DIFFERENTIATION AND PROTEIN SYNTHESIS 



105 



Fig. 47. Diagrams to contrast the development of a feather according to 

 the fusion of barbs or classical theory (A, B, C) and the concrescence 

 theory of Lillie and Juhn (D, E, F). In the classical theory the whole 

 feather arises from a ring of embryonic cells (i.e. the collar) surrounding 

 the base of the feather germ. According to the concrescence theory the 

 barbs, nos. 1-9, arise from the collar and as more cells are added to 

 the barbs from the rapidly-dividing collar cells, it will follow that they will 

 gradually approach the mid-dorsal line, and fuse with the dorsal-most 

 barb or rhachis. This of necessity becomes broader and takes on the 

 definitive shape of the rhachis. b, barb; co, collar; r, rhachis; s, sheath; 

 v, ventral (from Hosker, 1936). 



