HAIRS OF MONOTREMBS AND MARSUPIALS. 569 



For the purpose of showing the relationship of the inner root- 

 sheath we have represented in figs. 24 and 25, drawn under the 

 camera lucida^ (1) two entire follicles as seen in a longitudinal 

 section of the skin of Echidna ; (2) a portion of one of these on 

 a larger scale. The exact relationship of the inner root-sheath 

 to other parts of hair and its method of formation are matters 

 of fundamental importance in connection with the considera- 

 tion of the relationship which may exist between hairs and 

 other structures. 



The one point of fundamental importance is that the inner 

 root-sheath is a diflferentiation of the inner layers of the ori- 

 ginal epidermic follicle, and that as shown by various authors, 

 and most clearly perhaps by Mertsching (pi. iv, fig. 2) in a 

 section through the hair of a guinea-pig, the sheath is directly 

 continuous with the layers of the epidermis intermediate be- 

 tween tiie stratum corneum on the outside and the stratum 

 Malpighii on the inner side. This direct continuity is very 

 clearly brought out by adopting the method of staining already 

 indicated, and the appearance always presented in the case of 

 both large and small hairs of Ornithorhynchus and Echidna is 

 well seen in fig. 24. It will be seen tliat, in regard to this point, 

 our observations are at variance with those of Poulton, who 

 (p. 165) says ''The inner root-sheath is always present in the 

 developing hair, and is a structure of great importance, throwing 

 much light upon the corresponding sheath as it is described 

 in other mammals. As in the latter, the inner root-sheath 

 surrounds that part of the hair which is enclosed in the follicle, 

 but growing less rapidly it does not extend to the neck through 

 which the hair protrudes ; hence we do not find it at all in 

 sections of the upper part of the follicle (figs. 17 and 18)." 

 On reference to the figures indicated by Mr. Poulton, we think 

 that there is in them traces of the inner root-sheath to be 

 seen; in fig. 17 the wavy lines surrounding the old hair, and 

 evidently also passing round the succession al hair, are very 

 suggestive in this respect, as are also the wavy lines surround- 

 ing the old hair in fig. 18. In fact, judging by our own sections, 

 we cannot but think that the structures drawn, but not referred 



