370 



HISTOLOGY 



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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. 



Finally the cylinder cells 

 form concentric layers in each 

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



velopment of a down feather of the pigeon. The j:_ t -i _.._<. / F - ,,,_\ rpi 



cylinders are well defined, but have not yet cut off ' l P ari v ri g- 337 ) 



their inclosed intermediate cells, which now form sheath falls off and the pulp 



the longitudinal plates, from the epitrichial layer. 

 Letters same as in last figure. (After DAVIES.) 



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 



end arises a circular row of M. v. 



filamentous processes called 



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 



reab sorbs. 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 



cy.l. 



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.) 



