EARLY 



Cell Division in Relation to Reproduction - 43 

 LATE 



CENTRIOLES 

 ASTER- 



PROPHASES 



METAPHASES 



EARLY 



LATE 



NUCLEAR MEMBRANE 

 RE-FORMING 



ANAPHASE 



TELOPHASES 



Fig. 3-2. Diagram depicting modern concept of mitosis as observed in a typical animal cell 



system of protoplasmic fibers, collectively 

 called the spindle. The mitotic centers are 

 regarded as specialized parts of the cyto- 

 plasm, situated at the ends, or poles, of the 

 spindle, although these regions sometimes 

 are not very clearly delineated, especially in 

 plant cells. The fibers toward each end of the 

 spindle always converge, however, toward a 

 definitely localized center. Moreover, in 

 larger animal cells at least, radiating from 

 each mitotic center there is a stellate system 

 of protoplasmic fibers, collectively designated 

 as the aster (Fig. 3-2). Also in most, if not 

 all, animal cells, a pair of short rodlike 

 bodies, the centrioles, can be demonstrated 

 occupying a focal position in the mitotic 

 center (Figs. 3-2 and 3-5). 



The centrioles are "self-replicating" struc- 

 tures. They appear to focalize the dynamic 

 activities of the mitotic center during the divi- 

 sion of animal cells generally. The spindle 



always stretches between the centriolar re- 

 gions, and the plane of division regularly cuts 

 across the center of the spindle at right 

 angles to the spindle axis (Fig. 3-2). More- 

 over, the two short rodlike units that consti- 

 tute each pair of centrioles always lie at right 

 angles to one another. Each unit of the pair, 

 as seen at high magnifications in favorable 

 electronmicrographs, consists of a compact 

 bundle of 27 fibers (perhaps tubules), ar- 

 ranged concentrically in groups of three 

 (Fig. 3-5). Sometimes the units of each pair 

 of centriolar bodies replicate themselves very 

 early, toward the end of one mitosis, in prepa- 

 ration for the next; but in other cases replica- 

 tion is delayed until early prophase of the 

 next division. In any event, the sister pairs 

 of centrioles, after replication has occurred, 

 move to opposite sides of the prophase nu- 

 cleus, and the spindle begins to appear be- 

 tween the diverging sister pairs (Fig. 3-2). 



