THE KERATINIZED TISSUES 



67 



associated with a loss or reduction in teeth, and are typical of birds, turtles 

 and even some mammals (platypus). 



Embryonically the scale, like the feather, appears as a small out- 

 pocketing of the epidermis containing a dermal papilla whose formation, 

 as a denser gathering of cells, precedes the actual proliferation of epidermal 

 cells and is thought to induce and to control the epidermal changes. The 

 flat upper surface of the outgrowth gives rise to the hardened scale (Fig. 28). 

 The lower surface which may be more or less overlapped by the upper 

 scaly surface and constitutes an inter-scale region, usually consists of a 

 softer more normal epidermis, and imparts some flexibility to the entire 



.S (3 



■I ec 



Fig. 28. The reptilian scale structure. The scales S are thickened 

 epidermis and are not separate. The scaly layer S yields the /?-type X-ray 

 pattern and the interscale region I an a-pattern. m is the germinal matrix. 



integument. As mentioned above, the keratin of reptilian scales is of the 

 )8-type which is rather inextensible. Rudall (1949) has found, however, 

 that in the softer, flexible inter-scale region an a-type keratin tends to 

 predominate. The production of two molecular types of keratin from 

 neighbouring cells arising from the same germinal epithelium poses some 

 interesting questions of differentiation (p. 104). 



A continuous scaly epidermis may be so intensely hardened that it 

 cannot be shed by sloughing or simply worn off, but must be loosened 

 periodically and cast off as a unit. This is effected by a temporary cessation 

 of growth followed by the reformation of an entirely new horny layer 

 beneath the old. 



Horns 



The hollow horns of cattle, goats, sheep, etc., are horny sheaths covering 

 a bony core (Fig. 29 (a), (b)). They are not shed, but as they are worn 

 away they are renewed by the proliferation of a germinal layer. Such horns 

 do not branch although, owing to different rates of proliferation of the 

 germinal layers from one side of the horn to the other, they may grow in 

 graceful curves and spirals (Thompson, 1942). 



The antlers of deer, etc., are not strictly speaking horns, being bony 

 growths forming beneath a covering of hairy skin, the velvet, which dies 

 and is rubbed off leaving the naked bony antler. Pronghorns of certain 

 antelopes are permanent bony antlers extended by a thimble-like sheath 

 of true horn, which in this case is shed periodically like a scale and renewed 

 without loss of the bony core (Fig. 29a). 



