i6 Jones — Nestling Feathers. 



end of a first down of a fourteen-day chick embryo (47 to 50 and i, Figure 

 I, Plate I). 



Figure 47 represents two barb-vane ridges in cross section distal to 

 where the most distal barbules become attached to the barb. These two 

 barb-vane ridges show a decided reduction in the number of barbule cells 

 (bbl. cl) all of which are large. The cylinder cell layer (cl. cyl) has begun 

 to separate the barb vane ridge from the sheath cells. The axial plate (ax. pi) 

 is represented by large, irregular cells. There is a well developed basal 

 membrane (mb. ba) which confines the dermal pulp to the center of the 

 feather rudiment. In Figure 48 the barbule cells remain the same in num- 

 ber, but all of them are cut nearer their distal ends, and are therefore 

 smaller. The cylinder cell layer more nearly separates the barb-vane ridge 

 from the sheath cells. In Figure 49 barbule cells have disappeared, but the 

 barb rudiment persists as a row of cells within the complete sheath of cylin- 

 der cells; in the center four cells represent the medulla of the barb. Resid- 

 ual cells occur between the ridges, and a basal membrane still shuts the 

 pulp away from the barb-vane ridges and the spaces between them. In 

 Figure 50 nine barb-vane ridges are cut across at different distances from 

 their distal ends. They are in reality rings of cylinder cells enclosing cells 

 which are destined to form the extreme distal ends of the down barbs. 

 Residual cells fill the spaces between the barb-vane ridges, and a basal mem- 

 brane confines the dermal pulp to the central region of the feather. In 

 Figure 51 the feather rudiment is reduced to the feather sheath. and central 

 pulp separated by a series of residual cells. Barb-vane ridges have entirely 

 disappeared. 



