

MECHANISM OF CELL DIVISION 285 



The Mechanism of Cell Division is very much bet- 

 ter understood than the stimulus which brings it about. 

 In the great majority of cells division begins with the 

 nucleus, which undergoes a complicated process known 

 under a variety of names, nuclear division, mitosis, karyo- 

 kinesis. This process can be best studied in the growing 

 animal embryo or in the growing regions at the tips of 

 roots (Fig. 66) or stems. Before we can understand the 



Fig. 66. — Cells from the Growing Region of the Root Tip of an 

 Onion. Note carefully the different sizes and shapes of the cells 

 and that the chromatin is in a different stage of development in 

 different cells. 



process it will be necessary to consider the structure of 

 the nucleus. 



The Resting Nucleus in most cases consists of a sur- 

 rounding envelope known as the nuclear membrane, a 

 semi-fluid sap, a network of chromatin, and a nucleolus 

 (Fig. 67). The most important part, at least the most 

 permanent part, of it is the chromatin. In the cell which 

 is not in a state of division the chromatin appears in 

 stained specimens to consist of irregular threads which 

 frequently intersect. They are of very irregular size and 

 shape and almost always much thickened wherever they 



