38 ELEMENTS OF BIOLOGY 



tion determines the beginnings of organization of the new 

 individual. 



Function of Nucleus. The cell nucleus is indissociably linked 

 with the continuation of life of the cell, for if a fragment of a cell 

 does not contain a nucleus it is certain to die within a short time. 

 On the other hand, cell fragments containing a nucleus, or a portion 

 of the nucleus, may reorganize, recover, and live normally. Just why 

 the nucleus is such a vital part of the cell life is not definitely 

 known. The statement is frequently made that the nucleus is the 

 controlling centre of the cell; while this is true it gives us no picture 

 of how nor why the nucleus exercises this function. 



The nucleus is not a homogeneous mass of a single type of proto- 

 plasm but contains various types. Most prominent among the 

 nuclear contents is a substance which stains heavily with certain 

 types of stains and is therefore called chromatin (Fig. 4). When a 

 cell is about to divide, the first indications are in the behavior of the 

 chromatin, and the ensuing phenomena of division seem to be de- 

 vised to assure an equal division of the chromatin to the daughter 

 nuclei. By a complicated process that will be described in detail 

 later, the chromatin is concentrated into a characteristic number of 

 bodies called chromosomes and these divide equally, giving rise to 

 the same number of chromosomes in each daughter nucleus. All of 

 the evidence points to the conclusion that the inheritable qualities 

 of the cell are in some way associated with this substance, chromatin. 



Function of the Centrosome. The centrosome is not pres- 

 ent in some cells. Its real function is not known. It appears to act as 

 a leading element in cell division, for the centrosome is first to di- 

 vide, one portion eventually going to each of the daughter cells. 



Plastids. The various types of plastids in the cytoplasm ap- 

 pear to be centres of reactions. Thus in the cells of green plants a 

 type of plastid which contains a green coloring matter, chlorophyll, 

 is the locus of the reaction whereby the cell manufactures carbo- 



