. Elements of Comparative Anatomy. 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 hai
. Elements of Comparative Anatomy. 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, whi
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