No. 524] CHROMOSOMES AXD HEREDITY 



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division. This interpretation has met with some oppo- 

 sition. Every cytologist is familiar with the fusion of 

 the chromatin threads in the resting nucleus. If they 

 fuse, what guarantee is there that they will separate 

 again along the exact lines of union? If the separation 

 is not exact the materials of the chromosomes would, 

 before long, become completely intermixed. It is this 

 difficulty that has created a presumption against the 

 theory of the individuality of the chromosomes. 



Despite the supposed objection the fact remains indis- 

 putable that in cells where the chromosomes can he dis- 

 tinguished by their distinctive sizes, the same sized 

 bodies emerge after every supposed fusion in the resting 

 nuclei. The most convincing evidence for individuality 

 in this sense is that brought forward by Boveri's study 

 of the position and shape of the chromosomes as they 

 emerge from the nucleus at the two-cell stage of Ascaris. 

 He shows that there is often a remarkable agreement 

 between the chromosomes in the two sister cells which 

 can only be explained on the grounds that the chromo- 

 somes have retained in the resting stage the same form 

 and position that they had when they went into the rest- 

 ing nucleus, and this arrangement can be traced back to 

 the way in which the chromosomes divided in the segmen- 

 tation spindle. 



This evidence points to the conclusion that the central 

 part at least of the chromosomes has not been lost by 

 fusion in the resting stage. It is important to note that 

 we can not explain their reappearance after each resting 

 stage by means of the assumption that they differ chem- 

 ically and segregate according to their kinds of mate- 

 rials, because in each nucleus there are two chromosomes 

 of the same sort, one paternal the other maternal in 

 origin, but identical otherwise. The pairs may lie in any 

 position with regard to each other in the resting nucleus. 

 Hence like chromosomes they might often interlace, and 

 there is no guarantee that later these materials would 

 move into the two original chromosomes rather than con- 

 centrate around one of the two centers. It has been sug- 



