140 KERATIN AND KERATINIZATION 



general validity (Rashevsky, 1948), but it is one of the consequences of 

 multicellularity that they cannot be applied in a simple direct form. 



It seems more probable to many that each organ system itself produces 

 changes which automatically lead to its limiting its own proliferation. Such 

 a view is in harmony with modern theories of self-controlled mechanisms 

 which envisage control, in very general terms, as being effected by a " feed- 

 back " of information which introduces a limiting factor proportional to 

 the deviation from a norm. 



Physiological evidence that the entire population of cells in an organ 

 controls its own size is obtained from a variety of experiments in which 

 part of the population is removed. Partial hepatectomy is followed by a 

 burst of mitotic activity in the remaining tissue leading to a restoration in 

 size. That the influence causing the mitotic wave is carried by the blood 

 is shown by experiments in which hepatectomy is practised on one of two 

 rats whose blood supplies have been joined (parabiotic union). Mitosis 

 occurs in the second undamaged liver. These effects might be ascribed to 

 a stimulating substance (wound hormone), but the loss of an inhibitor is 

 indicated by the observations that mitosis can be induced in normal livers 

 if the blood is diluted by saline, and that regeneration itself is slowed up by 

 increasing the plasma concentration (Glinos, 1958). Weiss (1955) has 

 reviewed experiments in which the removal of one member of a paired 

 organ, e.g. the kidney, is followed by an increase in size of the remaining 

 member. Perhaps the best demonstration of the existence of control by 

 inhibitor productions is found in the experiments of Bullough on growth 

 control in the epidermis itself which will be described in the next section. 



Certainly numerous other factors, among them well-recognized hor- 

 mones, affect growth as is made clear in the reviews of Abercrombie (1957 

 and 1958) and Swann (1955, 1957 and 1958). Nevertheless, the possibility 

 exists that primarily control is based on hormones of the inhibitor-type and 

 that other hormones could operate by secondarily affecting the cells' 

 response to inhibition. These problems are returned to again below 

 (p. 146). 



The logistic growth curve is obtained from the exponential-type curve 

 simply by the addition of a further negative term (see above) which here 

 might be regarded as the " negative feed-back " term. The total growth 

 curve may be considered as the sum of the separate organ sigmoids. 

 Several proposals of this sort have been made, for example, by Morales 

 and Kreautzer (1945) and by Sock and Morales (1945). Weiss (1955) has 

 attempted to give these concepts a more definitely-biological basis and 

 recently with Kavenau (1957) has obtained solutions of a growth equation 

 which are sufficiently precise to be put to a quantitative test. They suppose 

 that each specific cell reproduces itself by a mechanism in which certain key 

 compounds act as catalysts (templates). The growth rate is proportional 



