94 THE STORY OF THE LIVING MACHINE. 



but from this typical form they may vary, becom- 

 ing irregular or elongated. They are sometimes 

 drawn out into long masses looking like a string 

 of beads (Fig. 24), or, again, resembling minute 

 coiled worms (Fig. 21), while in still other cells 

 they may be branching like the twigs of a tree. 

 The form and shape of the chromatin thread dif- 

 fers widely. Sometimes this appears to be mere 

 reticulum (Fig. 23) ; at others, a short thread which 

 is somewhat twisted or coiled (Fig. 26) ; while 

 in other cells the chromatin thread is an extremely 

 long, very much twisted convolute thread so com- 

 plexly woven into a tangle as to give the appear- 

 ance 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 



