HAIRS OF MONOTREMES AND MARSUPIALS. 563 



mammals generally/^ We may add that, in respect to this 

 point, Echidna agrees with Ornithorhynchus. The outermost 

 layers of nuclei, which are large and deeply stained, represent 

 the outer root-sheath, which, as usual at this level, is very thin 

 and at most two layers thick, the inner of the two layers being 

 represented probably by a very few small nuclei (fig. 18). 



Fig. 19 represents a section cut across the follicle close 

 to the top of the hair, that is about halfway along the length 

 of the follicle. In the centre is seen the hair, circular in out- 

 line ; in fact there is as yet no flattening of the hair to be seen, 

 though when oblique sections are cut — and it is not always 

 easy to cut true transverse sections — the appearance of a slight 

 flattening is produced. The hair is enclosed in the meshwork 

 of the inner root-sheath, which here has the form of a corneous 

 network containing nuclei which stain in the way already 

 described. The cuticle of tiie hair is well developed, and no 

 nuclei can now be seen in it at this level ; whilst the nuclei 

 which at a lower level represent the cuticle of the inner root- 

 sheath, cannot here be recognised. The outer root-sheath 

 contains apparently a single layer of deeply-stained nuclei, and 

 is well marked off' from the inner root-sheath, owing to the fact 

 that it is not corneous. The meshwork of the inner root-sheath 

 is rendered distinct, even at this stage, and still more so at later 

 stages, by the way in which it stains with indigo or picric acid. 

 Fig, 20 represents a slightly oblique section across the 

 upper end of the follicle, cutting at one side through the level 

 at which one of the first formed small hairs is given off. In 

 the centre lies the corneous network, and outside this is the 

 outer root-sheath. Two points of importance may be noticed. 

 The first is that there is no sharp line of demarcation between 

 inner and outer root-siieath ; in fact, in the upper part of the 

 follicle the two always merge to a certain extent into one 

 another, while in the lower part they tend to become, as develop- 

 ment goes on, more and more strongly marked off from one 

 another. The second is a fact of greater importance, viz. that 

 there is in this part of the follicle above the hair no distinct 

 lumen, much less any structure which could be described as an 



