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HANDBOOK OF PHYSIOLOGY. 



or threads then shorten and become thicker, while the convolutions, 

 which have become less numerous, arrange themselves in a series of con- 

 necting loops, forming the ivreath. The nuclear membrane and the 

 nucleolus disappear, the latter passing at times into the cell protoplasm 

 and disintegrating. The wreath then breaks up into V-shaped segments, 



Fine uniting' 

 filaments. 



FIG. 15. Stages of karyokinesis. Ulabl. ) A. Commencing separation of the split chromosomes. 

 B. The separation further advanced. ('. The separated chromosomes passing along the libres of the 

 achromatic spindle. 



the chromosomes, of which each species of animal has a constant and 

 characteristic number. This varies from two to thirty-six in the differ- 

 ent animals, but is sixteen in man. 



The two centrosomes migrate to the poles of the nucleus, while the 

 achromatic spindle which connects them occupies the long axis of the 



Remains of spindle. 



Line of separation 

 of the two cells. .. 



Antipole of daugh- 

 ter nucleus. 



Lighter substance 

 of the nucleus. 



Cell protoplasm. 

 Hilus. 



Fig. 16. Final stages of karyokinesis. In the lower figure the changes are still more advanced than 



in the upper. (Waldeyer.) 



nucleus. The chromosomes, becoming much shorter and thicker, gather 

 around the spindle in its equatorial plane, with their angles directed 

 toward the centre, forming the aster or monaster. 



The actual division of the nucleus is begun at this time (inetaphases) 

 by the splitting of each chromosome longitudinally into halves which lie 

 at first close together so that each seems doubled. Soon afterward the 

 fibrils of the achromatic spindle begin to contract, and thus separate the 



