DEVELOPMENT OF THE SKIN 573 



rows, one on either side of the shaft. From the barbs, smaller branches ex- 

 tend outward; the latter are the barbules (fig. 270E). An interlocking system 

 of hooks, the barbicels, enables the barbule of one barb to connect with a 

 barbule of the next barb. If these inierlocking hooks are disrupted mechani- 

 cally, the bird restores them while preening its feathers. 



b) Plumule or Down Feather. The plumules or down feathers form 

 an inner feathery coat which lies below the contour feathers in the adult bird. 

 They constitute the main insulating portion of the feather coat. In the down 

 feathers of the adult, the barbs arise in bouquet fashion at the distal end of 

 the quill. On the other hand, the nestling or first down feathers of the chick 

 or newly hatched birds of other species do not possess a quill, for the barbs 

 are attached to the distal ends of the apical barbs of the definitive feather 

 (fig. 270E). Therefore, two types of down feathers are found: 



( 1 ) the nestling down feather without a quill and 



(2) the later down feather which possesses a quill. 



The barbules in down feathers do not interlock, and a vane is not formed 

 (fig. 270D, E). 



c) FiLOPLUME OR Hair Feather. The filoplume or hair feather possesses 

 a long slender shaft which generally is deprived of barbs, although a tuft of 

 barbs may be present at the distal end. 



d) Distribution of Feathers on the Body. Feathers are not evenly 

 distributed over the surface of the body but arise in certain definite areas or 

 feather tracts, the pterylae. Between the pterylae are the apteria or areas 

 where the number of feathers are reduced or absent altogether. When feathers 

 are present in an apterium, they consist mainly of a scanty distribution of 

 downy and filoplumous feathers. 



b. Development of the Avian Skin 



1) Development of the Epidermis, Dermis, and Nestling Down Feather. 



When the epidermal tube in the chick embryo begins to form, it consists of 

 a single layer of cells of one cell in thickness. As development proceeds, this 

 single-layered condition becomes transformed into a double layer, so that at 

 48 to 72 hours of incubation a two-layered epidermis is realized. This condi- 

 tion consists of an outer layer or periderm, considerably flattened, and an 

 inner layer or stratum germinativum (fig. 269A, B). At 96 hours of incu- 

 bation in most parts of the developing integument, a primitive dermis is present 

 as a loose aggregate of mesenchyme below the two-layered epidermis. The 

 origin of a part of this mesenchyme from the dermatome is shown in figure 

 269C. At six days of incubation, mesenchyme is present as a definite dermal 

 condensation (fig. 269D). 



Between the sixth and eighth days of incubation, the epidermis and dermis 

 increase in thickness, and small, mound-like protuberances begin to appear 



