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CELLS, TISSUES, AND ORGANISMS 



for some time, and then were returned to a normal medium (sea 

 water ) , they did not merely recover from the block but divided directly 

 into four cells. Thus the blockage was not only reversible but appeared 

 to yield a small profit. The superficial image of what was happening to 

 bring about this four-way mitosis is shown in Figure 2. Here we see 

 that the centers at each pole of the original mitotic figure split, and the 



Figure 2. Formation of a 4-polar mitotic iigure in sea-urchin eggs blocked at metaphase 

 by mercaptoethanol. After various times in mercaptoethanol, the eggs were removed to 

 sea water for ten minutes, then "fixed" for isolation of the mitotic apparatus. The figures 

 showed isolated mitotic apparatus in phase contrast. 



A: 15 minutes in mercaptoethanol. No visible splitting of centers. 



B: 30 minutes in mercaptoethanol. Centers have begun to split. 



C: 45 minutes in mercaptoethanol. Four poles well defined. Chromosomes in metaphase 



plate. 

 D: 60 minutes in mercaptoethanol. A fully developed tetrapolar figure, three poles in 



focus. 



(After Mazia, Harris, and Bibring, 1960.) 



