370 



HISTOLOGY 



The two or three outer layers of the epidermis not involved in this 

 cylinder formation now form a stratified and partly cornified layer, the 



sheath layer, on the outside of 

 the rudimentary feather (Fig. 

 336). The outer edges of the 

 plates, where they touch this 

 sheath, become flattened and 

 broadened to partly cut the 

 cylinders off from the sheath 

 layer. This is more apparent 

 distally and disappears proxi- 

 ^ mally. 



cy. 



I. 



Finally the cylinder cells 

 form concentric layers in each 

 cylinder, and cornify in the 

 distal part (Fig. 337). The 

 sheath falls off and the pulp 

 tissue withdraws its blood 

 supply and dries up. This 

 leaves the cylinders free, but still connected at their basal ends with 

 the hardened base of the papilla which has not formed this part of its 

 epidermis into cylinders. We thus have, as a completed structure, a 

 quill-like base from whose distal 



bl. v. 



FIG. 336. A later stage, than Fig. 335, in the de- 

 velopment of a down feather of the pigeon. The 

 cylinders are well defined" but have not yet cut off 

 their inclosed intermediate cells, which now form 

 the longitudinal plates, from the epitrichial layer. 

 Letters same as in last figure. (After DA VIES.) 



circular 

 processes 



row of 



called 



end arises a 

 filamentous 

 the barbules. 



This down feather is soon 

 lost, usually by being pushed 

 out by the new or permanent 

 feather which takes its place. 

 This second feather, which like 

 all its successors is known as a 

 definitive feather, arises as a 

 new papilla that grows out of 

 the base of the old one which it 

 reabsorbs. Like the first pa- 

 pilla, it is a dermal pulp, covered 

 with a thick epidermis. This 

 epidermis, when the papilla is 

 large enough, begins also to 

 develop a series of longitudinal 

 folds much like those of the down feather rudiment. Several of the 

 folds on the dorsal side of the papilla continue straight and become 



FIG. 337. Transverse section of a young down 

 feather near maturity, int., longitudinal plates 

 of what were formerly the intermediate cells; 

 cy.l., cylinder cells which now surround the lon- 

 gitudinal plates. Other lettering same as in pre- 

 ceding figure. (After DAVIES.) 



