70 



KERATIN AND KERATINIZATION 



quill. When the thin skin forming the sheath and covering the feather 

 during growth bursts, the feather opens. 



The larger and more elaborate contour feathers in which the barbs 

 project from the sides of a shaft are formed in a more complicated way. 

 Up to a point development is similar to that of a plumule ; then the mid- 

 dorsal region of the germinal collar begins to proliferate more rapidly to 



Fig. 30. The parts of a typical flight (contour) feather. CA calamus 



a simple hollow cylinder, AR the rhachis, and B, barbs. The barbules 



(not shown) lock the barbs together. X-ray patterns are usually obtained 



from the calamus or the rhachis. 



form the rhachis. As before, the calamus (or quill) grows as a continuous 

 cylinder and fails to split into barbs. This question is returned to in 

 Chapter 3. 



Feathers are normally shed in an annual moult and replaced. The 

 first down or nestling feathers are replaced by juvenile feathers which 

 resemble true contour feathers growing from the same follicle. The same 

 follicle is thus capable of growing feathers of different kinds. 



