INTEGUMENT 



369 



cellular matter derived from the epidermis of the feather papilla. When 

 the feather is pulled out or is molted, the papilla is left behind, and 



FIG. 334. A second stage in the growth of a down feather, ep.l., epitrichial layer of epider- 

 mis; int., intermediate layer of epidermis; cy.l. t basal layer of cylinder cells of epidermis; 

 bl.v., blood vessel in papilla. (After DAVIES.) 



begins to form a new feather by the growth from its proximal dorsal 

 surface of a new papilla. 



The first feather to be formed by the embryonic papilla such as was 

 mentioned on the preceding page is called the down feather. The em- 

 bryonic papilla (Fig. 334) elongates, the while that it settles into the 

 skin. The whole structure now consists of an evaginated region of the 



bottom of an invaginated 

 region. The outermost epi- 

 dermal layer, however, does 

 not dip into the fundus but 

 reaches from the surface 

 directly up and over the 

 feather rudiment. 



The epidermis is very 

 thick on the papilla, and 

 soon begins to show a dif- 

 ferentiation. The basal 

 layer is thrown into longi- 

 tudinal folds whose inner 

 flexures are round, and 

 whose outer folds are 

 sharp and plate-like (see 

 Fig. 335). The mesodermal 

 tissue of the papilla extends 

 as a thin layer out into the 

 plates. The middle and basal layers of the epidermis are now divided 

 into a series of cylinders running lengthwise on the papilla. 



2B 



FlG. 335. Transverse section through a somewhat older 

 pin feather than that represented in Fig. 334. bl.v., 

 blood vessels in the papilla; ep.l., epitrichial layer of 

 epidermis; int., intermediate layer of epidermis; cy.l., 

 basal layer of cylinder cells. The longitudinal cylin- 

 ders are partly of cylinder cells. The longitudinal 

 cylinders are partly formed on the dorsal side of this 

 young structure. (After DAVIES.) 



