574 BALDWIN SPENDER AND GEORGINA SWEET. 



Our observations upon the Monotremes point very clearly 

 to the fact that in the most primitive mammals known to us 

 the inner as well as the outer root-sheath is a direct trans- 

 formation of certain parts of the follicle, and that during 

 development the hair formed on and from the bulb forces its 

 way up through the network, into which the inner root-sheath 

 becomes transformed. 



Giovannini,^ in dealing with the successional growth of the 

 hair in man, describes the inner root-sheath as formed first 

 above the tip of the developing hair, which subsequently 

 pierces the sheath in its upward growth. His figures are quite 

 compatible with the idea that when the downgrowth from the 

 old bulb to give rise to the new one is formed the inner part 

 of the new short follicle thus formed gives rise to the inner 

 root-sheath, which then occupies, exactly as it does in 

 Monotremes, the central part of the tube through which the 

 growing hair has to push its way upwards. 



The relationship of the various layers as indicated by 

 Mertsching and other workers on the one hand, and by our- 

 selves on the other, can be represented by the following 

 diagrams (pp. 575, 576). For the sake of clearness we have in 

 each case represented the hair as if it were slightly separate 

 from the walls of the follicle. 



In both diagrams the cuticle of the hair, and the layer with 

 which it is regarded as continuous, is indicated by the black 

 line, and if it be granted that the external surface of the hair 

 is to be regarded as morphologically equivalent in position to 

 the outside of the epidermic layer of the general body surface, 

 then it will hardly be denied that the diagram of the structure 

 as represented by ourselves, and as actually exists in Mono- 

 tremes, is, a priori, what might be expected to obtain. When 

 the hair has reached nearly to the summit of the follicle, and 

 the inner root-sheath is well cornified, there is, for the greater 

 part of its length, no distinct cuticle, the latter being repre- 

 sented merely by the clearly serrated edge of the sheath, but 

 near to the bulb this can be distinctly traced into connection 

 ' ' Arch. f. mikr. Anat.,' 1890, Bd. xxxvi, p. 528, pis. xxxv— xxxviii. 



