36 CELL DIVISION [CH. 



another; if their number is small there may be one circle, 

 with or without a chromosome in the centre; when the 

 number is larger there are two or more irregular concentric 

 circles. Where the chromosomes of one group differ in size, 

 there is no constant arrangement, though it often happens 

 that similar groupings recur repeatedly in different equa- 

 torial plates ; for example, if there is one very small chromo- 

 some it often takes up a central position, while larger ones 

 are usually near the circumference, and in some species, 

 especially when the number is small, similar (homologous) 

 chromosomes tend to lie side by side or near together. It 

 has been pointed out that the grouping of the chromosomes 

 in the equatorial plate corresponds very nearly with that 

 of centres of force which repel one another, such as a series 

 of magnets floated on water and all having the same pole 

 uppermost. Like the floating magnets, the chromosomes 

 are placed at nearly even distances from one another, and 

 their arrangement in a single circle, a circle with a central 

 group, or a series of concentric circles depends on their 

 number. Especially when the chromosomes are short and 

 of nearly uniform size, the figures of equatorial plates have 

 a remarkable resemblance to those given by physicists of 

 the groupings of various numbers of magnets floating in a 

 confined space, and this fact may have some bearing on 

 the theories concerning the mechanism of nuclear division 

 which will be briefly discussed later. 



When the chromosomes begin to divide in the metaphase, 

 their behaviour again depends on their shape. In the equa- 

 torial plate the long axis of each chromosome is perpen- 

 dicular to the axis of the spindle, but in anaphase the 

 chromosome lies lengthways on the spindle, parallel with 

 the spindle-fibres. t In the case of long, rod-shaped chromo- 

 somes, when the longitudinal split is completely formed, the 



