THE GROWTH OF EPIDERMAL STRUCTURES 143 



suppresses this development in adjacent cells and permits a second 

 reaction to arise in these cells. The products of the first reaction diffusing 

 from the " dominant " group of cells thus " induce " a second and 

 different reaction in neighbours. Possible fine structural evidence of this 

 form of induction in the epidermis and dermis has been described above 

 (p. 90). It would not be difficult to generalize the growth equations by 

 introducing terms expressing the interaction between cells assuming that 

 the anti-templates (or secreted differentiating mass) in a dominant early- 

 maturing group of cells can suppress similar development in less-advanced 

 cells. See also Waddington (1948). 



Also, no necessary place has been given to the fact that tissues are 

 organized in a cellular form. Certainly, although cell division introduces a 

 discontinuity in the output of a single cell, these irregularities would be 

 smoothed out when the output of a large non-synchronously-dividing 

 population is considered. Moreover, if adequate arrangements exist for 

 transport to and from sites of synthesis, the cellular habit does not in itself 

 seem essential for continued synthesis. For example, in insects relatively 

 enormous differentiated cells are common. Probably cell division is an 

 inherited act, originally developed to permit of replication and dissemi- 

 nation of the genetical apparatus, that occurs normally when the DNA is 

 duplicated and the cell has synthesized adequate amounts of the materials 

 required to provide the apparatus of division. These latter activities could 

 involve paths of synthesis distinct from those involved in the formation of 

 specialized products. 



That all authorities do not yet accept the necessity of control by in- 

 hibitor production is evident from a recent discussion on the growth of 

 proliferating tissues (Price, 1958). Obviously the possibility that stability 

 is maintained throughout multicellular organisms by the circulation of 

 inhibitors is a conception of far-reaching consequences. It implies, in 

 effect, the existence of a whole system of hormones which has escaped 

 notice. The already-known hormones and other growth influencing agents 

 would seem to effect the sensitivity of the cells to the circulating inhibitors 

 or act to influence the dispersal and disappearance of these. It is highly 

 desirable that an attempt be made to isolate these postulated inhibitors and 

 that their mode of action on cells be determined. It is evident that the 

 theory must be regarded as unproven until some of the postulated in- 

 hibiting substances have been isolated and the mode of action on the cells 

 observed directly (Bertalanffy, 1960). 



Periodic growth and cyclic activity 



Even in the adult when the size has become more or less constant, 

 certain organs undergo a periodic fluctuation in size and activity. Con- 

 spicuous among these are the sexual organs and with their changes are 



