VITAL PROPERTIES OF CELLS 



19 



vancing periphery. Division of such an enlarged cell into i\vo smaller 

 cells reestablishes the original and more favorable nuclco-cytoplasmir 

 dimensional relationship. 



(a) AMITOSIS. In typical amitosis the nucleolus first heroines bi- 

 lobed and then divides ( Fig. 24). This is followed by nuclear division. 



^ 





FIG. 25. SUCCESSIVE STAGES IN THE AMITOTIC DIVISION OF THE CILIATED CELLS 

 LINING THE VASA EFFERENTIA OF THE EPIDIDYMIS OF THE MOUSE. X 1500. 



- i . . 



each resulting nucleus enclosing one of the nucleoli. Nuclear division is 

 followed by cytoplasmic division. A centrosome is generally neither 

 active nor even visible during this process. This typical condition is 

 rarely realized. It was first described by Kemak (1841) for blood-cells. 

 Usually nuclear division is in- 

 dependent of nucleolar fission, 

 which may be lacking (Fig. 

 25). The nuclear fission pro- 

 ceeds variously by a medial or 

 submedial annular constric- 

 tion, or by progressive linear 

 indentation of some portion of 

 the surface. In certain in- 

 stances the division takes place 

 inside of the original nuclear 

 membrane. The nuclear prod- 

 ucts may be of unequal size, 

 and multiple (Fig. 26). Gen- 

 erally cytoplasmic division lags FlG ' 26. MULTINUCLEATED GlANT CELL, 

 n T n , . . . FROM THE \ OLK-SAC OF A 10 MM. PIG 



tar behind nuclear division, or EMBRYO. X 2000. 



may even fail to appear, thus 



producing hi- or multinucleate cells. Amitosis effects a mass division of 

 the nucleus ; neither spireme nor chromosome nor achromatic spindle, so 

 conspicuous in mitosis, appear. Until quite recently amitosis was gen- 

 erally regarded as a relatively rare and unimportant process. It was 



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r ' X 



6 



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