sensory bristle^ 



epidermal papilla 



sensory endings' 



Figure 8-9. Histology of a sensory organ or the scale of a lizard 

 (Agomo). (After Elios and Bortner, 1 957} 



corneum of soft keratin and a few layers of transitional cells 

 above the flattened (squamous) stratum germinativum 

 (Figure 8-10). The dermis is frequently filled with fat cells. 

 The only glands are the paired multisacculate uropygial 

 glands opening on a papilla above the tail. These glands 

 secrete an oily fluid used by the bird to condition the bill 

 sheath and the feathers. The bill sheath (Figure 8-11) is like 

 the nail or claw of the mammal; there are also scales and 



claws on the feet. Claws frequently occur at the tips of the 

 anterior and middle wing digits, at least in the young. 



The feathers are epidermal derivatives usually described 

 as modified scales. This homology is based on the observation 

 that, in the pigeon, feathers appear to arise out of the scales 

 of the lower leg. The scales of the pigeon leg are really scutes 

 formed by the fusion of several scales. Where scales and 

 feathers are both present, as in the Barn Owl, it is observed 

 that the feather is an interscale structure like the hair. This 

 similarity in scale relationship between feather and hair 

 does not indicate an homology, even though these structures 

 are analogous in that they insulate the body surface. The 

 feathers have two functions: they help in maintaining a 

 high body temperature and they are important for flight. 



The feathers are of two main types: plumes or pinnaceous 

 structures and plumules or down, including filoplumes. The 

 plumes are distributed on the body in oblique rows and con- 

 centrated into pterylae separated by areas free of such feath- 

 ers, the apteria. Down may cover the apteria and occur 

 among the plumes as well. In structure the feather is com- 

 plex, and in development it stems from a papilla enclosed 

 by a follicle not unlike that from which the hair develops. 



Amphibians 



Although it has a thin cellular stratum corneum (Fig- 

 ure 8-12), the skin of the amphibian is moist and fi-e- 



lower umbilicus- 

 ^Gxial blood vessel of dermal papilla 



Figure 8-10. Development of the feather. A, initial papilla; B, beginning of follicle formofion; C, 

 differentiation of feather parts which are identified in D and E. (After Witschi, 1956) 



222 



THE SKIN AND ITS DERIVATIVES 



