54 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



are placed diagonally across the barbules of the other side (side toward the tip of 

 the feather) of the adjoining barb, and that the former are provided with hooklike 

 projections which fit over and catch flangelike extensions of the latter. The 

 diagonal arrangement permits one hooked barbule to catch a number of flanged 

 barbules. By pulling on the barbs one can unhook the barbules, and by stroking 

 the barbs one can hook them up again. The latter process is the chief object of 

 the frequent preening of the feathers habitual with birds. In the rectrices and 

 remiges the hooklets are well developed throughout so that the barbs are inter- 

 locked over the entire feather; but in the coverts the lower barbules lack the 

 hooklets, and the lower barbs are loose and fluffy. In some birds, as the ostrich, 

 all of the barbules lack hooks, and the whole feather is fluffy. 

 Draw a contour feather, showing its parts. 



4. Development of the contour feather. — Contour feathers are similar in structure and 

 development to down feathers, except that in the latter the barbs spring directly from the quill 

 in a circle, while in the former they spring in a row from either side of a central axis. This is 

 due to the fact that in the development of the contour feather two of the longitudinal columns 

 formed by the stratum germinativum become much enlarged and fuse to produce the rachis, 

 the grooves between them remaining as the umbilical groove. This enlargement of these two 

 columns imparts an oblique position to the other columns, which become the barbs, since the 

 rachis grows faster than the barbs. The contour feather is rolled up inside the periderm, the 

 future dorsal or outer surface being outside next to the periderm, while the future ventral or 

 inner surface is inside next to the dermal papilla. When the periderm splits, the feather flattens 

 out. The quill is the base of the feather which failed to split into barbs; the white flaky 

 material noticeable inside the quill is the dried remains of the dermal papilla (see Fig. 14; 

 also K, p. 37, Fig. 28). 



5. Feather tracts. — Birds appear to be completely covered by contour 

 feathers, but actually the feathers are borne only by certain areas of the s kin 

 called feather tracts or pterylae, with featherless areas, apterylae, between them. 

 Observe the feather tracts on a demonstration specimen of a young bird and con- 

 sult also K, Figure 26, page 35, or P and H, Figure 1026, page 384. 



6. Structure of the filoplume. — The filoplumes are the "hairs" visible on a 

 plucked bird. Examine a prepared slide or remove a filoplume from a plucked 

 bird, mount on a slide in a drop of water, and examine under the low power. It 

 consists of a main axis bearing a few terminal barbs. It is a miniature degener- 

 ated contour feather. Draw. 



H. EXOSKELETON OF MAMMALS 



The exoskeleton of mammals consists primarily of hair, found in no other 

 vertebrates. Some mammals are provided with scales in addition. 



1. Structure of hair.— By examining your own skin determine that each 

 hair springs from a pit in the skin, known as a hair follicle. Remove a fine hair 

 from the under side of the forearm and examine with high powers of the micro- 

 scope. Observe by focusing on the surface of the hair the irregular wavy 



