CELLS IN DIVISION 



the second of these papers are sufficient to show an arrangement 

 within the interphase nucleus strikingly similar to the bouquet stage of 

 meiosis. Both authors agree that at stages of mitosis after prophase, the 

 chromosomes do not readily migrate cataphoretically. McGlendon 

 used a stronger current than did Hardy, and found that the spindle 

 and chromosomes could be moved as a unit, but that the relative 

 displacement of the chromosomes in anaphase was unaffected. 



One of the subsidiary objections that have been urged to the theory 

 that the chromosomes move in anaphase under electrical forces is that 

 on simple inverse square repulsion, it is unlikely that charges of sufficient 

 magnitude to account for the observed rate of movement could be 

 maintained in a conducting medium. This argument does not neces- 

 sarily apply to the orientations of the chromosomes in interphase or 

 in the early stages of mitosis. Harris^^'^ discovered a polarity in the 

 oocyte oi Echinus esculentus which affected the rate of fall of the nucleolus 

 through the nuclear sap; the velocity in opposite directions showed a 

 constant difference. He calculated that to account for this polarity, a 

 potential difference of 1*5 mv would be enough, which could be 

 maintained by an expenditure of energy roughly equivalent to 2 per 

 cent of that of the whole metabolism of the cell. 



It would be desirable to extend the electrical experiments of 

 McGlendon and Hardy to meiotic nuclei which were directly observed 

 in life, to see whether potential differences can affect either the migra- 

 tion of the centrosomes, or their apparent influence on the chromosomes 

 within the nucleus. 



THE spindle 

 Structure of the spindle 



The living spindle is a clear, apparently structureless body usually 

 visible only because it is outlined by the granules and inclusions of the 

 cytoplasm. Nevertheless, it is not structurelejs. It is birefringent; small 

 particles in it show unidirectional Brownian movement; on dehydra- 

 tion it shrinks more in width than in length, and on fixation it becomes 

 fibrous in appearance (Belar^*^). There is not the least doubt that it 

 consists of elongate particles arranged lengthwise, and the argument 

 that the fibrous appearance is an artefact because it only appears 

 after fixation, need not be taken seriously. It is certainly an artefact, 

 but as Darlington^*^ has pointed out, so is everything in a fixed cell. 

 Spindle fibres are not produced solely by fixatives, but by their action 

 on an elaborate structure of orientated protein molecules. The problem 

 is to decide what sort of orientated structure existed before fixation. 



The fact that the living spindle looks structureless in ordinary light, 

 shows, of course, that it does not contain microscopic fibres of high 

 density. The ordinary microscope, however, is not very sensitive to 



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