374 Edmund B. Wilson. 



examined represent two different families of Hemiptera (Penta- 

 tomidae and Lygaeidae) the idiochromosomes will probably be 

 found to be of wide occurrence in the group. 1 The only other case 

 known to me in any higher plant or animal of the unequal division 

 of a chromosome (or chromatin-body) in karyokinesis occurs in 

 Tingis clavata, regarding which Montgomery states that one of 

 the chromosomes of the first division "very frequently is seen to be 

 characterized in having its two components of very unequal vol- 

 ume" ('01, 2, p. 262). This author also observed a considerable 

 number of cases in which the "chromatin nucleoli" are unequal 

 in the rest stage of the spermatogoma, and he describes some forms 

 in which a similar condition appears in the growth-period of the 

 spermatocytes (e. g., in Trichopepla, Peribalus and Euschistus 

 tristigmus). In the last-named species he found that a separation 

 of the two unequal "chromatin nucleoli" takes place in the second 

 mitosis ('01, I, pp. 161, 162), but expressly states that they are not 

 joined together in the equatorial plate (op. cit., p. 162). It is evi- 

 dent from Montgomery's brief description that this phenomenon 

 is similar to, and probably identical with, the one that forms the 

 subject of this. paper. 



TERMINOLOGY. 



Since confusion may readily arise in the terminology, I wish to 

 define clearly the terms that will be employed throughout this 

 paper and its successors. I shall apply the term "chromosome" 

 to each coherent chromatin-mass, whatever be its form, mode of 

 origin or valence, which as such enters the equatorial plate. In 

 the case of compound or plurivalent chromosomes ("tetrads" or 

 "dyads") McClung's term "chromatid" may conveniently be 

 applied to each of their univalent constituents. I may call atten- 

 tion, in connection with this, to the fact that the valence of chro- 

 mosomes cannot be determined by mere inspection of their form. 

 In many Hemiptera, for example, the chromosomes of the first 

 maturation-division frequently show a dyad-like or dumb-bell 

 shape (typically the case, for example, in Euschistus, Lygaeus or 

 Coenus) even though in earlier stages they are plainly quadripar- 



'Since writing the above I have found the idiochromosomes in several additional genera. In 

 Mineus they are only slightly unequal, in Murgantia nearly as unequal as in Lygaeus. Nezara, 

 Mineus, Brochymena, Euschistus, Murgantia and Lygaeus thus show a progressively graded series 

 of stages in the size-differentiation of this peculiar pair of chromosomes. 



