HAIRS OF MONOTREMES AND MARSUPIALS. 571 



outermost layer of tlie epidermis, into which, at the mouth of 

 the cuticle, it merges. For the greater part of the length of 

 the follicle this cuticle of the inner root-sheath, though its 

 serrations render it very easily distinguishable, shows, in 

 common with the rest of the sheath, no cellular structure; 

 indeed Poulton (p. 166) says, " I gained the impression that 

 it is not a distinct and definite layer, but merely the condensa- 

 tion as it were of the innermost part of the inner root-sheath 

 upon the exterior of the hair and the moulding of its surface by 

 contact with the cuticle of the latter.*' If, however, the layer 

 be followed down to the bulb, then, where the cellular nature of 

 the sheath becomes evident, it is seen that the serrated cuticle 

 is directly continuous (fig. 22) with a special line of nuclei 

 which are distinct from those of the remainder of the sheath 

 and from those of the hair cuticle, next to which they lie, by 

 their very distinctly flattened appearance and dark staining. 

 Traced still further down into the bulb the flattened nuclei 

 become more rounded until they reach the lowest point at 

 which the nuclei of the hair cuticle can be distinguished as a 

 distinct layer, and at this point the two layers become con- 

 tinuous with one another {A, fig. 22). 



General Considerations. 



The figures and descriptions of Mertsching may be taken as 

 representing the relationship of the layers of the hair and 

 sheaths as most generally accepted. He shows the cuticle of 

 the inner root-sheath as running down into the bulb where it 

 turns back again in continuity with Huxley's layer. The cuticle 

 of the hair is directly continuous with Henle's layer. Of the 

 two layers into which the outer root-sheath resolves itself in 

 the lower part of the bulb, the outer one is continuous with the 

 medulla and the inner one becomes much expanded as it passes 

 upwards through the bulb and becomes continuous with the 

 cortical substance of the hair. 



In Ornithorhynchus and Echidna, on the other iiand, while 

 we have been unable to distinguish such complete and de- 

 finitely outlined cellular layers as Mertsching figures, the 



VOL. 41, PART 4. NICW SERIES. ER 



