m] MITOTIC FIGURE 31 



in the surrounding protoplasm, instead of being continuous 

 from one centrosome to the other; in other cases they are 

 thinner and less definite. The centrosome with its system 

 of rays is called an aster, and the two asters with the sheaf 

 of fibres connecting them are the achromatic or mitotic 

 spindle. 



When the nuclear membrane has disappeared, and the 

 spindle has been formed, the chromosomes are at first scat- 

 tered irregularly in its neighbourhood, but they soon move 

 to the equatorial or widest region of the spindle, where they 

 arrange themselves in a circle across it (PL II, /)). This 

 arrangement is very definite, the chromosomes all being in 

 one plane perpendicular to the axis of the spindle or line 

 joining the two centrosomes, and at this stage the chromo- 

 somes are said to form an equatorial plate. The details of 

 their arrangement differ according to their number and 

 shape, as will be described below. In the equatorial plate 

 stage it can sometimes be seen that to each chromosome 

 two threads or spindle-fibres have become attached, one 

 from each pole of the spindle, while other fibres run 

 continuously from pole to pole between the chromosomes, 

 and in some animals these fibres are said to be of different 

 thickness, those attached to chromosomes being thicker 

 and called "mantle fibres" (from their appearance at a 

 later stage), while those that run between the chromosomes 

 are thinner and called "interzonal fibres." 



When the chromosomes have arranged themselves across 

 the spindle in the equatorial plate, they next split longi- 

 tudinally into two similar halves, in such a way that a 

 spindle-fibre from one pole is attached to one half, and one 

 from the other pole to the other. Very frequently this longi- 

 tudinal split can be seen at a much earlier stage, especially 

 in those nuclei in which a spireme is formed; often before 



