INTEGUMENT 



371 



fused together as one. They grow extensively and become the rachis. 

 Figure 338 shows a cross section of a feather that shows this devel- 

 opment. 



The outer cylinders become arranged obliquely on the papilla pulp 

 so that they form two series each attached in a line, one to one side, the 

 other to the other side of the rachis. They now lie obliquely on the pa- 

 pilla, and as the rachis emerges from the skin it drags them in two rows 

 with it. They continue soft and developing on the bottom until the 

 head of the quill emerges with the last of them, thin and poorly developed, 

 as a ring of downy barbs which surround the superior umbilicus. 



FIG. 338. Transverse section of a large vane feather or permanent feather in an early stage 

 of development, v., point at which two of the longitudinal plates are united and enlarged 

 to form the vane or rachis. The remaining plates become specialized to develop the barbs 

 and barbules of the feather. (After DAVEES.) 



The growth of the individual cylinder into a barb of the definitive 

 feather is complicated by the fact that the latter develops, in the course 

 of its growth, the two series of barbules on its sides. It is the inner rod- 

 like region of the cylinder which hardens into the barb, while its outer 

 cells become arranged in slanting rows and harden into the barbules, 

 forming the hooks in one set and the plates in the other. Space forbids 

 us to go further into the details of this complicated development which 

 has been worked out by H. R. Davies and others. Figure 339 shows a 

 transverse section of a single barb in process of development. 



Scales of Fishes. Our last example of an integumental structure 

 used for mechanical protection is peculiar, in that it is developed in the 



