CHAPTER XIX 



NOTES ON THE DEVEI.OrMENT OF THE SMOOTH-BILLED AN I 



( Crotoph aga ani ) 



PTERYLOSIS 



The series of eiiibrvos examined embraced several stages 

 in the development of the down feathers in their papillae 

 state. The different tracts were found in their most simple 

 and primitive form, presenting excellent opportunities for 

 the study of their development in regard to the rate of 

 growth, pigmentation and transition to the adult stage. 



When the embryo has passed through about half its 

 incubation period, the papillae containing future down 

 sheaths show scarcely any pigmentation. None of the 

 sheaths, except the rectrices and their upper coverts, as yet 

 are pigmented. The rectrices, eight in number, are divided 

 into two groups by the pygostyle, the sheaths being 2 mm. 

 long and most heavily pigmented at their bases. The coverts 

 are smaller and less developed, the central pair being pig- 

 mented the most densely. 



All the feather tracts are pigmented in the embryo of 

 about thirty-six hours later development. The rectrices are 

 still the longest, though the femoral and humeral tracts have 

 made a rapid grow^th. The femorals are indicated by a single 

 row of sheaths directly in line with the rectrices and appear 

 to be a continuation of them. Proceeding anteriorly, they 

 divide into several rows and form a triangle, one side of 

 which is parallel to the spinal tract. The single row con- 

 tains more pigment than the remainder. (Fig. 102.) 



The spinal tract is narrowly cleft over the sacral and 

 dorsal portions, closing at the neck and lower pelvis. It 

 consists of a double line of sheaths which run together into a 

 single line over the scapular region, and where the cleft 



