HAIRS OF MONOTREMES AND MARSUPIALS. 587 



Fig. 19. — Section close to the tip of the developing hair. The follicle is 

 seen to be solid ; the cornified network which forms the inner root- 

 sheath {i. r. s.) tightly envelops the hair. Zeiss F, oc. 4. 

 Fig. 20. — Section close to the external end of the follicle, showing the 

 corneous and more open network of the inner root-sheath, the large 

 outer root-sheath, and an outgrowth from the main follicle to form the 

 follicle of a small hair. The main follicle is still solid, though the 

 central open network indicates the position of the future lumen. Zeiss 

 0-95, oc. 4. 

 Fig. 21. — Longitudinal section through the follicle of an embryo of Echidna, 

 showing the hair more highly developed. The inner root-sheath is more de- 

 finitely established ; the nuclei of the layers forming the cuticle of the hair 

 (c. h.) and of the inner root-sheath (c. i. r. s.) can be clearly seen in the lower 

 part of the follicle. The lumen is distinctly formed in the epidermis, and 

 below this the central part is occupied by disintegrating material. Imrae- 

 diately above the hair there is still the definite network formed by the inner 

 root-sheath, Zeiss, apert. 0"95, oc. 4. 



Fig. 22. — Longitudinal section through the lower part of the follicle of an 

 embryo of Echidna, in which the tip of the large hair has just appeared above 

 the surface. The outer root-sheath is well developed. The inner root-sheath 

 in the bulb region can, owing to the greater cornification of the outer part, be 

 distinguished into two layers, one {i.r.s.{) presumably the equivalent of 

 Henle's, and the other of Huxley's layer {i.r.s.„). The cuticle of the hair 

 fits closely on to the surface of the inner root-sheath, the serrations being 

 clearly marked. Traced down towards the bulb the outermost layer of the 

 inner root-sheath is directly continuous with a series of nuclei {c. i. r. s.) which 

 are flattened and stained deeply, and separated off in the lower part, where 

 they become more rounded, by a distinct line from the rest of the inner root- 

 sheath. At the point A this layer turns round and is continuous with the 

 layer which passes upwards into the well-developed cuticle of the hair (c. h.). 

 The cells of the latter are large and clearly outlined, the nuclei gradually 

 fading away as the layer is traced upwards into the strongly cornified part. 

 The medullary region has the appearance of opening up to admit possibly of a 

 secondary upward prolongation of the dermic papilla. Immediately above the 

 bulb is a constriction followed by a slight swelling of the hair. The cuticle is 

 equally developed on both sides. Zeiss C, oc. 4. 



Fig. 23. — Transverse section across a group of hairs of an embryo Echidna. 

 The follicle is at a slightly earlier stage of development than that represented 

 in Fig. 22, and the section is at some little distance below the epidermis. In 

 the central follicle the large hair is seen surrounded by the inner root-sheath, 

 the outer layers of which are not yet completely cornified. Four follicles in 

 which small hairs will be formed are cut through ; the one most to the right 

 hand is budding off a secondary follicle. Zeiss E, oc. 4. 



VOL. 41, PART 4. NEW SERIES. S S 



