Jones — Nestling Feathers. 13 



fact that relatively few of the intermediate cells are involved in the forma- 

 tion of the barb indicates that the residual intermediate cells represent the 

 missing barbules. 



The progress of transition which results in a so-called "quill" or tube 

 differs in some important particulars from that just given. In the early 

 stages of development no difference is recognizable, but at a little later stage 

 the whole mass of intermediate cells (Fig. 45, PL IV, cl. in) as well as the 

 sheath cells (cl„ tu) become much flattened, their nuclei elongated, and their 

 cell boundaries lost in a mass of fibrous tissue. Only the row of cells next 

 to the pulp, representing the cylinder cell layer, retains its characteristic 

 shape. At a still later stage in development, represented by Figure 46, in 

 which the epitrichial sheath is not shown, cornification of the outer rows of 

 cells, representing the region of the sheath cells, has taken place, only sug- 

 gestions of its original fibrous structure remaining. The outermost inter- 

 mediate cells have become almost wholly fibrous, narrow spaces representing 

 the position of the nuclei. The process of cornification now proceeds rap- 

 idly until practically all of the intermediate cells become cornified, and the 

 cylinder cell layer becomes fibrous. Figure 33, Plate III, represents the final 

 stage in development. That the formation of this horny tube is wholly dff- 

 ferent from the process by which the shaft and quill of the definitive feather 

 are formed, as described by Davies (p. 594 et seq.), is evident. Instead 

 of being a process designed for the accomplishment of a definite work — the 

 building of a shaft and quill — it appears to be due to a lack of differentiation 

 of the cell mass and a short cut to cornification of the tissues induced by a 

 reduced blood supply to this part of the feather during the period when the 

 cells would be showing differentiation if supplied with sufficient nourish- 

 ment. It is significant that this condition of a cornified ring instead of the 

 normal barb-vanes is more often found among the strictly altrical birds 

 which are hatched in a helpless condition. It is well known that the first 

 few days after the hatching of altricial birds are the most critical days of 

 their lives. During this critical period there appears to be no growth of the 

 down. An American robin which hatched on the fourteenth day of in- 

 cubation possessed the usual down upon the head and back. These downs 

 made no further growth. It was not until the fourth day after hatching 

 that the skin gave evidence of the beginning of the definitive feathers. On 

 the eighth day after hatching the skin surface was exposed to the drying in- 

 fluences of the air before renewed activity in the feather germ began. Dur- 

 ing this interval of four days the so-called "quill" was formed at the 

 proximal end of the down by the rapid drying of the imperfectly formed 

 barb-vane ridges. 



Although this horny cylinder seems often to be uniform in thickness for 

 its whole circumference, it frequently divides along lines continuous with the 

 barbs of the down, or may be made to so divide by gently rubbing it between 

 two hard surfaces, thus establishing the continuity between the down barbs 

 and the definitive feather barbs. 



