138 KERATIN AND KERATINIZATION 



times actually decreased the total thickness of the whole layer. The rate 

 of differentiation and of exfoliation, about which less is known, influences 

 the thickness of the intermediate layers and of the horny layer, respectively. 



General theories of growth 



More recently attempts have been made to refer the problem of growth 

 to a more fundamental basis by examining directly the behavioural 

 patterns of the individual cells. These investigations take two forms: one 

 is experimental, the direct observation of cellular activity in tissue culture — 

 some of this work has been already referred to — the other is theoretical 

 and attempts to develop an adequate general theory, which would relate 

 the growth of the whole organism to the growth rates of the constituent 

 cells and, further, would be competent to infer the characteristic stability 

 and homeostasis from the properties and interaction of these cells. At the 

 present time only tentative solutions of this problem have been made, but 

 it seems worthwhile to mention them here, not only in an attempt to 

 inquire how far epidermal growth may be included within the scope of a 

 more general discussion but also because it seems that the future develop- 

 ment and experimental verification of these theories will involve further 

 experiments on the epidermis. 



If the initial special events of cleavage and the blocking out of the early 

 embryo are omitted, and growth considered only after the point where the 

 embryo increases in weight, the bald facts demanding an explanation 

 according to Weiss and Kavenau (1957) are: 



(a) The increase in number of cells. 



(b) Their divergence into organ systems containing differentiated cells. 



(c) The quantitative facts of the growth curve, i.e. the sigmoid shape of 

 the plot of total weight of cell mass against time (Fig. 56). 



(d) The steady state which the organisms approach as the adult size 

 is achieved in which the various organs exist in equilibrium with each 

 other and with the external environment. In this state the several organs 

 have become specialized in the functions which they perform on behalf of 

 each other. The equilibrium is dynamic in the sense that it is maintained 

 by a constant intercellular communication to which considerations of a 

 cybernetical order are applicable. 



Observations made on cells isolated and cultivated in the absence of 

 other cells show that in these conditions cells gradually cease to produce 

 their characteristic products and assume a more generalized character. 

 Further, when mixed cell populations are grown together differentiation 

 again takes place. These very general findings are sufficient to prove that 

 differentiation is maintained by restraints exerted by one cell type on 

 another, either by direct contact or by exchange of their products through 

 the medium of their common humoral pool (p. 61). 



