THE FUNDAMENTAL PHYSIOLOGICAL ACTIONS 23 



we can see that if in the larger one the amount of material that can 

 enter through the surface is just sufficient to balance the dissimila- 

 tion, in the smaller one, with only half the mass to carry on dis- 

 similation processes, the relatively larger surface permits the en- 

 trance of more than enough material for a balance, and there is an 

 excess which can be built into the substance of the cell, the process 

 which constitutes growth. 



Cell Division. When the relation of surface to mass in the 

 growing cell reaches the point where there is no excess of assimila- 

 tion over dissimilation, growth necessarily stops. Since this rela- 

 tionship appears while both mass and surface are very small single 

 cells are always quite or nearly microscopic. To build up such 

 large structures as are present in the body cell multiplication takes 

 place. Whenever a cell which has reached the limit of its growth 

 divides, the greater relative reduction of mass as compared with 

 surface gives opportunity for excess assimilation to occur once 

 more and growth is resumed. 



Details of Cell Structure and Nuclear Structure. The peculiar 

 function of the cell nucleus appears to be the control of the processes 

 of assimilation. Assimilation involves the transformation into 

 cell protoplasm of the food substances which enter the cell; 

 furthermore, the differences which distinguish one sort of cell 

 from another sort, and in consequence, one sort of animal or plant 

 from another, are at bottom differences in the character of the 

 protoplasm; the nucleus is, therefore, the determining factor in the 

 establishment of definite species, and much study has been given 

 it in the hope of obtaining insight into the conditions upon which 

 specific cell differences depend. 



Microscopically the most striking feature of the nucleus as ob- 

 served in suitably fixed and stained preparations is an irregular 

 network of substance which has greater affinity for dyes than the 

 other constituents of the cell and therefore stains much more 

 deeply. Because of this affinity for coloring reagents the sub- 

 stance has been named chromatin. It occurs in the nucleus in the 

 form of excessively minute granules, strung like beads upon a 

 thread of different material, called linin, the whole twisted into 

 an irregular network (Fig. 6). 



During the process of cell division the chromatin network passes 

 through a remarkable sequence of events, of such a character as to 



