402 Experimental Zoology 
During the early divisions of the sperm-cells, the accessory 
chromosome behaves like an ordinary chromosome, 7.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 peculiarities. 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, like 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 almost 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 
