560 BALDWIN SPENCER AND GEOEGINA SWEET. 



are formed as outgrowths from the side of the large follicle, 

 and include, in the centre, a growth from the stratum lucidura 

 surrounded externally by a growth of the stratum Malpighii, 

 In the large follicle the central nuclei appear to be smaller 

 than those surrounding them, which latter are arranged in 

 roughly concentric circles, owing to the fact that they are cut 

 across their short axes. There is the earliest appearance in 

 the central part indicative of the formation of a network, which 

 becomes more definitely, in fact strongly developed in later 

 stages, as described by various observers in the case of dif- 

 ferent mammals. This section shows also the bilateral arrange- 

 ment of the large and small hairs. As yet only the rudiment 

 of one small hair is shown on either side, but from this will 

 subsequently be budded off (1) a second follicle giving two on 

 each side ; and from each of these, which remain proximally in 

 connection with one another and with the large follicle, are 

 budded off (2) the remaining follicles in which the small hairs 

 are developed, with the result that the large and small hairs 

 all have a common follicular opening. 



Stage 6 (figs. 17 — 20). — The most important feature of 

 this stage is the strong development of the inner root-sheath, 

 which is, as described already by Poulton, a striking feature of 

 the developing hair in Ornithorhynchus. It is equally strongly 

 developed in Echidna. The section figured is taken from 

 Ornithorhynchus. 



As compared with the last stage it will be seen that im- 

 portant changes have taken place in the follicle. The most 

 important one is that, except in the region of the bulb, the 

 inner part of the follicle now forms a network of corneous 

 material in which the now only faintly staining nuclei are 

 embedded. This network, which is in the case of Echidna and 

 Ornithorhynchus formed directly out of the central part of 

 the follicle itself, gives rise to the inner root-sheath. The 

 meshes of the network lie close together, and the strands 

 have at once, as regards the follicle, a longitudinal and a con- 

 centric arrangement — that is, as seen in longitudinal section 

 they appear to run irregularly along the length of the follicle. 



