THE CARRIERS OF THE HERITAGE 



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 L 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 proto- 

 zoan 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 opera- 

 tion and regenerate into complete individuals. Only 

 those pieces, however, which contained a fragment of 

 the nucleus regenerated into new Stentors, while pieces 

 of relatively large size which lacked a fragment 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 to- 

 gether within the nucleus into visibly definite structures 

 or bodies termed chromosomes. 



Throughout all the various cells that make up the 

 individuals of any one species these chromosomes ap- 



