146 T. H. MONTGOMERY, JR. 



These are chromosomes which preserve to great extent their 

 compact form during the whole growth period of the spermato- 

 cytes, and during the rest stages of the spermatogonia, and re- 

 tain throughout this whole period the deep staining characteristic 

 of the other chromosomes only during the height of mitosis. 

 Thanks to this peculiarity they can be followed with extreme 

 certainty from generation to generation, even during rest stages ; 

 and so are splendid evidence for the thesis of the individuality 

 of the chromosomes. 



Now there are two kinds of these. In the Orthoptera there is 

 an unpaired one in the spermatogonia, larger than the other 

 chromosomes ; in the Hemiptera they are paired in the spermato- 

 gonia, and usually smaller than the other chromosomes. Other- 

 wise in their behavior they are very similar in these two groups 

 of insects. To include both these kinds the name " heterochro- 

 mosomes," as expressing a difference from the other chromosomes, 

 can be advantageously applied; and this would include (i) 

 the "accessory chromosomes" (unpaired in the spermatogonia), 

 and (2) "the chromatin nucleoli " or "small chromosomes" 

 (paired in the spermatogonia). McClung regards them as sex 

 determinants ; I have considered them to be chromosomes that 

 are in the process of disappearance, in the evolution of a higher 

 to a lower chromosomal number. 



Now these can be followed from generation to generation with- 

 out in the Hemiptera undergoing those profound changes which 

 characterize the other chromosomes after a mitosis. In the Figs. 

 15-23 they are the chromosomes marked N, n ; Figs. 16 and 

 17 show them in the spermatogonic and first spermatocytic mi- 

 toses of Anasa; Figs. 18, 19 for the same stages in Corizits ; 

 Figs. 20, 21 for Trichpcpla ; and Figs. 22, 23 for the spermato- 

 gonic monaster and late prophase of the first maturation respec- 

 tively of Protcnor. In all these cases they can be recognized in 

 mitosis by their much smaller size. 



Recently I have found them to occur in the same number and 

 form in the ovogonia ; Fig. 15 shows a pole view of the chro- 

 mosomal plate in the ovogonium of Anasa ; Fig. 16 a similar view 

 of the spermatogonium of the same species, and the chromosomes 

 marked N and n are found to correspond exactly. 



