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BIOLOGY: GENERAL AND MEDICAL 



distinct dot. As the chromatin increases in quantity 

 prior to division it comes to fill the greater part of the 

 nucleus and the nucleolus disappears. Soon the chroma- 

 tin increases in filamentousness until a single long some- 

 what spirally coiled thread the spirem is found. When 

 examined under most appropriate conditions this thread 

 has a fuzzy appearance which is supposed to depend 

 upon an incomplete differentiation of the chromatin from 

 the linin, which clings to it. Soon, however, the linin 



v. VL vii. viii. 



FIG. 31. Karyokinesis of a cell with four chromosomes. I, Resting nucleus 

 with centrosome above. II, Formation of the spirem, and beginning division 

 into chromosomes; division of the centrosome and formation of the nuclear 

 spindle. Ill, Elongation of the nuclear spindle to the poles of the cells; division 

 into chromosomes complete. IV, Longitudinal cleavage of the chromosomes. 

 V, Separation of the chromosomes. VI, Complete separation of the chro- 

 mosomes which are clustering about the centrosomes, forming daughter stars. 

 VII, Double spirem formation and beginning division of the cytoplasm. VIII, 

 Daughter cells resulting from the completed changes. (After Schaeffer.) 



separates completely, after which it is observed that 

 the chromatin no longer forms a single thread, but has 

 broken into a number of segments of uniform length, 

 which are the chromosomes. These bodies are of great 

 interest from many points of view, and are believed by 

 Weismann and others to endow the offspring of the cell 

 with its chief hereditary impulses. The number of 

 chromosomes is exactly the same in all cells of the same 

 kind, though it varies in different kinds of cells. In 

 complex organisms there is also a difference between 



