

V 



Cellular Structure and Activity 



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Fig. 10. Mitosis in the whitefish blastula (from Turtox slides) 



seem to fall outside of the general picture 

 (but cf. DeLamater and Mudd, '51). 



The divisions of the blastomeres in the 

 whitefish blastula are useful to describe the 

 morphological aspects of mitosis in an ani- 

 mal cell (Fig. 10). Following Strasburger 

 the process is subdivided into four phases: 

 During prophase the centrosome divides (Fig. 



10^); the centrioles are not visible in these 

 preparations but can be demonstrated in slides 

 stained with iron-hematoxylin (Fig. 11). The 

 chromosomes now appear embedded in eosin- 

 ophilic material that later forms the spindle 

 (Figs. lOA and B) and finally the nuclear 

 membrane breaks down (Fig. IOC). While 

 the spindle takes shape the chromosomes be- 

 come oriented in the equatorial plane (Fig. 

 107)): metaphase. During anaphase the 

 chromosomes move toward the spindle poles 

 and the spindle elongates (Figs. lOE, F, G). 

 Finally, in telophase the nuclear membrane 

 reappears, in this case around each chomo- 

 some separately, so that karyomeres are 

 formed which later may fuse into a single 

 nucleus. The cytoplasm is subdivided by 

 the cleavage furrow (Fig. lOH). 



This account of mitosis in the whitefish 

 blastomere has been pieced together from 

 various stages in fixed and stained cells. Is 

 it possible to follow the process in the living 

 cell as it divides? If we place the blastula of 

 a whitefish egg during cleavage in calcium- 

 free Ringer's solution and gently squeeze it 

 in a rotocompressor,* the cells separate and 

 a single layer of cells can be obtained. For 

 some time the blastomeres continvie to divide 



Fig. 1 1 . Prophase in blastomere of whitefish, stained 

 with iron-hematoxylin to show centriole. 



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* Obtained from Biological Institute, Philadel- 



