HAIRS OF MONOTREMES AND MARSUPIALS. 555 



latter being formed of striated fibres is that of the muscles 

 moving vibrissse,^ 



Development of the Hairs. 

 So far as essential points are concerned, the development of 

 large and small hairs alike agrees in both Ornithorhynchus and 

 Echidna. As in all mammals, so in the Monotremata the 

 hair is developed in a solid downgrowth of the epi- 

 dermis. It is possible that owing to the remarkable develop- 

 ment of the inner root-sheath an indefinite central cavity may 

 be formed at a somewhat earlier stage than that in which the 

 follicle becomes tubular in other mammals, but a prolonged 

 investigation of a large series of sections comprising various 

 stages has led us to the conclusion that it is incorrect to say 

 that the hairs of either Ornithorhynchus or Echidna are 

 developed in tubes open to the- exterior, "in" to use Mr. 

 Poulton's words, " a tubular and not a solid downgrowth from 

 the exterior." The down^'rowth is at first a solid one 

 exactly as it is in other mammals, and it is only at 

 some considerable time after the hair has been 

 formed that there is developed any lumen opening to 

 the exterior. Exactly as in all other mammals so in the 

 Monotremata the developing hair has to push its way up 

 through the centre of a solid follicle. 



For the sake of convenience we have taken eight stages, the 

 structure of the hair and its follicle in each one of these being 

 represented in the figures. 



Stage 1 (fig. 9). — We have not, unfortunately, been able 

 to secure a young enough embryo to demonstrate the earliest 

 appearance of the follicular downgrowth. The section of the 

 earliest stage which we have so far been able to secure, and 

 which is figured, is, however, sufficiently early to indicate 

 without any doubt that, in Monotremes, the commencement of 

 the development of the hair and its follicle is fundamentally 

 identical with that of all other mammals. There is nothing 



' Owen, ' Coiiip. Aiiat.,' vol. iii, p. 621. See also Keneage Gibbes, ' Quart. 

 Joum. Mic. Sci.,' vol. xxiv, p. 193. 



VOL. 41, PART 4. — NEW SERIES. Q Q 



