112 THE STORY OF LIFE'S MECHANISM. 



or coiled (Fig. 26) ; while in other cells the 

 chromatin thread is an extremely long, very 

 much twisted convolute thread so complexly 

 woven into a tangle as to give the appearance 

 of a minute network. The nucleii differ also in 

 the number of nucleoli they contain as well as in 

 other less important particulars. Fig. 26 will 

 give a little notion of the variety to be found 

 among different nucleii ; but although they 

 thus do vary most remarkably in shape in the 

 essential parts of their structure they are alike. 



CENTROSOME. 



Before noticing the activities of the nucleus it 

 will be necessary to mention a third part of the 

 cell. Within the last few years there has been 

 found to be present in most cells an organ which 

 has been called the centrosome. This body is 

 shown at Fig. 23, g. It is found in the cell 

 substance just outside the nucleus, and com- 

 monly appears as an extremely minute rounded 

 dot, so minute that no internal structure has been 

 discerned. It may be no larger than the minute 

 granules or microsomes in the cell, and until 

 recently it entirely escaped the notice of micro- 

 scopists. It has now, however, been clearly 

 demonstrated as an active part of the cell and 

 entirely distinct from the ordinary microsomes. 

 It stains differently, and, as we shall soon see, it 

 appears to be in most intimate connection with 

 the centre of cell life. In the activities which 

 characterize cell life this centrosome appears to 

 lead the way. From it radiate the forces which 



