ORGAN DEVELOPMENT IN VERTEBRATES 259 



acting at a very late stage. The induction is not effective on non-follicular 

 ectoderm; and it is very remarkable that when a feather is induced in 

 this way, the details of its structure (for instance, whether it is typical of 

 the breast or the back) is determined not by the dermal core which in- 

 duced it but by the place of origin of the follicular epidermis which 

 responded to the inducing stimulus. (For a longer discussion of feather 

 formation see Waddington 1952a). 



Hairs also are formed from follicles which possess both ectodermal and 

 mesodermal components. Much less is known about their structure and 

 the inducing actions, if any, which go on within them; but since one 

 special type of hair, namely wool, is one of the major raw materials of 

 industry, a great deal is known about its development in other respects. 

 Perhaps the aspect which is of most interest to general embryology is the 

 study of the various shapes which may be taken by wool fibres, a subject 

 which has been largely opened up through the pioneer investigations of 

 Dry (1933-34). There are many different types of wool fleeces; each of 

 them is characterised by a particular array of fibre-types, which occur 

 with particular frequencies and can be distinguished by their length, 

 thickness and the sequence of curves along them. It has long been known 

 that the follicles occur in groups in the skin; within each group, the fol- 

 hcles develop in series, first the central primary, then the lateral primaries, 

 finally one or two waves of secondaries. Fraser (1952) has recently pre- 

 sented evidence that each type of fibre is formed from some particular 

 type of foUicle; and he has elaborated a theory which shows how one 

 could account for the shapes of the fibres, which differ in the series of 

 curvatures along their length, by the interaction between a varying 

 growth rate and a regular periodic chance in the direction in which the 

 fibre is pushed out. If all the fibres grew at a constant rate, they would all 

 have a regular wavy form. But Fraser suggests that actually they grow 

 at a rate which is dependent on the efficiency of their follicle in competing 

 with other follicles for a limited quantity of available substrate; and this 

 efficiency is supposed to change according to the time of origin of the 

 folhcle. In the various breeds of sheep there are differences, not only in 

 the relative numbers of the primary and secondary follicles, and in their 

 density per unit area, but also in the curve which relates the efficiency to 

 the time of origin of follicle (Fig. 12.4). In this way, the different shapes of 

 the fmal structures can be accounted for in terms of physiological processes. 

 The theory, although still in need of further testing, is a good example of 

 the kind of mechanism one has to search for in the attempt to explain the 

 facts of structure in terms of functional activities. 



The formation of hairs, which usually grow very rapidly in comparison 



