STUDIES ON CHROMOSOMES 93 



first spermatocyte-division, and passes with one half of the bivalent 

 to one pole. Since the spermatogonial number in Leptynia (36) 

 is an even one and twice that of the separate chromosomes present 

 in the first spermatocyte-division, it may be inferred that the 

 X-element is already united with one of the ordinary chromo- 

 somes in the spermatogonia, though Sinety does not state this. 

 Somewhat later McClung ('05) discovered essentially similar rela- 

 tions in the grasshoppers Hesperotettix and Anabrus (fig. 6, 

 c-e),and in case of the first named form was able to establish the 

 important fact that it is always the same particular bivalent with 

 which the X-chromosome is thus associated. In respect to the 

 intimacy of this association, a progressive series seems to exist, 

 since in Leptynia it seems to take place in the spermatogonia, in 

 Hesperotettix only in the prophases of the first spermatocyte- 

 division, while in Thyanta the union is only effected after the 

 first division is completed. 



Finally, the recent observations of Boring ('09), Boveri ('09) 

 and Edwards ('10) seem to establish the fact that in Ascaris megalo- 

 cephala the X-element, whether in the diploid groups or in the 

 maturation-divisions, may either appear as a separate chromo- 

 some (which has the usual behavior of an accessory chromosome) 

 or may be indistinguishably fused with one of the ordinary chromo- 

 somes. 



These relations may, of course, be the result of a secondary 

 coupling; and I myself formerly so interpreted them ('09c). But 

 in view of what is seen in Thyanta or the reduvioids we may 

 well keep in mind the possibility that they are expressions or 

 remnants of a more primitive association, like that which I have 

 assumed for an original XY-pair. Whatever be their origin, the 

 effect is the same a definite linking of the X-chromatin with 

 that of one of the other pairs. 



Fig. 7 shows, in purely schematic form, the general conception 

 of these relations that has been suggested above, the X-chromatin 

 being everywhere represented in black. A is the primitive XY- 

 pair from which all the other types may have been derived. By 

 simple reduction of such a pair arises the ordinary or typical 

 idiochromosome-pair (B) ; and from either A or B may be derived 



