HAIRS OP MONOTREMBS AND MARSUPIALS. 579 



If we trace the development of the hair iu Ornithorhyuchus 

 we find that at an early stage, before there is any appearance 

 of a hair in the follicle, the base of the latter is somewhat 

 flattened into a plate-like structure, indicating possibly the 

 original development of the hair rudiment out of a bilaterally 

 symmetrical structure; but it is not until the radially sym- 

 metrical bulb with its dermic papilla has been formed that we 

 can see any trace of the hair itself, and the latter has at first, 

 and until it is well established, a radial symmetry with a per- 

 fectly circular outline in transverse section. Subsequently, and 

 as a secondary modification, there arises a flattening which 

 produces in the scale-like part of the hair a bilateral symmetry, 

 but this is in no sense a primitive feature of the hair. So far 

 as we have any evidence, every hair is primarily a radially 

 symmetrical structure, — that is, we never find the hair com- 

 mencing to grow until the bulb of the follicle from which it 

 grows has acquired a radial symmetry. In Echidna the radial 

 symmetry is emphasised in some of the large hairs, which 

 become modified into spines, and lost iu others which become 

 flattened, the latter feature being especially emphasised in 

 Ornithorhyuchus ; but both of these are very clearly secondary 

 developments, and cannot in any way be regarded as represen- 

 tative of the structure of the primitive mammalian hair. 



In the same way the remarkably developed inner root-sheath 

 of the large hairs of Monotremes is not to be regarded as a 

 primitive feature ; it is simply a secondary feature of no 

 phylogenetic importance, and is to be associated with the strong 

 development of all the various parts of the follicle. The 

 modification of the large hair requires, as it were, that all the 

 various parts of the walls of the follicle should be strengthened 

 and stiff"ened, and that at the same time the root of the hair 

 should be tightly held. These requirements are met by the 

 coruification of the inner root-sheath, that is of the walls of 

 the follicle, and also by the way in which it grasps the hair 

 and has its surface strongly imbricated so as to oppose the 

 pulling out of the latter, which might otherwise be easily sepa- 

 rated from the soft structures of the bulb. 



