166 INTRODUCTION TO CYTOLOGY 



inconstancies in chromosome number is to be found in the chromosome 

 constrictions, which under certain conditions become especially pro- 

 nounced and temporarily divide one or more of the chromosomes of the 

 group into loosely connected smaller parts. This suggestion, which 

 Sakamura supports with much direct evidence, is probably one of the 

 most fruitful which has been made in this connection. 



The theory of chromosome individuality is believed by McClung 

 and Hance to be strengthened, rather than weakened, by such instances 

 of numerical variation as those described above. McClung emphasizes 

 the point that the composition of a given chromosome can be fully under- 

 stood only if something is known of its genetic history, for what appears 

 as a chromosome may often be either an aggregation of two or three 

 chromosomes, or, on the other hand, only a portion of the true chromo- 

 some individual. How widely this interpretation may be applicable to 

 other reported cases of numerical variation and to chromosome structure 

 in general cannot at present be stated, but it promises to lead to signifi- 

 cant results. 



Discussion and Conclusions. — The author's views on the subject of 

 the individuality of the chromosomes can be most effectively stated in 

 the words of McClung (1917): 



1 . . . the practical matter before us is to decide whether the metaphase chromo- 

 somes of two cells are individually identical organic members of a series because 

 they were produced by the observed reproduction of a similar series of the parent 

 cell, or whether the resemblance is independent of this genetic relation and due to 

 chance association of indifferent materials, or to a reconstituting action of the cell 

 as a whole." 



"If it were possible for chromosomes to reproduce themselves and still pre- 

 serve their physical configuration unchanged, there would probably be little 

 question of their continuity and individuality — the demonstration would be self- 

 evident. But it happens that the necessities of the case require that each newly 

 produced chromosome should take part in the formation of a new nucleus, through 

 whose activities the cell as a whole and each chromosome, individually, is enabled 

 to restore the volume diminished by the act of division. During this process the 

 outlines of the chromosomes become materially changed and in their extreme 

 diffusion can no longer be traced in many cases. Because of our limitations in 

 observational power the} r appear to be lost as separate individuals and we are 

 thus deprived of the simple test of observed continuity. Later, in the same cell, 

 there reappears a series of chromosomes severally like those which seemed to 

 disappear during the period of metabolic activity. We confront two alternative 

 explanations for this reintegration of the chromosomes; either they actually 

 persist as discrete units of extremely variable form, or they are entirely lost as 

 individual entities and are reconstituted by some extrinsic agency. There is no 

 other possible explanation and we must weigh the facts for one or the other of the 

 alternatives. 



All the facts which indicate order and system in chromosome features speak 

 for the former, those which demonstrate variability and indefiniteness, for the 



