428 F. R. ROWLEY ON SOME POINTS IN THE STRUCTURE 



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probably the reason why this stage is more frequently met with 

 than any other, and, during the interval, the two halves of each 

 of the chromosome loops are separated from one another, and 

 arranged in readiness for distribution to the two daughter 

 nuclei. The two centrosomes are visible as pale spherical bodies 

 lying close to the polar surfaces of the spindle. 



The division of the nucleus is heralded by a change in the 

 shape of the equatorial ring, whose polar surfaces, where in 

 contact with the central spindle, are drawn out in the direction 

 of the poles. The divided chromosomes are now finally parted 

 by a line of division at right angles to the axis of the spindle, 

 and the two halves begin to recede from one another. 



The nuclei having arrived at the poles of the spindle, a notice- 

 able feature is the appearance of two large masses between the 

 nuclei and their respective centrosomes (Fig. 10, s/>). According 

 to Lauterborn these are really the polar ends of the spindle 

 separated from the more central portion by constriction, due to 

 the diminution in size of the central aperture in each chromo- 

 some ring. 



Up to this point the interzonal fibres have been very sharply 

 and clearly defined, but with the first appearance of a line of 

 division in the cytoplasm, they assume a wavy aspect with a 

 slight thickening in the equatorial part. Shortly afterwards 

 complete partition of the cell contents takes place (Fig. 11), and 

 the fibres then vanish entirely, perhaps passing through the 

 central aperture of the chromosome ring into the remnant of the 

 polar end of the spindle. The nucleus viewed endways at this 

 stage exhibits a slit-like central aperture in place of the approxi- 

 mately circular one noticed in the equatorial ring. 



Division of the cell is now^ to all intents and purposes complete, 

 and the nuclei have only to undergo a process of reconstruction 

 as they pass into the resting condition. At first they are curved 

 in shape, with their concave surfaces turned towards the broad 

 end of the cell near to which they are still lying (Fig. 12). The 

 chromatin is now present in a granular condition, the central 

 cleft being surrounded by a dense zone, from whence fibrils, 

 studded with darkly-stained granules, pass at regular intervals 



