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ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



row of barbules are provided with booklets (hamuli) which hook over 

 the proximal barbules belonging to the next barb, and it is this which 

 causes the barbs to cohere together to form the flat vane. When these 

 hamuli are not present the feather is downy — the individual barbs 

 remaining loose. This condition is usually to be seen in the barbs 

 nearest the quill (Fig. 184, A). 



From near the base of the rachis 

 there springs the after-shaft or hypo- 

 rachis (as), like a small replica of the 

 rachis and vexillum. This varies 

 greatly in size and in the Ostrich 

 or Emu is of practically the same 

 size as the rachis and vane. 



as. 



Fig. 184. 



A, Contour feather from a fowl, seen from the inner side ; B, abnonnal feather from a Golden 

 Pheasant in which the process of shedding at the preceding moult has not taken place, as, After- 

 shaft ; b, barbs ; c, quill ; r, rachis. 



The feather in the early stages of its development resembles the 

 rudiment of a reptilian scale but during subsequent stages undergoes 

 complicated changes which need not be described. As development goes 

 on it becomes sunk in its follicle and growth takes place at its inner 

 end. The growth of the feather takes place in spurts at the periods of 

 moulting. At this period the growing zone close to the tip of the quill 

 becomes active, the feather rapidly increases in length, and the pre- 



