104 KERATIN AND KERATINIZATION 



half spiral in the cylinder. During the formation of the continuous 

 cylinder of the calamus, which completes the feather, growth is continuous 

 around the entire ring of the collar. 



This description follows the classical accounts due to Strong (1902) and 

 Davies (1889). More recently somewhat different views were advanced by 

 Lillie and Juhn (1932 and 1938) which envisaged an actual migration of 

 growing tissue tangentially along the collar to enter the mounting rhachis as 

 suggested by the drawing Fig. 47. Their view has been contested by 

 Hosker (1936) and 'Espinasse (1939) in particular, and it seems it cannot be 

 held in its extreme form. Lillie and Juhn make the point that the rhachis 

 appears an independent growth to which barb material secondarily 

 becomes attached (Lillie, 1942). Certainly surgical experiments prove that 

 the capacity to generate rhachis is a special differentiation of the dorsal 

 portion of the collar. Following removal of the ventral half of a follicle the 

 entire feather may be regenerated; on the other hand the removal of the 

 dorsal half leads to the regeneration of a feather lacking a rhachis. The 

 actual relevance of some of these data to Lillie and Juhn's theory is not 

 immediately apparent. An interesting discussion of these questions will be 

 found in Waddington's book (1952). 



According to the concrescence theory the rhachis is formed by a process 

 of concrescence of the continually-growing right and left halves of the 

 collar, the levels from apex to base being formed successively (Fig. 47). 

 The forming barbs are carried along with the constantly-streaming halves 

 of the collar to their definitive positions at the sides of the shaft with con- 

 sequent change of orientation. As the series of barbs move dorsally (nos. 

 1-15, Fig. 47D), new barbs (nos. 16-25, Fig. 47E) take their origin in 

 the space thus provided at the ventral surface of the collar. 



The cells of the germinal collar closely resemble in their cytological 

 features those of the hair bulb (Mercer, 1958). The basement membrane 

 is typical and the basal layer cells, which abut it, form a columnar-like 

 epithelium. They are strongly basophilic and their cytoplasm abounds in 

 clusters of small dense particles of the same kind as described in the 

 germinai cells of hair and skin. Differentiation becomes apparent in the 

 cell layers immediately above the basal layer with the appearance of wispy 

 filaments in the cytoplasm. Although these filaments are known to consist 

 of a jS-type keratin (p. 16), their appearance, their manner of formation and 

 the cytology of the cell, seem exactly similar to the cells of the hair follicle 

 forming filaments of an a-keratin (Plate 12A). 



The production of the j3-keratin type of structure in the epidermis 

 of birds and reptiles poses some interesting questions, which have 

 been considered by Rudall (1949). Both a- and £-type keratins are 

 produced in these structures by cells which originally belonged to the same 

 primary ectoderm; the later development of two cell-types, distinguished 



