ELEMENTARY VITAL PHENOMENA 



193 



h. Indirect Cell-division 



By far the great majority of all animal- and plant-cells follow the 

 mode of the so-called indirect or mitotic cell-division, in which the 

 protoplasm is simply constricted, while the nucleus undergoes 

 very remarkable and typical changes of great regularity. Different 

 authors have distinguished different stages and have designated 

 them by different names. Two phases in nuclear division can be 



D E F 



Fig. 72. — Scheme of mitotic cell-division. (After Flemming.) 



very generally recognised — a progressive one, in which the changes 

 reach their height, and a retrogressive one, in which the two 

 nuclear halves that arise from the division go back to the 

 " resting-stage " of the nucleus, which latter term designates the 

 condition in which the nucleus shows no phenomena of division. 

 A picture will put before oar eyes the important phenomena 

 of nuclear division better than all classifications and descriptions 

 (Fig. 72). 



To begin with the resting nucleus about to undergo division, it 

 is seen that the chromatic substance, which, as is well known, 



