THE CELL AND CELL DIVISION 37 



variety of structures and materials differing chemically and 

 functionally; these may not all be directly visible as organized 

 structures. The only constantly differentiated substance is the 

 nuclear material which is usually contained in a definitely 

 formed body, the nucleus, though it may be scattered through 

 the protoplasm. There are many reasons for believing that 

 primitively the nuclear substance was not thus organized into 

 a definite nucleus, but that it was distributed through the 

 cytoplasm in the form of small granules, as it is still in many 

 of the simplest organisms, and that gradually these became 

 aggregated into fewer larger masses. The Protozoa show many 

 stages in the gradual enlargement and numerical reduction of 

 the nuclear elements, but in the Metazoa the nuclear material 

 is nearly always collected into a single body. The nucleus is 

 to be regarded as a specialized portion of the protoplasm of 

 the cell, highly differentiated in structure, chemical compo- 

 sition, function, and behavior. All cell activities seem to 

 involve mutual interaction between the nucleus and the 

 remainder of the cell and neither is able long to function 

 normally without the other. But the action of the nucleus is 

 primary and directive, to a large extent controlling and regu- 

 lating cell activities and cell life as a whole. In most cases 

 the nucleus is a spherical or ovoid body of fixed form; in some 

 very active cells it may be elongated or of irregular form, or 

 even branched, ramifying all through the cell; in a few rare 

 instances the nucleus may be amoeboid (Figs. 14, 15). 



Typically this complex center of cell activity shows much the 

 same fundamental structure as the remainder of the protoplasm, 

 which in distinction from the nucleus is called the cytoplasm. 

 The nucleus is limited by a definite nuclear wall or membrane 

 formed either from the cytoplasm or from the nucleus itself. 

 The substance of the nucleus as a whole is termed the karyo- 

 plasm. The equivalent of the spongioplasmic reticulum of the 

 cytoplasm is here termed the linin network, and the hyalo- 

 plasmic ground substance is known as the nuclear sap, or 

 karyolymph, or paralinin. The chief distinction of the nucleus 

 is the presence of a very special nucleo-protein substance called 



