478 GROWTH 



removed from any spot, epidermal cells from the edge of the 

 wound creep upon the denuded spot and form a covering layer — 

 a surface-tension phenomenon. The stretching of the contents 

 of the surrounding cells so produced, causes a rapid series of cell 

 divisions, i.e. growth under stimulation of stress takes place 

 (Chap. XVII.). That the cause of the increased cell division is the 

 stretching of the cells is borne out by the fact that the larger the 

 area to be covered (within limits) the greater is the tension and 

 the more rapid the process of forming a new skin. When the skin 

 is formed and the interna,! pressure is altered from one of stretching 

 to one of compression due to the crowding of the proliferating 

 cells, growth slows down to normal. It is noteworthy that, before 

 this return to a normal rate of cell division can take place, distinct 

 pressure must be exerted by the epithelial cells on one another, 

 i.e. excessive formation of epithelial cells occurs. Is it possible, 

 from this and similar experiments, to consider that cell pressure is 

 one of the limiting factors in growth ? 



There are, as we have seen, two main epochs of growth, each 

 followed by a slowing down of velocity. In the first case, in early 

 life, the slowing down is temporary. This may be correlated with 

 (a) the fact that the increase in weight during this period is due 

 in great part to a deposition of fat which is absorbed during the 

 subsequent period of slower growth, and (6) to the changes in 

 glandular functions, etc., which usher in the second period of very 

 rapid growth. This final period is followed by a complete arrest 

 of positive growth. The increased weight is due to muscle and 

 organ building — protein is laid on and the percentage of water 

 decreases. No further change resulting in increased metabolic 

 activity takes place after this. Cell pressure now developed is 

 not relieved. 



The physical chemistry of negative growth will be considered in a later 

 chapter. 



V. Growth and Form. 



One cannot leave this subject without a brief reference to the 

 relationship existing between growth and form. Form is deter- 

 mined hi/ the specific rate of growth in various directions; i.e. as 

 D'Arcy Thompson puts it, form is a function of time. If a spherical 

 organism grew symmetrically, its form would not alter, but 

 because of its complexity, growth is not uniform in all directions. 

 There are structural differences in protoplasm which set up 

 unequal resistances to growth. One part may be more viscous 

 than another, or may have a higher surface tension and so on. 

 Although it has been pointed out that the presence of external 



