102 



KERATIN AND KERATINIZATION 



number, are indicated by their distance from the central rhachis. The 

 collar is at C. The barbs form a subdivision of the cylinder by clefts as may 

 be seen more clearly in (b) where following Lillie the cylinder is shown cut 

 along its ventral side, spread out flat, and viewed from inside the follicle. 

 Cell movement is everywhere vertical, i.e. parallel to the rhachis, and, in 

 the area of the collar giving rise to the rhachis, is continuous; in those 



Fig. 45. Illustrating rhachis and barb formation in the feather follicle, 

 (a) A short length of the feather cylinder adjacent to the collar, (b) the 

 cylinder opened out, (c) a section cut across (b). The rhachis R grows 

 steadily upwards. The bases of the barbs move inwards to meet the 

 rhachis as a result of the migration of the growing areas but cell movement 

 is always vertical. 



regions giving rise to barbs, however, growth is restricted to the areas 

 shown in (c), separated from each other by clefts which thus give rise to a 

 series of ridges on the inner face of the cylinder. In nesting feathers the 

 ridges ran parallel to the rhachis (itself here only a barb) and the feather 

 when open appears as tube with a slotted end. The growing areas of the 

 collar are here stationary relative to the rhachis. In forming adult (contour) 

 feathers the discrete growing areas generating barb ridges migrate towards 

 the rhachis (Figs. 46 and 47) and ultimately meet it and join the barb to the 

 rhachis. Only the sites of growth migrate inwards, the movement of cells 

 remains vertical, and the effect is to produce the series of spiral grooves 

 inside the cylinder as shown in Fig. 45, each completed barb describing a 



