26 



THE CELL AS A FUNDAMENTAL UNIT OF LIFE 



In the meantime, the nuclear membrane fades from view and the 

 chromosomes appear to be free in the cytoplasm. The centrosome, if 

 present, is forming the spindle figure which is to function in the division 

 of the chromosomes. The two little dot-like centrioles within the 

 centrosome begin to spread apart and spindle fibers appear between 

 them to form the spindle figure. Delicate microdissection studies in 



# 



% 





Courtesy General Biological Supply House 



Fig. 2.4. Photo of mitosis in the eggs of the whitefish. A resting cell, prophase, 

 metaphase, and telophase can be seen in this group. 



which extremely tiny needles are inserted into the cell indicate that these 

 are not true fibers, but instead represent lines of force within the 

 protoplasm. It has been suggested that the fiber-like appearance is 

 due to the lengthwise orientation of molecules in these lines of force. 

 As the spindle figure becomes larger it spreads down over the center 

 of the cell and engulfs the chromosomes. The chromosomes then be- 

 come arranged in the form of a disc within the center of the spindle 

 at the end of the prophase. 



