DEVELOPMENT OF THE SKIN 575 



definitive feather are developed as one continuous process, and cannot be 

 regarded as two separate feather growths (Jones, '07, p. 17). When the chick 

 hatches, the outer horny sheath around the differentiated down feather dries 

 and cracks open, and the barbs and barbules of the down feather spread out 

 into fuzzy tufted structures (fig. 270D). Later, as the definitive feather emerges 

 from the surface of the skin, the down-feather barbs appear as delicate tufts, 

 attached to the distal ends of the barbs of the definitive feather (fig. 270E). 



2) Development of the Contour Feather. The development of the contour 

 feather is more complicated than that of the nestling down feather described 

 above. Its development may be divided into early or primary and later or 

 secondary phases (Lillie and Juhn, '32). The formation of barbs during the 

 early phase consists in the elaboration of barb and barbule rudiments without 

 a shaft rudiment. This type of development resembles somewhat that of the 

 down feather. The secondary phase of contour-feather development is con- 

 cerned with the formation of a shaft, as well as the barb and barbule rudiments. 



a) Formation of Barbs During the Primary or Early Phase of 

 Contour-feather Formation. During the first phase of contour-feather 

 formation, the barbs are formed in two different orders. The first order of 

 barb rudiments arises more or less simultaneously (Lillie and Juhn, '32); they 

 are practically of the same size, about equal in number on either side, and 

 dorsally placed. After this first set of barb rudiments is formed, a second order 

 of barb rudiments arises in seriatim with the youngest barb rudiments, located 

 more ventrally. (See first and second sets of barb rudiments in fig. 270D.) 

 Both of these sets of barb rudiments eventually give origin to the barbs at the 

 apical or distal end of the feather. As a shaft is not formed during the period 

 when these two sets of barb rudiments are developing, i.e., during the first 

 phase of definitive, contour-feather formation, these barbs later become asso- 

 ciated with the forming shaft as the latter develops during the next or second 

 phase of feather formation. 



b) Secondary Phase of Contour-feather Formation. Following the 

 formation of the barb rudiments mentioned above, the second phase of feather 

 formation is initiated. It consists in the formation of the shaft and the further 

 development of barb ridges and barbules. The development of the shaft is 

 effected by the migration dorsalward of the collar cells (fig. 270E), which 

 produces a continuous concrescence and fusion in the middorsal line of the 

 two dorsal ends of the barb-bearing collar. This fusion of the collar cells 

 forms the rudiment of the shaft as indicated in figure 270D. This concrescence 

 of cells, however, establishes only the rudiment of the shaft, for it is apparent 

 that the development of the shaft results from two sets of processes: 



( 1 ) the concrescence of a segment of the shaft rudiment at a particular 

 point in the middorsal line of the feather rudiment and 



(2) the elongation and growth of the rudiment material thus established. 



