CELLS IN DIVISION 



We are better able to realize the nature of this problem now that it is 

 understood that the unit of structure in the living aster and spindle is 

 not the microscopically visible ray, but the sub-microscopic micelle; 

 though the shift of the inquiry to a lower order of magnitude reveals 

 how formidable are the difficulties in the way of investigation. There 

 is some evidence that nucleic acids may be found at the centre of 

 orientation; this is suggested by the basophily of the centriole, and 

 also by some of the model experiments of Fisgher.^os He impregnated 

 pieces of elder pith with protein solutions which were then fixed and 

 sectioned. 'Astral figures' were common within the coagulated protein 

 of these preparations, and frequently the remains of the nucleus of the 

 pith cell was seen at their centres {Figure 39). The fact that in the 

 development of the spindle, the centromere also serves as an orientating 

 centre is in accordance with this hypothesis. Yuasa^^*' ^^^ ^^^ finds 

 that although the centrosome of many cryptogamic plants is derived 

 from the nucleus, it is Feulgen-negative ; he considers it to be of the 

 same nature as the nucleolus. If it is generally true that under appro- 

 priate circumstances a granule of ribonucleoprotein can orientate the 

 surrounding cytoplasm, this would possibly explain why accessory 

 cytasters can be so readily induced in the sea-urchin egg. It seems, 

 however, that a nuclear component is also involved, for as Fry^^^ says 

 *if eggs are immature, neither sperms nor artificial agents produce 

 asters, since they do not arise before the germinal vesicle has released 

 nuclear substances into the cytoplasm'. 



CHROMOSOMES AND THE ACHROMATIC FIGURE 



Close study of time-lapse films of chick and mammalian cells in mitosis 

 shows that towards the end of prophase two changes happen at the 

 same time : at opposite ends of the nucleus the asters appear, and then 

 the chromosomes begin to move. The further sequence of events varies 

 in detail; either within the still intact nuclear membrane the paired 

 chromatids congregate opposite each aster (Plate XII (16) ), or, if the 

 membrane is at once broken down, the spindle is completed and the 

 chromosomes are then drawn towards the whole achromatic figure 

 (Fell and Hughes224) (Plate XI (14)). 



Cells in meiotic division provide other and better known examples of 

 the orientation of chromosomes towards asters, which are all the more 

 remarkable because the influence, of whatever kind it is, must be 

 exerted across an intact nuclear membrane. In the bouquet stage of 

 meiotic prophase the ends of the chromosomes at zygotene or pachytene 

 are bunched together opposite the centrosome (Figure 40) ; but this is 

 not invariably seen, and is commoner in animals than in plants. The 

 chromosomes in the bouquet usually consist of thin loops with their 



III 



