Jones — Nestling Feathers. 15 



ening of feather development, and therefore proves this first feather to be the 

 first down. The fact that the shaft rudiment of the first definitive feather 

 of the duck does not make its appearance until after the seventeenth day of 

 incubation, but that it does appear before the duckling is hatched, and that 

 the first feather continues to grow for many daj^s after the hatching, prove 

 that in the development of the first feather of the duck there are stages of 

 development which are exactly paralleled during the development of the 

 down and first definitive feather of other birds. These stages may be briefly 

 summarized for the sake of comparison. 



From the fifth or sixth day of incubation to about the twelfth day of 

 incubation the first feather is a slender outgrowth of the epidermis with a 

 dermal core. From the twelfth to the seventeenth day an invagination at 

 the proximal end of the first feather occurs and a feather pocket is formed. 

 On or about the eighteenth day that portion of the feather which lies within 

 the skin surface begins to enlarge. About the twenty-third day the shaft 

 rudiment is formed by the coalescing of two barb-vane ridges. From this 

 time onward barb-vane ridges are becoming merged into the shaft rudiment 

 and new barb-vane ridges are being developed in the epidermal wall of the 

 feather opposite to the shaft rudiment. At the limit of growth of the feather, 

 barb-vane ridges cease to develop and a quill is formed by a process fully 

 described by Davies for the definitive feather (p. 597, et seq.). 



These stages of development and growth cover the whole period of the 

 development and growth of the first down and the first definitive 

 feather of most birds, but of only the first feather of the duck. The first 

 feather of the duck is therefore a combination of the first down and 

 the first definitive feather of other birds, and undoubtedly represents the 

 primitive relation between these two structures. The first down is 

 morphologically the distal end of the first definitive feather. This relation 

 is very marked in Figure 58, Plate V, in Figures 70 and 71, Plate VI, and 

 in Figures 97 and loi, Plate VII. Figure 85, Plate VI, represents a condi- 

 tion found where a plumulaceous definitive feather follows a first down. 

 Except for the distinct growth mark where barbules are lacking, 

 there is scarcely a difference in structure between the barb-vanes of the two 

 parts of this feather. In a pennaceous feather the barbs and barbules are 

 shorter and stiffer. Most definitive feathers are partly pennaceous and 

 partly plumulaceous, so that in a single feather are combined the rigidity of 

 the pennaceous definitive feather and the soft fluffiness of dow^n. 



Davies makes the statement that in the course of development of the 

 down the pulp tissue pushes in between the barb-vane ridges and 

 finally comes to surround the ridges almost completely, only a thin layer of 

 cells serving to attach the ridge to the feather sheath as the peritoneum at- 

 taches the digestive tube to the body wall (p. 581). Only one of his fig- 

 ures (7) partially supports this statement, and it is diagrammatic. Strong 

 did not find such a condition in the definitive feather, and I have failed to 

 find it at any stage in the development of the first down. I have 

 prepared Figures 47 to 51, Plate IV, to illustrate the conditions at the distal 



