and while there is no doubt that half chromosomes can separate without 



any apparent spindle, the nature of the force still remains unexplained. 



The biochemical approach to the problem of the spindle seems more 



Figure 6-4. Phase-contrast Photomicrograph of Mitotic Apparatus Iso- 

 lated from Eggs of the Sea Urchin, Strongylocentrotus pmpiiratus Using Cold 

 Ethanol-digitonin. (Courtesy of Dr. Daniel Mazia, University of California, 

 Berkeley. ) 



likely to provide precise information concerning both its nature and 

 function. Mazia and Dan (1952) have developed a method of isolating 

 the mitotic apparatus from sea urchin eggs (Figure 6-4) which has 

 permitted extensive studies to be made on the chemical composition of 



MECHANICS AND PHYSIOLOGY OF CELL DIVISION / 145 



