28 FOUNDATIONS OF BIOLOGY 



cytoplasm. But superimposed upon this, as it were, is the 

 highly characteristic nuclear material, or CHROMATIN, which 

 takes various forms during different phases of cell activity 

 but generally, in a 'resting' cell, gives the appearance of a 

 network of tiny granules with one or more dense 'knots' of 

 chromatin (KARYOSOMES) . Later we shall describe some of 

 the important changes in chromatin arrangement, but it is 

 sufficient at this time to emphasize that the nucleus is a 

 differentiated area of the cell protoplasm which is the arena 

 of the chromatin. Frequently there is a conspicuous round 

 achromatic body within the nucleus known as the NTJCLEOLUS. 

 Cytoplasm and nucleus, looked at from the functional view- 

 point, represent a physiological division of labor within the 

 confines of the cell. Experiments have shown that they are 

 mutually necessary for cell life; the removal of the nucleus 

 putting an end to anabolic processes assimilation, repair, 

 and growth and thus leading rapidly to death. Accord- 

 ingly the nucleus may be considered as the center of the 

 synthetic activities of the cell, and the cytoplasm, if not as 

 the area in which destructive processes are chiefly involved, 

 at least as a neutral region which translocates material 

 toward and away from the nucleus. All the evidence points 

 to the nucleus as the " controlling center in cell activity, and 

 hence a primary factor in growth, development, and trans- 

 mission of specific qualities from cell to cell, and so from one 

 generation to another." 



B. ORIGIN OF CELLS 



With regard to the origin of life on the Earth absolutely 

 nothing is known. But all the evidence at hand tends to 

 show that, at the present time at least, living matter never 

 arises except under the influence of preexisting living matter. 

 That is, protoplasm grows cells grow and, having attained 



