THE GROWTH OF EPIDERMAL STRUCTURES 



141 



to the concentration of these intracellular templates which constitute the 

 generative mass. This mass is being constantly converted into a non- 

 reproductive mass which is the differentiated product. These reactions are 

 further supposed to be accompanied by the formation of inhibitors (anti- 

 templates) which block the templates, and thus may potentially act as 



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Fig. 57. Illustrating Weiss and Kavenau's model (1957) for a cell 

 showing control of growth by the production of an inhibitor. The cell 

 contents are divided into generative mass G and its product the differ- 

 entiated mass D. In this example the inhibitor I is supposed to be 

 produced by D and its action is fed back to control the processes of 

 growth and reproduction (reproduced by permission). 



growth regulators by an intracellular negative feed-back (Fig. 57). In 

 order to effect a control over the whole distribution of a cell type, Weiss 

 and Kavenau assume (a) that the inhibitor molecules diffusing from the 

 cells enter the common humoral pool and thus reach other cells, (b) carry 

 a " tag " or " label " enabling them to be recognized by other cells of the 

 same type and (c) that they have a normal rate of degradation or loss 

 which, in equilibrium, balances their rate of production (Fig. 58). 



These assumptions are biologically acceptable and sufficiently general 

 and simple to permit of mathematical expression. It is clear, without 

 attempting to formulate and solve the growth equations, that since they 

 contain negative feed-back terms they will lead to a system which will 

 automatically regulate its own size. Weiss and Kavenau set up differential 

 equations for their system and applied a solution of these to the case of the 

 growth of a chicken. The quantitative agreement between theoretical and 

 experimental data is surprisingly close. 



One feature of systems stabilized by feed-back should be pointed out. 

 Since a delay in time occurs between the despatch of a signal from one part 



