Skin and Its Derivatives 



509 



indicated by the lack of alteration in the 

 epithelial cells. Rapid growth and prolifera- 

 tion of the dermal cells soon cause the over- 

 lying epidermis to protrude, forming the 

 characteristic protuberances or feather germs 

 comprising the various feather tracts (see 

 Fig. 193A,B,C). Before hatching, each der- 

 mal papilla sinks beneath the surface of the 

 skin in a tube-like follicle lined with epi- 

 dermis. Later proliferation of the epidermal 

 cells overlying the dermal papilla gives rise 

 to the embryonic region from which the 

 parts of the definitive feather arise. For in- 

 formation concerning the structure of the 

 definitive or permanent types of feather and 

 its relationship to the fully differentiated 

 skin, see Figure 194 A and B. 



The effect of removing the ectoderm, from 

 the upper surface of a 3-day wing bud, on 

 the origin of feather germs of the chick 

 has been tested by Saunders and Weiss ('50). 

 In the absence of ectoderm the prospective 

 dermis is found to be incapable of organiz- 

 ing either a typical corium or dermal papil- 

 lae. A causal interaction of the inductor type 

 between the ectoderm and subjacent meso- 

 derm in feather germ formation is suggested 

 by these experiments. Evidence of a more 

 crucial nature has come from some recent 

 transplantation experiments of Cairns ('51), 

 in which he was able to induce feather 

 germs in a region of the wing skin ectoderm 

 which normally produces no feather germs 

 by implanting mesoderm from the leg bud 

 of 4-day embryos. 



The most conclusive evidence that induc- 

 tive principles are operative in feather de- 

 velopment has come from the beautiful and 

 systematically executed experiments of Lil- 

 lie and Wang ('41, '44) and Wang ('43) on 

 the papillae of regenerating feathers of the 

 adult fowl. When a feather is shed naturally 

 through the process of molting, or when it is 

 plucked, the dermal papilla, covered by a 

 thin layer of epidermal ("regeneration") 

 cells, is left behind in the base of the tubu- 

 lar follicle. From this epidermal component 

 is formed a thick ring of embryonic cells, 

 the "collar" (Lillie and Juhn, '32), which 

 gives rise to all of the epidermal parts of the 

 regenerating feather. If the dermal papilla 

 is removed, a feather is never formed from 

 the epidermal cells of the follicle wall which 

 grow over the site of extirpation. 



The dermal papilla is, therefore, essential 

 for a feather-forming epidermal response. 

 By means of a variety of skillful transplan- 

 tations, Lillie and Wang succeeded in dem- 

 onstrating clearly that the mesodermal or 



dermal portion of the feather papilla func- 

 tions as a feather "inductor" and determines 

 the symmetry and orientation of the result- 

 ing feather. The specificity of epidermal re- 

 sponse, i.e., the specific type of feather in- 

 duced, whether breast or saddle, was found 

 to be dependent upon the tract origin of the 

 overlying epidermis. For example, a saddle 

 feather papilla from which the epidermal 

 cells have been entirely removed, trans- 

 planted into an "empty" breast feather fol- 

 licle, induces a feather of the breast type 

 from the epidermal cells of the breast fol- 

 licle wall which grow over it, and vice 

 versa. Breast or saddle papillae from which 

 the epidermal cells have not been removed 

 retain their specificity when transplanted 

 into empty follicles of either tract. 



The Hair. Hairs, like feathers, are highly 

 keratinized epidermal outgrowths which 

 arise also in a definite time-space sequence. 

 The first series of primordia are uniformly 

 spaced. As noted previovisly for feather pri- 

 mordia, so here, a second series arises to each 

 side of the first and definitely oriented in 

 relation to it, and so on in transverse rows 

 until the number characteristic of the species 

 is laid down. All of the hair papillae are 

 formed during embryonic life or shortly after 

 birth; hence the number and arrangement 

 are the same in the adult as in the embryo. 

 Growth of the connective tissue of the dermis 

 later on, however, does tend to disrupt the 

 earlier more orderly arrangement and to 

 make the linear order somewhat more diffi- 

 cult to discern. 



The first primordia to appear are those 

 of the sensory hairs or vibrissae on the face 

 around the nose and mouth. These arise very 

 early in gestation, long before there is any 

 indication of hair primordia elsewhere on 

 the body surface. In their early develop- 

 mental stages the sensory hairs differ in 

 certain respects from the general body hairs. 

 The future site of a sensory hair is fore- 

 shadowed by a sub-epidermal condensation 

 of the mesodermal cells of the prospective 

 corivim, the primordium of the dermal 

 papilla. This precedes any appreciable 

 change in the overlying epithelium. Rapid 

 proliferation of the mesodermal cells raises 

 the epidermis, forming rows of papilla-like 

 elevations easily observable in surface view 

 (Fig. 195/1). These elevations (Hockerchen) 

 have been described by many of the early 

 investigators for various mammals — cat, 

 sheep, pig, rabbit. In sections they are strik- 

 ingly similar to down feathers (cf. Figs. 

 1P3.4 and 195A). This initial stage is of 



