64 Introductory Biology 



semlbroken lines of force, known as the aster, which emanate out into 

 the cytoplasm. The centrosomes also form a small, lightly staining set 

 of fibers, known as the spindle, between them as they migrate. This 

 spindle becomes more clearly defined in later stages. The asters and 

 spindle both stain lightly and may be called the achromatic (ak ro- 

 mat' ik) figure or amphiaster (Gr. a, without; chromatin, staining well). 

 The spindle eventually occupies the position where the original nucleus 

 has been. 



P4i * _ T^ 



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Fig. 21. — Photograph of a section of the embryo of a whiteftsh showing many 

 figures of the various stages of mitosis. Note particularly the chromosomes, spindle, 

 asters, etc. (Copyright by General Biological Supply House, Inc., Chicago.) 



The nuclear membrane disappears about this time. The chromatin 

 granules within the original nucleus lose their netlike appearance and 

 form a specific number of bodies known as chromosomes. Each cell of a 

 specific species of animal or plant has a definite number of characteristic 

 chromosomes, provided the cell is normal. The chromosomes carry the 

 genes or determiners of heredity. 



Finally, the chromosomes migrate in an orderly manner toward the 

 middle of the original nucleus and arrange themselves on the center of 



