290 ON GROWTH AND OVERGROWTH 



trolling part, not merely in cell division, but also in the function 

 of the cell. With activity the nuclear chromatin becomes used 

 up and discharged into the body of the cell, there to combine 

 with other substances to build up prezymogens and other bodies, 

 which are eventually discharged as the specific secretion of the 

 cell. This being the case, we can readily understand that the 

 higher specific functions of the cell cannot be carried on by a 

 nucleus whose nuclear material is being utilized to its fullest in 

 mitosis. The process of cell division and the performance of 

 the higher functions of the cell are incompatible, and the cell 

 engaged in the active performance of its special functions cannot 

 undergo division. 



Yet, paradoxically, under normal conditions after birth, it 

 is the actively functioning tissue that undergoes hyperplasia 

 and takes on increased growth. This is to be explained in most 

 cases by the fact that the new units of that tissue are derived, 

 not from the actively functioning cells, but from mother cells 

 present therein ; in other cases, probably by the fact that such 

 hyperplasia occurs where the activity is not continuous, but 

 interrupted, growth occurring in the intervening periods of rest. 

 One may quote, for example, the late Sir James Paget's well- 

 known illustration of the growth of a corn on the foot being due 

 to an interrupted irritation of the skin by the pressure of a tight 

 boot, that irritation which otherwise would lead, if continued, to 

 atrophy, being removed at night, leads to hyperplasia of the 

 epidermis. 



For a cell to divide actively there must be at least temporary 

 arrest of the specific, as distinct from the vegetative functions. 

 Or, conversely, arrest or disturbance of the specific functions 

 of the cell, if of such a nature as not to arrest vegetative activity, 

 favours cell multiplication. The nuclear activities, unemployed 

 in one direction, become diverted into another ; the nuclear 

 material, not being discharged and converted into the specific 

 secretions of the cell, tends to become heaped up, and, accumu- 

 lating up to a certain point, is then used in mitotic processes. 



INFLUENCE OP ENVIRONMENT ON CELL ACTIVITY 



While heredity surely plays an important part in determining 

 the structure of the cell, we are forced to see that, underlying 



