MULTIPLE CHROMOSOMES 537 



dissimilar cells from a single cell that renders a knowledge of the 

 finest details of their structure and beha\ior of the utmost im- 

 portance. That this behef is well founded would seem to be 

 clearly indicated by cumulative e\ddence from many sources. 

 The fundamental concept involved is the Roux-Weismann hypo- 

 thesis that the chromatin is the idioplasm, which is differentially 

 organized and linearly arranged. As an indication of this dif- 

 ferential organization and linear arrangement, the existence of 

 definite aggi'egates of the chromatin substance into more or less 

 thread-like chromosomes is regarded as most important. 



Details of chromosome structure and beha\dor are significant, 

 therefore, as indexes of the precision of organization in the ma- 

 terial of which they are composed. Since it is the existence and 

 perpetuation of this definite series of differentiated materials 

 that is primarily required by the hypothesis, it is conceivable, 

 and possible, that it may vary in the nature of its aggregates 

 into definite masses (chromosomes) without affecting the manifes- 

 tation of its various specific effects, except in their combinations. 

 It is not the existence of a certain number of these aggregates 

 that is of first importance, but the presence of the varied materials 

 which enter into their composition. While this is true, it is to be 

 expected that a somewhat exact correspondence should be main- 

 tained between the ultimate units of differentiated substances 

 and the units of higher order into which they are assembled. 

 But such a fundamental arrangement may be maintained in the 

 presence of both lower and higher numbers of chromosomes 

 through secondary combinations into units of still higher value 

 in one case, or through duplications of the normal series in whole 

 or in part, in the other. Numerical variation is not of itself 

 prima facie e\ddence of altered organization — it must be shown 

 that something of the complete series is lost, or new and unrep- 

 resented materials added, in order to demonstrate the existence of 

 altered organization. The maintenance of the morphologically 

 recognizable units of the original series in the face of changed 

 conditions is indeed added proof of the exactness and stability 

 of structural conditions in the chromatin substance. E\'idence 

 of a very important character in support of this position is fur- 



