THE GROWTH OF EPIDERMAL STRUCTURES 145 



of cytomembranes; and (b) mucous cells with an elaborate system of 

 y-cytomembranes and few RNP particles (r.h.s. Fig. 59) (Burgos and 

 Wislocki, 1958; Nilsson, 1959; Schulz et al., 1958). Cyclic behaviour 

 of this character is in effect a cyclic change in differentiation. Like 

 differentiation in the more stable sense, it is a consequence of the inter- 

 action between one group of cells (organ) upon another. It was precisely 

 this interaction which was omitted for simplicity in the simplified theory 

 of growth outlined above (p. 138). When interaction is permitted, periodic 

 phenomena can arise spontaneously from the effects of intercellular 

 communication when there is a time lag between the transmission of a stimulus 

 from one organ and the return of a counter stimulus from the other. Such 

 questions are currently discussed under the heading of cybernetics. 



t + T 



Fig. 60. Illustrating the possibility of generating cyclic activity by a feed- 

 back link between two organs A and B. A signal from A stimulates the 

 development of B whose secretion returns with phase lag T to repress 

 the activity of A (see text). 



The possibility of oscillations can be understood most easily from a 

 consideration of the ideal situation depicted in Fig. 60. We suppose two 

 organs A and B. The secretion SA of organ A stimulates the growth of 

 organ B, whose secretion SB is capable of inhibiting the activity of A. 

 Suppose the secretion SA reaches a threshold value at a time t and initi- 

 ates a growth phase in B, which after a further time T leads to the release 

 of SB by the organ B. This inhibitor SB in turn now acts on and suppresses 

 the action of A ; with the consequent fall in ^4's activity, the stimulus to 

 B falls off and its activity declines again, leading to a fall in the inhibitor 

 SB and the recommencement of activity in A. Thus cyclic activity is 

 set-up both in A and B with a difference in phase introduced by the time 

 lag T (Fig. 60). 



In the language of cybernetics, we say that the two organs are controlled 

 by a feed-back linkage, and that the oscillation is made possible because 

 the feed-back signal from B to A is out of phase with that from A to B. 

 The feed-back from B to A is negative; that from A to B is positive. 



As described, the feed-back signals (SA and SB) are " hormones " and 

 the lag between them is introduced by the time of maturation of a target 

 organ B. This illustration was chosen because it corresponds, in the type 



