540 



CELL MECHANICS 



are constantly recognisable), in their cyclical staining properties, and 

 in their correlated division and repulsion cycles. They differ from 

 the centrosomes, however, in the timing of their division and in its 

 effects. How this comes about we can discover from their relations 

 with the spindle. 



Table 76 



Relationship of the State of the Centromere to the Movements 



of the Chromosomes 



State. 



I. Unpolarised. 

 [a) Inert (no 

 centric re- 

 action) . 



(6) Co -orientated. 



2. Polarised and 

 auto-orientated . 



Occurrence. 



Univalent at first meta- 

 phase. Daughter uni- 

 valent at second meta- 

 phase. 



Configurations of 

 chromosomes united 

 by chiasmata at first 

 metaphase. 



Mitotic or second meta- 

 phase chromosome or 

 univalent at first 

 anaphase. 



Behaviour. 



Lies on the spindle, but has no 

 other relationship with it. No 

 division. No movement 



except from body repulsions. 

 May pass to polarised state at 

 first anaphase. 



Two or more are orientated 

 with respect to one another 

 and to the spindle and move 

 in accordance with this 

 orientation at anaphase. No 

 division. 



Characteristic orientation and 

 congression on equatorial 

 plate. Always precedes 



division. Incapable of co- 

 orientation, even when 

 chromosomes are united by 

 pseudo-chiasmata at mitosis. 



We have already seen how they modify the shape of the spindle at 

 metaphase and anaphase, acting on it in a similar way to the centro- 

 somes. Indirect evidence of this action may be obtained from 

 irregularities of meiosis. In many plants the spindle has been 

 found to control in some way the formation of the wall between 

 the daughter-cells formed at mitosis. Its changes of shape can be 

 seen in the living cell to follow the growth of the partition (Belar, 

 1929 h). It is found that in such plants when pairing has entirely 

 failed a cell-wall or cell-partition may be formed between a pole of 

 the spindle on one side and the whole body of the chromosomes on 



