THE INTEGUMENT AND THE EXOSKELETON 



53 



up. (See further, K, p. 36; P and H, pp. 369-73; Wd, pp. 25-28.) It should be noted that 

 the development of a feather is similar to that of a reptilian scale, involving a dermal papilla for 

 nutritive purposes and an ectodermal thickening. 



3. Structure of a contour feather. This is the common type of feather 

 which covers the bodies of birds. Obtain one and identify the following parts. 

 The short, bare, hollow portion the lower end of which is inserted into the feather 

 follicle of the skin is the calamus or quill; the remaining expanded portion, con- 

 stituting most of the feather, is the vane. The quill bears two openings, the 

 inferior umbilicus at its proximal end, which was inserted in the skin, and through 

 which in the early stages of the feather, the dermal papilla passes; and the superior 

 umbilicus at the junction of vane and quill on the ventral surface of the feather. 

 From the superior umbilicus protrudes a more or less well-developed accessory 



periderm 



umbilical 

 groove rachis 



\ \ 



stratum 

 corneum 



FIG. 14. Diagrams of the development of the down and contour feathers. A, dermal papilla. 

 J5, cross-section of a later stage of the dermal papilla showing the thickenings of the stratum germina- 

 tivum. C, cross-section of a later stage; each thickening has separated to form a barb; the dermis in 

 the center of the papilla has degenerated into the pith; the stratum corneum forms the periderm or 

 sheath of the down feather. D, section across a developing contour feather, showing the two enlarged 

 thickenings or barbs which become the rachis and the oblique course of the other barbs. (A-C from 

 Wiedersheim's Comparative Anatomy of Vertebrates, courtesy of the Macmillan Company; D after 

 Kingsley's Comparative Anatomy of Vertebrates, copyright by P. Blakiston's Son and Company.) 



feather called the aftershaft, consisting of only a few tufts in some birds but in 

 others of a complete feather, nearly or quite as large as the primary feather. 



The vane consists of a central axis known as the shaft or rachis which is continu- 

 ous with the quill and of a sort of web or membrane springing from each side of the 

 rachis. Extending along the ventral surface of the rachis is a groove, the 

 umbilical groove. The web or membrane of the feathers is obviously composed 

 of a large number of parallel, obliquely placed rays, adhering to each other. 

 These rays are the barbs, and each barb bears side rays called barbules, exactly 

 as in the down feather. The barbules interlock, causing the barbs to adhere to 

 produce an unbroken surface. To see the method of interlocking of the barbules 

 it is necessary to examine a small piece of the feather under the microscope. It 

 will then be noted that the barbules of one side of each barb (side toward the quill) 



