HAIRS OF MONOTREMES AND MARSUPIALS. 565 



and correspouding to those which will be more fully described 

 in the next stage. The nuclei of the outer root-sheath are 

 seen in parts to be proliferating, so as to give rise to the more 

 strongly developed and definite outer root-sheath of the more 

 highly developed hair. 



Stage 8 (figs. 22 — 26). — The figures represent the 

 structure of the large hairs, the tips of which have just 

 appeared above the surface, and are drawn from sections taken 

 from the back of an Echidna measuring 5'5 cm. in length. 

 They may be regarded as representing the structure of the hairs 

 when the various parts are well developed ; and before, owing 

 to the great growth of the hair and extreme eornification and 

 subsequent absorption of the softer parts, the structure and 

 relationship of the diflerent layers is more difficult to determine. 

 In fig. 22 we have represented a longitudinal section which, 

 apart from the special modifications which take place subse- 

 quently to form, on the one hand the flattened hairs of both 

 Ornithorhyuchus and Echidna, and on the other the strongly 

 developed spines of Echidna, may be regarded as representing 

 the details of the typical structure of the hairs, large and small 

 alike, of both animals. It will be noticed, in the first place, that 

 the dermic papilla appears at this stage to lead up very distinctly 

 in the direction of the medulla, the nuclei of the cells at the 

 apex being elongated in the direction of the length of the 

 hair. Whether there be at this stage any direct continuation 

 of the dermic cells into the medulla of the hair, we are unable 

 to state with certainty ; and, despite the suggestive appearance 

 of the section figured, which is only one of very many in which 

 the same appearance is presented, we are strongly of opinion 

 that there is no such continuity. In early development the 

 medulla — which is very difficult to distinguish as a definite 

 structure at this stage, though it becomes more prominent in 

 the adult hair — is without any doubt whatever formed entirely 

 from the structure of the bulb, the dermic papilla taking no 

 part whatever in its formation. With the subsequent breaking 

 down of the central cells of the hair and the formation of an 

 open space, an upward growth of the dermic, papilla may take 



