THE SPERMATOGENESIS OF THE MYRIAPODS. 213 



separation of the chromatid occurring along the equatorial plane 

 at the place of constriction. This very evidently represents a 

 cross division of the chromosome. 



In the division figures of one half of the second spermatocytes, 

 all the chromosomes are of one type (the dumb-bell form), the 

 accessory chromosome not being present. Thus it will be readily 

 seen that the cells arising from two spermatocyte mitoses are di- 

 vided into two classes of equal numbers those which possess 

 the accessory and those which do not. Similar phenomena have 

 been observed in the cells of a number of insects by Henking, 

 '90, Paulmier, '99, McClung, 'oo, '02, de Sinety, '01, and others. 



Regarding the function of the modified chromosome, two 

 theories have now been advanced. Paulmier in his paper on 

 Anasa puts forth the theory that the " small chromosome repre- 

 sents characteristics which are being eliminated from the race." 

 He bases this conclusion entirely upon the failure of the ele- 

 ment to divide in one spermatocyte division. Montgomery in 

 his later papers adopts the conclusions of Paulmier and believes 

 with him that it is a chromosome undergoing the process of 

 elimination. 



McClung, '02, however, in a paper in which he considers in 

 detail all of the reported observations upon the accessory chromo- 

 some, formulates an hypothesis which ascribes a very different 

 function to this element. He maintains that the mere fact of the 

 unequal apportionment to the spermatozoa would not necessarily 

 indicate that the element is degenerating, and in addition there 

 are other facts which militate strongly against such a conclusion. 

 The extreme nicety with which this element is excluded from 

 contact with the others in most stages, especially in the sper- 

 matogium, would seem to indicate a very different and much 

 greater significance. This exclusiveness, taken in connection 

 with the fact that exactly one half the spermatozoa contain the 

 accessory chromosome, suggests the theory that it has to do with 

 the determination of sex, as this is the only respect in which the 

 progeny are divided into two classes of equal numbers. Although 

 no positive proof is advanced to support this theory the author 

 establishes in a very logical manner the probability of the acces- 

 sory chromosome representing such a function. It seems to 



