9 Jones — Nestling Feathers. 



tern, gull, and duck embryos, all of which showed essentially the same char- 

 acteristics represented by these figures. 



B. The Barbs and Barbules. 



Differentiation of the epidermal wall of the nestling down begins as 

 soon as the down papilla has passed distally beyond the region of cell divis- 

 ion. It therefore follows that the distal end of the down papilla is more de- 

 veloped than any region proximal to it. Cross sections at different distances 

 from the distal end will therefore give different stages in the development 

 of the down structures. 



At the level (7, Fig. i, PI. I) where cell division has just been com- 

 pleted one finds the condition represented by Figure 7, Plate I. The dermal 

 nuclei are numerous and lie closely pressed against the layer of cylinder 

 cells (cl. cyl), being separated from it by a basal membrane (mb. ba). A 

 layer of cubical intermediate cells (cl. in) rests upon the cylinder cells, but 

 peripheral to this layer the intermediate cells have become elongated and 

 irregular, becoming the sheath cells (cl. tu). 



Figure 8 illustrates the condition at the level 8 in figure i, where the 

 basal membrane has pushed in dividing cylinder cells and intermediate cells 

 into groups. Figure 9 shows the barb-vane ridges completely separated 

 from each other except at their extreme base. The cylinder cells are much 

 flattened and elongated and their nuclei are far apart. The intermediate 

 cells are definitely arranged into three groups — a central row of cells indi- 

 cate the position of the so-called axial plate (ax. pi) and on each side of this 

 plate a series of barbule cells (bbl. cl). 



It will be at once evident that the differentiation of the down barbs 

 and barbules is so similar to the differentiation of the same structures 

 so fully described by Strong (1902) for the definitive feather that to follow 

 the steps of differentiation farther would be only to duplicate what has al- 

 ready been done. Some minor differences should be mentioned. 



1. The down rudiment has a much shorter diameter and fewer ridges 

 than the definitive feather. Figure 4, Plate I, represents a semidiagrammatic 

 cross section at about the middle of a down from the Common Tern {Sterna 

 hirundo), and Figure 5, a similar cross section of a definitive feather from 

 the same species. The down has a sort of radial symmetry, the ridges being 

 approximately equal in size, while the definitive feather has a decided bilat- 

 eral symmetry due to the shaft rudiment X and the wedge-shaped piece 

 opposite the shaft rudiment where the two vanes will separate when the 

 sheath falls away. 



2. The down barbules are never provided with hooked barbicels (Fig. 

 52, PI. IV), hence the down has a loose, light texture, in contrast to the firm 

 texture of most definitive feathers. Although hooked barbicels are wanting 

 in the down, several of the distal barbule cells send out one or two filamen- 

 tous prolongations from their distal ends (Figs. 52, 55, PI. IV) which resem- 

 ble the unhooked barbicels of the definitive feather barbules (Fig. 54, PI. 



