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CELL-DIVISION 



join one another. Chromatin is called by this name 

 (derived from the Greek word for color) because its sub- 

 stance colors easily and beautifully when various dyes 

 are applied to it. The stuff of which it is composed is 

 a very complex nucleo-protein, or more likely an aggrega- 

 tion of many slightly different complex proteins. The 

 nucleolus is also a protein body. It appears to be some- 



FiG. 67. — A Typical Plant Cell, from the root tip of the Spider- 

 wort (Tradescantia virginica). c.tv., cell wall; cyt, cytoplasm; n, 

 nucleus; chr, chromatin; n.«., nuclear sap; n', nucleolus; r, vacuole 

 filled with water and dissolved substances. 



what less complex in its chemical structure and is not a 

 pennanent organ of the nucleus. It is thought by many 

 to be rather of the nature of a supply of food for the 

 growth of the chromatin than an actual living organ. It 

 is certainly not an absolute necessity, since many nuclei 

 do not possess nucleoli. Still less is known about the 

 chemical nature of the nuclear sap. It seems likely that 

 it is a colloidal solution of proteins, but whether it is liv- 

 ing or not is not yet determined. The nuclear membrane 

 is now known not to be a part of the nucleus proper but 

 merely a thin membrane produced by the action of the 

 nuclear sap on the surrounding protoplasm. 



The Behavior of the Chromatin in preparation for 

 and during nuclear division is most interesting and impor- 



