Jones — Nestling Feathers. i i 



surface, or even by slight projections from the surface, representing rudimen- 

 tary barbules, as in this figure. 



Figure 142 represents a common condition found among all of the water 

 birds except the Anseriformes. This is undoubtedly the typical condition, as 

 I hope to demonstrate in the following detailed discussion of the develop- 

 mental processes which take place in the region of transition from nestling 

 down into first definitive feather. 



Before discussing the development of the feather in the region which is 

 supposed to lie between the down and the definitive feather, I will sketch 

 in a general way the passage of a first down barb-vane into the definitive 

 feather barb-vane or vanes. Figures 10 to 46, Plates II and III, have been 

 prepared to illustrate the three conditions represented by Figures 140, 141, 

 and 142. 



Proceeding from the down at the level 10, Figure 2, Plate I, where the 

 down is complete and only waits the loss of the sheath to expand, we will fol- 

 low first the process which results in the condition illustrated by Figure 141, 

 Plate IV, as shown in Figures 10 to 20, Plate II. 



In Figure 10, Plate II, two down barbs are shown in cross section with 

 their barbules cut at different distances from their attachment to the barbs. 

 Cellular structure has been entirely obliterated by the cornification of the 

 cells. Figures 10 to 20 were taken at intervals of nearly 36 microns prox- 

 imally toward the distal end of the definitive feather. Barbules rapidly 

 disappear and none are formed in the region of transition. The last vestige 

 of a barbule will therefore be its base where it is attached to the barb. The 

 medulla of the barb disappears and the shape of the barb in cross section be- 

 comes somewhat circular. 



In Figure 12, remnants of nuclei begin to indicate the cellular 

 origin of the barbs, and in 14 scattered cells are found in one side of 

 the barb. In 15 the cells predominate, and in 16 cornification has entirely dis- 

 appeared. The larger barb becomes irregular, in 17 there is an evident 

 division in progress, which is completed in 18. In 19 the three cellular masses 

 begin to assume the character of barb-vanes, and in 20 they are unmistakably 

 barb-vanes with cylinder cells, barbule cells, and axial plate. One of the 

 down barbs has remained single and has passed directly into one definitive 

 feather barb, while the other has divided and merged into two definitive 

 feather barbs. 



Figues 21 to 28 illustrate the passage of a down barb from a 21 -day 

 chick embr>'0 into two definitive feather barbs, after complete cornification of 

 all cells. No barbules are shown. 



Figures 29 to 39, Plate III, were taken from a 21-day chick embryo 

 ready to hatch, at somewhat irregular intervals, from the region marked 29 

 to 39 in Figure 3, Plate I. Figures 29 to 33 are entire cross sections of the 

 down. In Figure 29 seven down barbs are seen in cross section (brb), and 

 two masses of cornified material (hr) which are the fused proximal ends of 

 the several remaining down barbs. Figures 30, 31, 32, and 33 illustrate the 

 further fusion of the proximal ends of the barbs into cornified material (hr) 



