EPIDERMAL STRUCTURES. 



97 



expand so as to form the vane. One point needs a little more 

 detail. On that side where the shaft is to be formed are two 

 longitudinal thickenings (Fig. 105) ; with growth these become 

 larger and bend inwards to meet each other. Near the tip the 

 result is a solid rod (Fig. 105, A), but farther down the ingrowth 



FIG. 105. Diagrammatic sections through a developing contour-feather : A at 

 about the middle of the vane, B near the base of the vane, and C through the quill, 

 after Davies. b, barbs ; fs, feather sheath ; / J , / 2 , different portions of the pulp 



cavity. 



includes a space (Fig. 105, B and C), so that the proximal por- 

 tion of the shaft is hollow. The umbilicus is formed by these 

 ingrowing ridges. As will readily be understood, the so-called 

 dorsal and ventral sides of the feather correspond to the outer 

 and inner surfaces of the epidermis of the feather papilla. 



At regular intervals the bird sheds or molts its feathers, the 

 old ones dropping out, while new ones 

 arise to take their place by a repetition 

 of the process just described. 



Hair is as characteristic of mammals 

 as feathers are of birds. In its formation 

 the epidermis apparently takes the initi- 

 ative, the result being the formation of a 

 solid ingrowth of epidermis into the un- 

 derlying derma, the deeper end of which 

 becomes cupped (Fig. 106) to accommo- 

 date a small collection of dermal cells, 

 the rudiment of the hair papilla. Next a 

 circular depression appears in their in- 

 growth, separating a central portion, the 

 future hair, from the surrounding epidermis which forms the 



FIG. 106. Two stages 

 in the early development 

 of the hair of the mouse, 

 after Maurer. D, derma ; 

 E y epithelial hair-forming 

 cells ; F, follicle ; /, hair 

 papilla. 



