IJ^TEGUMENT OF VERTEBEATA. 



419 



and claws found at tlie ends of tte limbs. There are indications of 

 these in tlie Amphibia themselves (Salamander) ; they are generally 

 present in Eeptiles and Birds ; nails of this kind are not unfrequeutly 

 retained even on some of the fingers of the bird's hand, which is 

 used as an organ of flight. They are much larger in many Mammals, 

 where they form hoofs, 



Epidermal Structures. 



§ 321. 



Other differentiations beside the cornified structures already 

 mentioned affect the epidermis. The most important of these are 

 feathers and hairs; and that on account of their distribution in 

 the two higher classes of the Vertebrata, and of their peculiar 

 appearance. It is usual to regard 

 them as organs very closely allied^ 

 as they have many points in com- 

 mon. They seenij nevertheless, to be 

 divergent structures. The earliest 

 rudiment either of a feather 

 or of a hair is a thickening of 

 the epidermis; that is to say^ 

 it forms a knob-like thicken- 

 ing (Fig. 216, A), into which 

 there grows a papilla from the 

 cutis. This process is small in the 

 case of the hair, but larger in that 

 of the feather. They resemble 

 those elevations which are found 

 in the Reptilia. The first sign of 

 the feather is the growth of the 

 knobs into papilliform processes 

 {B G, feather processes), which are 

 made up of an outer epidermal 

 layer (G e), and a subjacent papilla. 

 The arrangement also of these first 

 rudiments of the feathers in de- 

 finite areas (feather-tracts, pterylia) 

 is much the same as that of the 

 scales in Eeptiles. The featherj 



therefore, is, in this simple condition, a mere process of the epidermis 

 and subjacent cutis. The depression of the embryonic feather which 

 carries the cuticular papilla, and the consequent formation of_ a 

 "feather follicle," is a later phtenomenon, as is also the differentia- 

 tion of the feather into rachis and vexillum. This differentiation 

 does not obtain until the feather sheath is protruded, which sheath 

 is itself an epidermal layer derived from the earhest rudiment. 



2t ^ u 



Fig. 216. Diagrams of the earliest 

 rudiments of feather and hair. In 

 section. A Thickening of the epider- 

 mis. B Uprising of a papilla. C Pro- 

 cess of the feather. I) Depression of 

 the epidermis. E Differentiation of 

 the rudiment of the hair. F Hair- 

 follicle and hair, e Epidermis, p Hair. 

 j/ Boot of the hair, ws Eoot-sheath. 

 rj Sebaceous glands. The corium is 

 represented by the dark lines. 



