14 



INVEETEBEATA 



CHAP. 



cases there was one more chromosome than in others. Let us denote 

 these numbers by the formulae x and x-vl. Further investigation 

 revealed the fact that this extra chromosome, termed the accessory- 

 chromosome or hetero-chromosome, does not pair with any other 

 chromosome before the first maturation division : in this division it 

 divides longitudinally into two, whilst in the second, maturation 

 division it does not divide at all, but passes to one pole of the spindle, 

 and is thus distributed to one of the two spermatids resulting 

 from that division (Fig. 6). All the ^ripe eggs showed a number of 

 chromosomes equal to the number in those 

 spermatids which had the extra chromosome. 



The zygotes which resulted from the con- 

 jugation of these eggs with the two kinds of 

 spermatozoa would show therefore 2 a; + 2 and 

 2 a; + 1 chromosomes iu their nuclei. Examina- 

 tion of the tissues in the adult insect showed 

 that the dividing nuclei of males possessed 

 2 a; + 1 chromosomes, whilst those of females 

 had 2 a; + 2 chromosomes. It seemed, therefore, 

 probable that in the case of these insects, this 

 abnormal chromosome contained some material 

 which determined the production of the female 

 sex in the zygote to which it passed. 



In other cases it was found that the 

 spermatids all contained an abnormal chromo- 

 some, but that this abnormal chromosome was 

 in some spermatids large and in others small. 

 It was shown that in the spermatocyte of the first order both 

 abnormal chromosomes were present in the same nucleus, and that 

 they both divide in the first maturation division by longitudinal 

 splitting ; but that before the second division they fuse together to 

 form a bivalent chromosome, and that in 

 the second maturation division they again 

 separate from one another, and that one 

 proceeds to each daughter nucleus ; so that 

 in the case of these chromosomes alone the 

 second maturation division is a "reducing 

 division," whereas in the case of all the 

 others the first maturation division is the 

 reducing division. These abnormal chromo- 

 somes are termed idiochromosomes (Fig. 7). 

 When two idiochromosomes are present, 

 the developing eggs always carry the larger 

 one when maturation is complete, and the 

 nuclei in the tissues of the adult female have 

 two large special chromosomes; whereas the nuclei in the tissues 

 of the adult male carry one large and one small idiochromosome. 

 Hence it is evident that the female grows from a zygote which has 



Fro. 6. — Second matura- 

 tion division of tlie 

 male germ cells of 

 Protencfr belfragei. 

 (After Wilson.) 

 /i, Accessory chromosome. 



Fig. 7. — Second maturation 

 division of the male germ 

 cells of Euschistus vario- 

 laris. (After Wilson.) 



idX, the small idiochromosome ; 

 i(Z2, the large idiochromosome. 



