Jones — Nestling Feathers. 17 



VI. summary. 



1. The development of the barbs and barbules is similar in the first 

 down and the definitive feather. 



2. The barbule cells of the down never develop hooked barbicels, 

 but some of the distal cells may give off filamentous barbicels from their 

 distal end. These filamentous barbicels are homologous with the barbicels 

 which are developed from the definitive feather barbule cells. 



3. The first down has no shaft. The barb-vanes which compose the 

 first down are continuous and separate through the entire length of the 

 down. 



4. The first down has no quill. The down barb-vanes pass without 

 interruption into the first definitive feather barb-vanes, either as entirely sep- 

 arate barb-vanes or else as a more or less homogeneous horny cylinder which 

 can be split along lines continuous with the barb-vanes by pressing or rub- 

 bing the so-called "quill" betw^een two hard surfaces. The horny cylindri- 

 cal connection between the down and the first definitive feather is frequent 

 among birds whose period of incubation is less than seventeen days. 



5. No shaft is formed at the extreme distal end of the first definitive 

 feather. The rudiments of a shaft begin to appear several millimeters prox- 

 imal to the distal end of the feather by the coalescing of two or more barb- 

 vane ridges. 



6. The dermal pulp tissue is confined to the central region of the 

 first down and does not separate the barb-vane ridges from each other nor 

 from the layer of sheath cells at any stage of development. This is also true 

 of the definitive feather. 



7. The first down and its succceeding definitive feather are pro- 

 duced by one continuous growth, and therefore cannot be regarded as two 

 distinct feathers. The first down is the plumulaceous tip of the first defini- 

 tive feather. 



8. Barbules are rarely entirely lacking in the first down. A typical 

 down barbule is made up of a number of lengthened barbule cells placed 

 end to end, the resulting barbule being long and slender, and not differing 

 from the barbules of plumulaceous definitive feathers. 



