CELL DIVL'^ION 



73 



it is not made of cliroiuatin, but of linin and fibers from the cytoplasm 

 resemliling linin, it may also be called the achromatic figure. That i)or- 

 tion of the entire division figure which is made of chromatin is called the 

 chromatic figure. The lines of the asters and of the spindle have by some 

 been considered to be fibers. By others they are thought to be granules 

 arranged along lines of force as iron filings arrange themselves in a mag- 

 netic field. Because of the supposed arrangement of the granules along 

 these lines they have the appearance of fibers. Still other observers 

 think the apparent fibers may represent lines of flow of the interal- 

 veolar substance of the protoplasm, which is in more or less straight lines 

 due to rearrangement of the alveoli. The centrosomes, with their radiat- 

 ing fibers, migrate to opposite poles of the nucleus. The spindle fibers 

 may persist and penetrate among the coils of the chromatin thread, or 

 the original spindle fibers may disappear and new fibers arise. 



Fig. 39. — -Dividing cell with the chro- 

 mosomes arranged in an equatorial posi- 

 tion, as seen from one of the poles. Late 

 prophase. 



Fig. 40. — Dividing cell with chromo- 

 somes arranged in an equatorial plate, the 

 same stage as Fig. 39, but viewed from 

 the side. Late prophase. 



Division of the centrosome may occur as indicated above or in many 

 species it may occur during or soon after the conclusion of the telophase 

 preparatory to the next division. 



Equatorial Plate. — -If the chromatin thread has not broken into frag- 

 ments at an earlier time-it now segments into a number of pieces, called 

 chromosomes (Fig. 38). The chromosomes come to lie in the plane of 

 the equator of the spindle. What force brings them into this position 

 is not determined. Some investigators think that certain fibers of the 

 spindle are attached to the chromosomes and to the mass of cj'toplasm 

 immediately surrounding the centrosome and that these fibers pull the 

 chromosomes into position. Others think that they may migrate by 

 some sort of amoeboid movement, while others suggest that electro- 

 magnetic forces are responsible for the movement. Seen from a polar 



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