Studies on Chromosomes. 375 



tite; and such dyad-like forms, agreeing both in mode of origin and 

 in fate with actual tetrads, may occur in the same equatorial plate 

 with obviously quadripartite forms (cf. Fig. 2e}. Conversely it 

 will be shown beyond that bivalent and univalent chromosomes 

 occurring in the same equatorial plate may exactly agree in form, 

 though having a wholly different mode of origin. 



The purely descriptive term "idiochromosomes" (peculiar 

 or distinctive chromosomes) will be applied to the two chromo- 

 somes, usually unequal in size, which, as stated above, undergo a 

 very late conjugation and subsequent asymmetrical distribution to 

 the spermatid-nuclei. These bodies, as already stated, are iden- 

 tical with some of those to which Montgomery ('01, '04) has 

 applied the term "chromatin nucleoli." This use of the latter 

 term is, however, undesirable, since the accessory chromosome 

 also appears in the growth-period (of Orthoptera and some 

 Hemiptera) in the form of a chromatin-nucleolus. I shall, there- 

 fore, employ the latter term in a broader sense to designate any 

 compact deeply staining chromatin-mass, present in the resting 

 nucleus, which afterward contributes to the formation of the 

 chromosomes. When, as in case of the accessory chromosome, 

 or the idiochromosomes, such a chromatin-nucleolus represents 

 a single chromosome or pair of chromosomes it may conveniently 

 be called a "chromosome-nucleolus"; but I think this term should 

 be restricted to the resting nuclei and cannot appropriately be 

 applied to the corresponding chromosome of the division-stage. 

 Especially large or small chromosomes may be designated as 

 "macrochromosomes" or "microchromosomes," irrespective of 

 their behavior. 



DESCRIPTIVE. 



In the following account Lygaeus and Coenus will be taken as 

 types, a brief comparison of the other forms being added. Some 

 of the latter especially Brochymena and Nezara present features 

 of peculiar interest which I hope to make the subject of a special 

 study hereafter. 



I. The Maturation Divisions. 



Lygaeus and Coenus show an extremely close agreement in the 

 general history of the chromosome-group, and especially in the 

 behavior of the idiochromosomes; though the earlier history of 



