THE KERATINIZED TISSUES 77 



It would seem that a better knowledge of the factors influencing the 

 relative rates of proliferation of dermal and epidermal elements when 

 these are co-operating to form an organ is necessary. The recent develop- 

 ments in the culture of hairs and feathers in vitro gives promise that this 

 may be obtained. 



In the course of their development all these special structures are 

 necessarily related and more light is thrown on their relations by embryo- 

 logy than by an examination of the mature structures and their arrange- 

 ment. Fleischhauer (1953) has shown, for instance, that regional hair- 



Fig. 34. The hypothetical scale-hair-gland complex. H-T the head-tail 

 line, S, scale; h, hair; g, gland; m, muscle. On the lower right-hand side 

 a view looking down on the unit. A further element not shown in this 

 diagram is the " hair disk " found in some cases behind the hair according 

 to Pinkus (1905). 



streams can be detected before the hair germs appear. From spreads of 

 foetal human skin he concluded that the first hairs develop at roughly 

 fixed distances from each other in a quasi-hexagonal arrangement. New 

 anlagen appear between these when a critical distance is reached owing 

 to growth. This is a pattern which might be expected theoretically if we 

 regard these primordia as successful centres of proliferation which are 

 able to repress like developments in their immediate neighbourhood. 

 The two lateral hairs completing the " trio group " next appear in a line 

 at right angles to the main body axis. The three hairs already slope 

 backwards. The explanation usually offered for trio formation (above) is 

 that the hairs develop as though they were growing out from under 

 the free edge of a scale as indeed they do on the tails of rodents. There 

 is much to suggest in these facts that the scale, the hair group and its 

 associated glands together form a unit (Fig. 34) which in most mammals 

 has degenerated to the hair group and glands, the " pilosebaceous unit " 



