Jeffries.] 210 [February 7, 



The mucous cells are as yet unaltered. As the papilla grows, new 

 thickenings are formed until from twelve to twenty exist (fig. 

 7). As the number of folds increase they become more and 

 more cut off from the rest of the epiderm, so that they almost 

 form columns. This is true of the tips only, the greater part of 

 the folds remaining triangular in section (fig. 7). The differ- 

 ence is due to the lack of barbules at the tips of the barbs. 



With the above changes in form are associated certain histo- 

 logical changes. The mucous layer ceases to produce new cells. 

 Those already formed suffer various changes according to their 

 position. The outer ones help to form the sheath of the feather 

 which will be described later; the cells in the centers of the folds 

 simply thicken their walls or form them. Here it is to be noted 

 that most if not all the cells forming the surface of the barb-tips 

 are derived from the mucous layer. The cells between the folds 

 begin to degenerate about this time. 



At a more advanced stage the development is complicated by 

 the formation of barbules. This causes the ridges to increase 

 considerably in size and offers a difficult problem to the investi- 

 gator. The barbs are formed by the cells at the angle of the 

 thickenings, as seen in section, while the cells on the sides arrange 

 themselves in columns (fig. 8) which bend slightly towards the 

 tip of the papilla and ultimately form the barbules. The cells 

 nearest the surface are the smallest. These cells, as yet with 

 distinct nuclei and nucleoli, rapidly elongate and loosen their 

 connection with the cells of adjacent columns. By this means 

 large intercellular spaces are formed, or rather the intercellular 

 substance is much increased. The elongation of the cells of the 

 columns begins at the ends, that is, at the parts destined to form 

 the tips of the barbules. This growth in length of the barbules 

 of course necessitates an alteration in the position of the bar- 

 bules. The tips of the barbules slowly push their way towards 

 the tip of the feather-sack or papilla; a change which would 

 burst the feather-sack were it not that the cells of the barbules 

 decrease in transverse section in proportion as they elongate. 

 While the above changes are going on in the mucous cells and 

 those next to that layer, the ceils in the middle of the thicken- 

 ings shrink and form a sort of connective tissue. The pulp also 

 becomes very vascular and assumes an adenoid structure. Thus 



