6 Jones — Nestling Feathers. 



feather germs out by means of the down were ineffectual. Likewise, barb- 

 vanes pulled out were useless for any other study than that of the complete 

 down, the proximal portion invariably being torn. 



Plate V represents down attached to definitive feathers photo- 

 graphed enlarged. Plates VI, VII, and VIII were made by direct printing 

 upon sensitive paper. In the latter case the feathers were placed upon a plate 

 of clear glass, and a sheet of sensitive paper was pressed down upon them 

 with the sensitive face against the feathers. The whole preparation was 

 then exposed to the light in the regular printing out photographic process. 

 Developing papers were used, but these gave less satisfactory results. Of 

 course the prints thus obtained give very little or no color patterns. The 

 feathers act as opaque objects upon the sensitive paper. This method served 

 admirably where the photographic method must fail because the color con- 

 trasts between the down and the definitive feather make any sort of 

 a background act as a neutralizing agent for one or the other. Dividing the 

 background to suit the two contrasting elements spoils the picture. 



In the further discussion of this subject it will often be necessary to 

 speak of the barbs and barbules collectively, and for this purpose I shall em- 

 ploy the term barb-vane. The feather-vane is made up of a median shaft 

 with barbs attached on each side. The barb is the median axis of what I call 

 a barb-vane, and the barbules are arranged on the barb as the barbs are sit- 

 uated on the shaft. Likewise, during the earlier developmental stages of the 

 down, before barb and barbule elements are clearly defined, the ridge from 

 which these structures are to result will be called barb-vane ridge. 



IV. the development of the nestling down. 



A. The Feather Germ. 



Davies (1889) seems to be the last investigator to study the develop- 

 mental stages of the nestling down. His is by far the most thorough and the 

 most accurate treatment of the subject, but he gave no attention to the devel- 

 opment of the barbs and barbules. He gave an extended review of the lit- 

 erature up to the time of his work, pointing out the contribution which each 

 successive author made to the subject. 



According to Davies (p. 571), the down first appears in the pigeon 

 embryo on the fifth day of incubation as round white spots in the two- 

 layered skin. These two layers he terms "epitrichial," outer (PI. I, Fig. 6, 

 Ep) and "mucous," inner (PI. I, Fig. 6, Mp). These white spots are pro- 

 duced by groups of dermal cells (Drm. pp) closely pressed against the epi- 

 dermis, and cause slight prominences upon the skin surface. Division of the 

 mucous cells which lie over these groups of dermal cells now begins longi- 

 tudinally, causing an arching of the epidermis, and the cavity in the skin 

 thus formed becomes filled with dermal cells which are also rapidly dividing 

 (PI. I, Fig. 6). At this time the epitrichial cells undergo transverse divis- 



