DEVELOPMENT OF THE SKIN 571 



bony plates. Horny claws develop upon the digits of the appendages in turtles, 

 crocodiles, and lizards. 



4. Birds 



a. Characteristics of the Avian Skin 



The skin of the bird is more delicate than that of the reptile. The epidermal 

 layer is thin with a highly cornified external surface. The dermis is composed 

 of an outer compact layer below the epidermis, and beneath the latter is a 

 vascular layer. Below the vascular layer is another compact layer of con- 

 nective-tissue fibers, and between this layer and the deep fascia is the charac- 

 teristic adipose (fatty) layer (fig. 269G). Extensive cutaneous glands are not 

 developed. However, the two uropygial or preening glands at the base of the 

 tail are common to most birds, although they are not present in the ostriches. 

 In certain gallinaceous birds, such as the common fowl, modified sebaceous 

 glands are present around the ear. Scales, resembling the reptilian type, are 

 developed on the distal parts of the legs, while feathers present a feature 

 characteristic of the avian skin. 



1) Kinds of Feathers. Feathers are of many kinds, but they may be grouped 

 under three major categories: 



(1) plumae (plumous or pennaceous feathers), the most perfectly con- 

 structed type of feather, tilling the role of contour feathers, 



(2) plumules (plumulae or plumulaceous feathers), making up the under 

 feather coat or down, and 



(3) filoplumes or hair feathers. 



Of all the epidermal structures developed in the vertebrate group, feathers 

 appear to be the most ingeniously constructed. They possess to a high degree 

 the qualities of lightness, strength, and toughness which serve to protect a 

 delicately constructed skin from cold, moisture, and abrasion. 



2) General Structure of Feathers: a) Pluma or Contour Feather. The 

 plumous feather consists of a rachis (shaft or scape) and a vane. The proximal 

 portion of the rachis or shaft is the quill or calamus. The latter is hollow but 

 may contain a small amount of loose pith. It has an opening, the inferior 

 umbilicus, at its base. The quill resides in a feather follicle, a deep pit sur- 

 rounded by epidermal tissue projecting downward into the dermal part of the 

 skin (fig. 270D, E).. Above the quill is the expanded "feathery" portion of 

 the feather, called the vane. At the junction of the quill and the vane is a 

 small opening, the superior umbilicus, to which is attached, in some contour 

 feathers, a secondary, smaller shaft, the aftershaft or hyporachis, together 

 with a group of irregularly placed barbs. 



The shaft of the vane of the feather is semisolid, with its interior filled 

 with a mass of horny, air-filled cavities. Extending outward from the shaft 

 in this area are lateral branches or barbs (fig. 270E). The barbs form two 



