196 McCLUNG. [Vol. II. 



While there exist these main points of agreement between 

 the observations on Pcntatoma and XipJiidiiini, minor differ- 

 ences may be noted. Thus there seems to be no reason to 

 suppose that the accessory chromosome oi^Xiph^diuJn arises 

 by the direct transformation of one of ^he ordinary ones, al- 

 though such a change may be possible. This does not argue 

 against the chromatic origin of the body, however, for it is 

 almost certainly modified chromatin, but in XipJiidiiini it arises 

 during the resting stage and may represent derivative substance 

 from one or all the chromosomes. 



Again, its relative importance is much greater and its be- 

 havior more marked in XipJiidiitni than in Pcntatoma. The 

 constancy of form and structure appears to be less pronounced 

 in Montgomery's object. As far as can be told, the staining 

 reaction is essentially the same in both objects, and shows the 

 same constancy that Wilcox noted for his "nucleolus." The 

 final disposition of the body is a question that has not been 

 decided in any case, and is one of great importance. It will, 

 perhaps, require a knowledge of the steps in fertilization to 

 decide positively the true character of the accessory chromo- 

 some. 



Regarding the name to be applied to this structure, it would 

 seem much more reasonable to class it with the chromosomes 

 than with the nucleoli. The indefinite character of the latter 

 group of bodies makes it desirable to avoid confusing the 

 nomenclature by the addition of any more varieties to those 

 already existing. But more important than this is the fact 

 that during the time that chromosomes exist as such in the 

 cell, the accessory chromosome is practically indistinguishable 

 from the others in its behavior. It is a unit, a chromatic unit, 

 constant in character and nearly typical in origin, transforma- 

 tion, and final disposition, as is believed, and corresponds well 

 to the definition of a chromosome given by Montgomery, i.e. : 

 "A chromosome is each separate chromatin element (chromatin 

 microsomes imbedded in, or sheathed by, linin) formed in the 

 prophases of mitosis by transverse segmentation of the spireme 

 thread, or which, in those cases where a continuous spireme is 

 not formed, segregates as a separate element from the chro- 



