Studies on Chromosomes. 533 



one-half receiving the larger and one-half the smaller idiochro- 

 mosome. They are not ordinarily associated with a heterotropic 

 chromosome, the single known exception being Banasa. The 

 idiochromosomes are known to occur in Lygaeus, Coenus, Podisus, 

 Trichopepla, Mineus, Nezara, Murgantia, Brochymena and 

 Banasa and are doubtless of much wider occurrence. 



3. The "accessory" or heterotropic chromosome is certainly 

 in most Hemiptera — and I believe will be found to be in all — 

 unpaired in the spermatogonia, and its behavior is throughout 

 that of a univalent body. It fails to unite in synapsis with any 

 other chromosome, and persists throughout the spermatocytic 

 growth-period as a chromosome-nucleolus. During the earlier 

 part of this period it resembles the idiochromosome bivalent (or 

 the univalent large idiochromosome) in being attached to a large 

 plasmosome from which it afterward separates.^ This chro- 

 mosome divides in only one of the maturation-divisions, passing 

 undivided to one pole of the spindle in the other. The latter 

 division is usually the second (Pyrrochoris, Anasa, Protenor, 

 Alydus, Chariesterus, Syromastes, Harmostes, CEdancala), but 

 in Archimerus and Banasa it is the first. In either case one-half 

 the spermatozoa receive one more chromosome than the other 

 half. 



From the foregoing it will be seen that Montgomery correctly 

 identified the chromosome-nucleolus in the growth-period of 

 such forms as Euschistus, Coenus, Podisus, Brochymena, Tricho- 

 pepla or Nezara, which possess the idiochromosomes. He was, 

 however, at fault in the conclusion that it gave rise to a small 

 bivalent in the first division, the small chromosome of this division 

 being always a univalent that is not at this time paired with its 

 (usually) larger fellow; and further, owing to a failure to discrimi- 

 nate between these bodies and the paired microchromosomes of 

 the Anasa or Alydus type, he describes and figures the spermat- 

 ogonial groups in most of these forms as containing a symmetrical 

 pair of "chromatin-nucleoli." Owing to his having overlooked 

 the constant separateness of the idiochromosomes as univalents 

 in the first mitosis he has also, I believe, been misled in several 



'It is doubtless a similar condition that has led Moore and Robinson ('05) in the case of Periplaneta, 

 to conclude that the "accessory'' chromosome is nothing but a "nucleolus." These observers have 

 evidently studied the phenomena in a very superficial manner. 



