ELEMENTARY VITAL PHENOMENA 



195 



This mode of mitotic nuclear division is the same in the different 

 forms of cells, almost without exception and even to the finest 

 details. But the division of the cell as a whole does not always 

 proceed in exactly the same manner. Deviations from the type 

 occur in various cases, especially in the division of egg-cells that 

 contain much nutrient material (yolk). With O. Hertwig ('92) 



FIG. 74. /. Division of the frog's egg. P, Pigmented surface of the egg ; pr, protoplasmic pole ; 

 d, pole rich in yolk ; sp, nuclear spindle. (After Hertwig.) //. Unequal division of the egg 

 of a worm (Fabricia). A, Protoplasmic pole ; V, pole rich in yolk. (After Haeckel.) 



all known forms of cell-division can be conveniently classified under 

 four types 



I. Total division. 



a. Equal division, 

 ft. Unequal division. 

 c. Gemmation. 

 II. Partial division. 



III. Multiple division. 



IV. Reducing division. 



In total division, the protoplasm of the daughter-cells is com- 

 pletely divided by a partition, so that complete cells always result 

 from the division. But certain differences are here noticeable. 

 In one case, that of equal division, the daughter-cells are entirely 

 equal, as in the type described above (Fig. 72, F). In another 

 case, that of unequal division (Fig. 74), the two daughter-cells are 



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