96 GENERAL BIOLOGY 



themselves to the chromosomes. Through the pull 

 exerted by the contraction of these fibers the chro- 

 mosomes are swung into an equatorial plane. Not 

 all the fibers radiating from the centrosomes go to 

 make up the spindle proper; other radiations at 

 the sides the so-called mantle fibers attach 

 themselves to the cell-wall or lose themselves in 

 the cytoplasm. 



The next step consists in a longitudinal splitting of 

 each chromosome and the shortening of the spindle 

 fibers attached to each side so that the two half 

 chromosomes separate. That this splitting of the 

 chromosomes is only accompanied by and not caused 

 by the fastening on them of the spindle fibers is evi- 

 denced by the fact that in many instances the chro- 

 matin aggregate in the skein or " spireme " stage 

 splits precociously before the spindle has formed or 

 the spindle fibers have become attached . The divided 

 chromosomes move toward the poles of the spindle 

 (or are dragged toward them), and in the interval 

 between may be seen fibrils of the spindle in the 

 midst of which a row of granules often appears, 

 foreshadowing the formation of the new cell- wall. 

 When the chromosomes have moved to each pole of 

 the spindle, the reverse of the preparatory changes 

 previously described begins to take place, and the 

 individual chromosomes fuse together into a spireme 

 which eventually breaks up into a mass of granules 

 characteristic of the original " resting stage." 



The stages involved in mitotic cell division may be 

 made clearer by a diagrammatic summary. 



