16 GENETICS 



wall or membrane which serves to separate one cell 

 from another. Within the protoplasm there may 

 be a considerable amount of non-living substance 

 in the form of salts, pigments, oil-drops, water, and 

 other inclusions of various kinds. 



The nucleus is to be regarded as the headquarters 

 of the whole cell, since changes which the cell under- 

 goes seem to be initiated in it, while cells deprived of 

 their nuclei cannot long survive. A single instance 

 will serve to show the vital part which the nucleus 

 plays in the life-history of the cell. In 1883, Gruber 

 found that after rocking a thin cover-glass back and 

 forth in a drop of water containing a collection of the 

 protozoan Stentor, which has a long chain-like nucleus, 

 these tiny animals could thus be cut into fragments, 

 which would in some instances recover from the 

 operation and regenerate into complete individuals. 

 Only those pieces, however, which contained a frag- 

 ment of the nucleus regenerated into new Stentors, 

 while pieces of relatively large size which lacked a frag- 

 ment of nuclear substance very soon disintegrated. 



The nucleus, it should be said, is made up of more 

 than one substance, a fact that is easily demonstrated 

 by processes of staining, in which certain dyes, 

 through chemical union, stain a part but not the 

 whole of the nuclear substance. The part most 

 easily stained is called chromatin, that is "colored 

 material," and during certain phases of cell life the 

 chromatin masses together within the nucleus into 

 visibly definite structures or bodies termed chromo- 

 somes. 



