576 INTEGUMENTARY SYSTEM 



As the shaft is laid down progressively from apex to base, the continuous 

 concrescence of the collar cells and gradual formation of the shaft rudiment 

 along the middorsal plane of the feather germ bring about the formation of 

 the shaft (Lillie, '40; Lillie and Juhn, '32, '38), beginning at its apex and 

 progressing baseward. 



As the collar material is fed into the developing shaft rudiment dorsally, the 

 bases of the barbs, which are located in the collar or germinative ring, are 

 carried continuously dorsalward and eventually become located along the sides 

 of the shaft (fig. 270E). Also, the first set of barbs, which was formed in the 

 first phase of contour-feather formation, becomes attached along either side 

 of the developing shaft in the same way that the later barbs become attached. 



In the formation of the barb, the apical or distal end of the barb is laid 

 down by cellular contributions from the collar. Following this, more basal 

 or proximal portions of the barb are elaborated by cellular deposition from 

 the collar cells. The base of the barb thus remains attached to the collar 

 as the barb rudiment elongates, while the apex maintains its position in the 

 midventral line. As the base of the barb and the collar material to which it 

 is attached move dorsalward toward the forming shaft, as observed in the 

 previous paragraph, the base of the barb comes in contact with and fuses 

 with the rachis or shaft, whereas the ventral extremity, i.e., the distal end 

 of the barb, remains associated with the mesodermal pulp along the ventral 

 aspect of the developing feather (fig. 271 A). The barb thus comes to form 

 a half spiral around the developing feather within the external horny sheath 

 (fig. 270E). As successive barb rudiments are laid down, the previously 

 formed barbs are moved progressively distad along with the mesodermal core. 



c) Formation of the Barbules and the Feather Vane. During the 

 period when the barbs are being formed, the side branches of the barbs or 

 barbules are developed by the formation of groups of cells along either side 

 of the barb (fig. 271B, C). Each of these groups of barbule cells differentiates 

 into a barbule. A barbule thus represents a group of cells, specialized to form 

 an elongated structure as shown in figure 27 ID. After the distal end of the 

 feather extends markedly beyond the surface of the skin, the horny sheath 

 breaks, and the barbs and barbules expand to form the vane of the feather. 

 In doing so, the barbules interlock by means of barbicels which develop on 

 the barbules, located on the side of the barbs facing toward the apex of the 

 feather (fig. 27 ID). 



d) Later Development of the Feather Shaft. During its develop- 

 ment, the shaft gradually enlarges in the direction of the base of the feather. 

 When the feather approaches its mature length, the shaft has enlarged to 

 the extent that it comes to occupy the entire basal portion of the feather rudi- 

 ment. As the last condition develops, barb formation becomes less exact until 

 finally it is suppressed altogether. When this stage is reached, the contained 

 dermal pulp within the base of the shaft begins to atrophy, starting at the end 



