Jones — Nestling Feathers. 



PLATE III. 



All figures are from a 20-day chick embryo. 



Figs. 29 to 33. Entire cross sections of down, x 141. All cellular struct- 

 ure has disappeared. 



Figs. 34 to 39. Cross sections of the feather, passing proximally into the 

 distal end of the definitive feather. 34, 35, and 39. x 618. 36, 37, 

 and 38. X 323. 



Figures 29 to 39 form a series to illustrate the passage of the down into 

 the definitive feather when a so-called "quill" is formed, that is, when 

 the individual down barbs are lost in a horny ring, at Fig. 3, Plate I. 



Figs. 40 to 44. Cross sections at X in Figure 2, Plate I, at different 

 stages in the development of this region, to illustrate the reduced dif- 

 ferentiation of the cells which results in a barb devoid of barbulcs. 40, 

 41, and 42. X 371. 43 and 44. x 516. 



