. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. 32 COMPARATIVE MORPHOLOGY OF VERTEBRATES. the primitive type. The oldest fossil bird, Archtzopteryx, had well developed contour feathers. Except in the ostriches, penguins, and toucans, feathers are not distributed everywhere on the surface of the body, but are gathered in feather tracts (pterylae), separated by apteria in which no contour feathers and but few down or hair feathers occur. These vary in their arrangement in different groups of birds and are of systematic im- portance (fig. 23). Complicated as th
. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. 32 COMPARATIVE MORPHOLOGY OF VERTEBRATES. the primitive type. The oldest fossil bird, Archtzopteryx, had well developed contour feathers. Except in the ostriches, penguins, and toucans, feathers are not distributed everywhere on the surface of the body, but are gathered in feather tracts (pterylae), separated by apteria in which no contour feathers and but few down or hair feathers occur. These vary in their arrangement in different groups of birds and are of systematic im- portance (fig. 23). Complicated as they are, feathers are probably derived from scales, and the section of lizard skin (fig. 17) might well represent an early stage in the develop- ment of a feather. A down feather begins as a thickening of the corium, pushing the epidermis before it. By continued growth this forms a long, finger-like papilla,. FIG. 24.—Stereogram of developing down feather, bv, blood-vessels entering pulp; c, corium; ep, epidermis; /, feather follicle; p, pulp (mesenchyme) of developing feather; per, periderm; r, rods of epidermis, which later dry, separate, and form the down. projecting from the skin. The corium extends into the outgrowth, carrying blood- vessels with it, while an annular pit, the beginning of the feather follicle, forms around the base of the papilla. Next, the corium or pulp of the distal part of the papilla forms several longitudinal ridges (fig. 24) which gradually increase in height, growing into the epidermis and pressing the Malpighian layer above them against the periderm. As a result the stratum corneum is divided distally into a number of slender rods arising from the base (quill), which at last are only held together by the periderm. Then the pulp retracts, carrying with it the Mal- pighian layer. With the blood supply removed, the epidermal parts dry rapidly, the periderm ruptures, allowing the rods to separate to form the down. A contour feather has much the same
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectanatomy, bookyear1912
