VITAL PROPERTIES OF CELLS 



19 



vancing periphery. Division of such an enlarged cell into two smaller 

 cells reestablishes the original and more favorable nucleo-cytoplasmic 

 dimensional relationship. 



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

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



tm m 



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

 LINING THE DUCTULI EFFERENTES OF THE EPIDIDYMIS OF THE MOUSE. X 1500. 



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 Remak (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 

 far behind nuclear division, or 

 may even fail to appear, thus 

 producing bi- 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 



FlG. 26. MULTINUCLEATED GlANT CELL, 

 FROM THE YOLK-SAC OF A 10 MM. PIG 

 EMBRYO. X 2000. 



