THE INTEGUMENT ti71 



a groove runs the length of its lower surface. This is the umbilical 

 groove. The vane consists of lateral branches or barbs on either side, 

 which have, in turn, still smaller side branches called barbules. These 

 latter usually have small hooks at their sides and tips. These hooks 

 interlock to give iirmness and continuity to the whole vane. In down- 

 feathers, where hooks are lacking, the barbs arise directly from the end 

 of the quill, the barbs do not interlock, and no vane is formed. Hair- 

 feathers consist of long slender shafts \vith a few terminal barbs. 



Archaeopteryx, the oldest known fossil bird, had well developed 

 contour feathers. In most birds, feathers are not equally distributed, 

 but are gathered in tracts known as pterylae (Fig. 398), and separated 

 by apteria or featherless regions where there are but few down or hair 

 feathers. These feather-tracts vary, however, in different groups of 

 birds, but are used to a considerable extent in classification. 



There is a great similarity in the method in which the integument 

 develops in the different type forms we are studying (Fig. 394). For 

 example : "A down-feather begins as a thickening of the corium, push- 

 ing the epidermis before it. By continued growth this, forms a long, 

 finger-like papilla 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 which gradually increase in height, growing into the 

 epidermis and pressing the Malpighian layer above them against the 

 periderm. As a rule, the stratum corneum is divided distally into a num- 

 ber 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 Malpighian layer. With the blood-supply removed, the epidermal 

 parts dry rapidly, and the periderm ruptures, allowing the rods to sepa- 

 rate, forming the down." 



Contour-feathers are quite like down-feathers in their development 

 up to a certain point. 



It is to be remembered that the dorsal and ventral sides of the 

 feather were the outside and inside of the stratum corneum of the 

 papilla. Scales of lizard skin show extreme similarity in their develop- 

 ment to the feather just described (Fig. 395). Many smooth muscle 

 fibers act to elevate the feathers in the corium of birds, and there are 

 also tactile or sense organs. The colors of feathers depend partly upon 

 red, yellow, orange, brown, and black pigment deposited in them, but the 

 iridescent colors are due to interference spectra. 



MAMMALS 



Mammals have a relatively thicker skin than other vertebrates (Fig. 

 393). There are many glands and considerable hair, except in a few 

 orders such as the whales and sirenians. There are likewise horns and 



