574 INTEGUMENTARY SYSTEM 



in certain areas (fig. 269E). Each elevation is produced by a mass of cells, 

 known as the dermal papilla, which pushes the epidermal layer outward (fig. 

 269E). The initial dermal papillae represent the beginnings of the feather 

 rudiments. At eleven days of incubation, many feather rudiments have made 

 their appearance. Each rudiment consists of a central, mesenchymal (dermal) 

 core or pulp, surrounded externally by epidermal cells. The dermal pulp is 

 supplied copiously with small blood vessels (fig. 269F). The epidermal cells 

 at this time are beginning to be arranged into longitudinal columns of cells. 

 These longitudinal cellular columns represent the initial stages of barb- 

 rudiment development (fig. 270A). This condition of the developing feather 

 marks the beginning of the first or the "nestling down" feathers. 



At 12 to 14 days of incubation, the feather rudiment increases considerably 

 in length and begins to invaginate into the dermal layer at its base (fig. 270B). 

 This invagination of the base of the feather rudiment marks the beginning of 

 definitive feather formation (Jones, '07). In the developing feather from 14 

 to 17 days of incubation, two general regions are indicated. These regions of 

 the developing feather are: 



(a) a region from the surface of the skin to the distal end of the feather 

 germ where the barbs and barbules of the nestling down are being 

 formed (fig. 270B) and 



(b) a proximal region below the surface of the skin where the barbs and 

 barbules of the definitive feather begin to differentiate (fig. 270B). 



After the seventeenth day, the differentiation of the definitive feather pro- 

 ceeds rapidly (fig. 270C, D). 



From the fourteenth to the seventeenth days, the barbs of the nestling down 

 feathers elongate slightly by adding new ridge material at the basal end of 

 each ridge (fig. 270B, C). The length of the barb rudiments of the down 

 feather thus increases as the feather rudiment grows outward from the surface 

 of the skin. As the barb rudiments elongate, they differentiate into the barbs 

 and barbules (fig. 27 IB, C). (See Davies, 1889; Strong, '02.) At about 

 eighteen days of incubation, such a feather may be removed, and the distal 

 portion of the horny sheath may be ruptured with a needle. Following the 

 rupture of the horny sheath, the enclosed barbs will spread out as shown in 

 the distal part of the developing feather in figure 270D. 



At eighteen to twenty days of incubation, feather development in the chick 

 may be represented as shown in figure 270C and D. A distal or nestling- 

 down-feather region and a proximal definitive-feather area are present. Barbs 

 and barbules of the definitive feather differentiate in the proximal area. A 

 real quill is not established at the base of the nestling down feather, although 

 a horny cylinder may intervene between the base of the down feather and 

 the barbs of the definitive feather (fig. 270D). (See Jones, '07.) Thus, in the 

 chick and most birds, the first or nestling down feather and the succeeding 



