12 Jones — Nestling Feathers. 



until all trace of individual barbs is lost (33). Passing proximally along the 

 feather, Figure 34 shows this cornified material again dividing into some- 

 what irregular but elongated masses. Figure 35 shows these masses becom- 

 ing more regular in form and arrangement; Figure 36 shows them assuming 

 the form of a cornified barb with outlines of barbules appearing; and Fig- 

 ures 37, 38, and 39 show them as definite definitive feather barb-vanes, Fig- 

 ure 37 well toward complete cornification of the elements, and Figure 39 

 much less cornified because this figure was taken from the pulpy tip of the 

 definitive feather. It should be borne in mind that Figures 29 to 39 are 

 rather oblique than exact cross sections, nearly in the line X in Figure 3, 

 Plate I. All exact cross sections through the region of transition would 

 show a broken cornified ring (Fig. 29, hr), because, as I have already in- 

 dicated, the plane of growth is always oblique, as pointed out by Davies 

 and Strong for the definitive feather. This obliquity is shown in Figure 3, 

 Plate I, in the line X, which corresponds roughly to the angle which the down 

 makes with the skin surface. In a plane parallel to the transverse axis of 

 the down, the posterior surface is less developed. The cells here were re- 

 tarded in their division while the papilla was turning backward, whereas a 

 more rapid division of the cells took place on the anterior surface. 



Figures 10 to 20, Plate II, prove that at the time when the barb vane 

 ridges are beginning to be differentiated as such at the distal end of the def- 

 initive feather, practically all of the barb-vane elements at the proximal 

 end of the down have become cornified and their cellular nature obliterated. 

 Hence, in order to trace the cytology of the transition from the down barb 

 into the definitive feather barb it became necessary to study feathers at differ- 

 ent stages of development. We will first consider die case illustrated in 

 Figure 139, Plate IV, where no barbules are formed at the extreme prox- 

 imal end of the down just before it branches into definitive feather barbs. 



At X in this transition region, at an early stage in development, the 

 epidermal cells are arranged as represented by Figure 7, Plate I. In other 

 words, division of the mucous cells has proceeded in the usual manner — there 

 is no indication that a fully formed feather will not be developed at this 

 point At a little later stage of development (Fig. 40, PI. Ill) barb-vane 

 ridges (brb. crs) are indicated by a more rapid proliferation of the interme- 

 diate cells (cl. in) at certain places, where the cells are alike in shape and 

 size, and differ from the surrounding cells. Figure 41 illustrates a still later 

 stage of development, at which a group of similar intermediate cells (brb. 

 crs) has become definitely separated off from those cells which surround it. 

 Figure 42 shows the differentiation further progressed. Figure 43 shows the 

 cornification of the cells begun and the retreat of the barb ridge toward the 

 periphery, and Figure 44 cornification of the barb and almost complete oblit- 

 eration of cellular structure. The whole epidermal wall is here far toward 

 maturity. It is at once evident that a decreased differentiation of the inter- 

 mediate cells in this region has resulted in a barb devoid of barbules. The 



