BILL AND HAIRS OF ORNITHORHYNCHUS PARADOXUS. 187 
the outer root-sheath, has of course disappeared, while the 
inner root-sheath forms a thick fibrous network round the base 
of the hair. No longer confined in the narrow space between 
hair and outer sheath, its fibres would be looser and their free 
ends would curl up. 
Giovanni has recently (“ De la regeneration des poils aprés 
Vépilation,” ‘Archiv f. mikr. Anat.,’ 1890, p. 528) published 
most beautiful illustrations of the earliest phases and growth of 
the successional hair in man. He shows in pl. xxv, figs. 138, 
19, 25, that the tip of the inner root-sheath is the first product 
of the bulb, and that the hair itself is formed later and pierces 
it (pl. xxvii, figs. 1,6,11). ‘The appearance suggests a possible 
homology of inner root-sheath with the sheath of a developing 
feather, but the specialised character and great size of the 
former in Ornithorhynchus favour rather the homology with 
the appendicular parts. The growth of the inner root-sheath 
round the base of the hair is to be explained by the shortening 
of the cone round which both the primitive hair and sheath are 
here supposed to have developed. The final result of such 
shortening leaves the hair to develop in the axis and the sheath 
