THE GROWTH OF EPIDERMAL STRUCTURES 159 



to the germinal level could thus be effected through this mechanical link. 

 A control over growth in the bulb could be produced by the movement of 

 the bulb into regions differing in inhibitor content or simply by mechanical 

 deformation. Three periods are involved : (a) the period of oscillation pro- 

 duced by the curved fibre passing through the upper levels; (b) the time 

 for the fibre to pass through the zone of hardening; and (c) the period of 

 the growth cycle of the bulb. All these periods are variable within limits 

 and it is likely that the coupled oscillation, which evolves and is stabilized 

 by feed-back, results from a selected resonance between all three (p. 146). 



The successful transfer of control from the curved fibre in the upper 

 reaches of the follicle back to the lower levels required that the fibre be 

 stiffened before it passes into the curved upper levels. A soft fibre would 

 not produce the postulated oscillation. In confirmation of this, Marston 

 (1946) has shown that in the event of incomplete keratinization, as occurs 

 in sheep deficient in copper, the wave-form is poorly developed and its 

 frequency (number of waves produced per unit time) is lower as would be 

 expected from a weaker fibre. The above explanation of crimp formation 

 has some features in common with that given by Auber (1950) and 

 Wildman (1932). However, Auber's assumption on which his explanation 

 rests must be rejected. He supposes that the a-structure of the fibrils is 

 produced by an actual contraction in length produced during hardening 

 and that the contraction is greater on the more highly-keratinized inner 

 face {para) of the fibre. There is ample experimental evidence (p. 21 1 et 

 seq.) that the a-structure is present in the fibrils as originally formed and 

 owes nothing to the subsequent chemical changes occurring during 

 keratinization. 



A characteristic relationship between crimp form and tip shape in the 

 various classes of wool fibres was first described by Dry (1926 and 1928) 

 and more recently discussed by Fraser (1951) (p. 149) who has tried to 

 explain it in terms of interfollicular competition. Fraser develops the idea 

 of a competition between follicles for the materials needed for growth, and 

 suggests that their efficiency in this competition depends in part on the 

 time of origin of the follicle. In these terms he gives an explanation of the 

 formation of the first few curves at the tip of a wool fibre which precede 

 the establishment of the regular crimp form. The concepts of competition 

 and follicular efficiency seem to overlap and supplement those based on 

 inhibition; both ideas stand in need of further analysis and testing (see 

 also p. 149). 



Allometric growth 



When the different parts of an organism are compared it is usually found 

 that they grow at different rates and that the proportions of the organism 

 thus change as life continues. A formula which has often been found to 



