THE GENERAL OUTLINE OF MITOSIS 17 



where there are considerable size -differences it is 

 always the smaller chromosomes which occupy the 

 centre of the spindle (Fig. 46).* All these types of 

 arrangement can be explained if the generalized 

 repulsion between the surfaces of the chromosomes 

 referred to earlier persists throughout metaphase, 

 keeping the chromosomes with their associated 

 spmdle elements at a certain distance from one 

 another. 



In the case of the peripheral chromosomes it is 

 clear that they are attached by a single short region 

 to the spindle, so that their long arms float freely in 

 the cytoplasm outside the spindle, perhaps covered 

 by a layer of spindle-substance.^^ The region of 

 attachment corresponds to one of the constrictions 

 seen during prophase ; this constriction, the spindle- 

 attachment, is thus a permanent cell-organ which, 

 although visually similar to the other or secondary 

 constrictions, behaves entirely differently. Although 

 the smaller central chromosomes are usually entirely 



* There are a few exceptions to this rule. The clearest 

 case is that of the Tree Cricket Oecanthus longicaiida "' where 

 the smaller chromosomes take up a peripheral position, with 

 the larger ones in the centre. The same thing happens in 

 many hybrids such as that between the moths Biston hirtarius 

 and Nyssiazonaria, where the chromosomes of the two parent 

 species are different in size and the large ones of hirtarius 

 take up a central position in the hybrid. ^^ 



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