

 chromosome are thus set aside, and my attempt to compare the 

 idiochromosomes with the m-chromosomes was made on incorrect 



