402 Experimental Zoology 



During the early divisions of the sperm-cells, the accessory 

 chromosome behaves hke an ordinary chromosome, i.e. it divides 

 into equal parts at each division. But in the two final divisions 

 of the male cells, — the spermatocyte divisions, — the accessory 

 chromosome shows certain pecuHarities. It can sometimes be 

 distinguished from the other chromosomes by its slightly differ- 

 ent affinity for stains, by its inertness in one of the divisions, by 

 its forming at times a separate vesicle, and especially by the 

 fact that at one of the spermatocyte divisions it fails to divide 

 when the other chromosomes divide. It passes bodily to one 

 pole of the spindle ; hence only half of the spermatozoa contain 

 the accessory. 



It has been shown by Wilson with great probability that the 

 failure of the accessory to divide, when the others do so, is due 

 to the fact that it has no mate, as have all the other chromosomes, 

 so that when the other pairs separate the accessory remains 

 single. It has been suggested by other observers that the acces- 

 sory is a double chromosome, Hke the other pairs, but that it 

 fails to separate into its components when the others so separate. 

 This view seems improbable, because Wilson has found a con- 

 tinuous series of species in some of which the accessory has a 

 partner of equal or of slightly smaller size, while in others the 

 difference in size is very unequal — one being almo'st a vanish- 

 ing chromosome. We can imagine that one step farther would 

 lead to the complete disappearance of the mate of the acces- 

 sory, so that it would have no pair. Whether the disappear- 

 ance is really due to its vanishing away, or to the absorption 

 by its mate, we cannot even surmise with any degree of 

 probability. If the latter view should prove correct, the 

 accessory is a double or fused chromosome, but not formed 

 by temporary union as are the others at one time, but by a 

 permanent fusion. 



McClung offered the suggestion that the difference between 

 the two kinds of spermatozoa, with and without the accessory, 

 is connected with sex determination. His argument was that 

 since there are two kinds of sexual individuals, and two kinds 



