198 EDWARD B. POULTON. 
The reference letters correspond to those of Fig. 19. In the centre of the 
hair-cells is a cavity in which the papilla was doubtless contained, for in 
Ornithorhynchus the papillee extend above the bulb into the ueck (compare 
Fig. 23). The inner root-sheath (7. 7. s.) is apparently divided into two layers ; 
which, however, are less sharply marked off from each other than in Fig. 23. 
The cellular nature of the inner part is apparent. The upper surface of the 
hair is rendered very distinct by its immensely thickened cuticle and by the 
absence of pigment. 
Fic. 21.— x rather over 400 diameters. Transverse section of large hair in 
tail of young Ornithorhynchus. The pigmented hair-cells constitute the 
thickness of the shaft, and no medulla is visible. A superficial pigmentless 
layer is probably the-cuticle. The inner root-sheath (7.7. s.) is exceedingly 
thick and made up of fibres which are polyhedral and irregular in section, and 
its innermost part appears to be condensed into a continuous layer imme- 
diately surrounding the shaft of the hair. The outer root-sheath (o. 7. s.) is 
very distinct. A lymph space separates the last layer from the hair-sac. 
Fic. 22.—x rather over 400 diameters. A transverse section of a rather 
small hair from the tail of the same animal, taken at a somewhat deeper level 
immediately above the bulb. The inner root-sheath is here divided into two 
well-marked layers, the inner of which is not corneous and stains very deeply 
in logwood. Ata still deeper level this would be true of the whole sheath. 
In other respects the sections are essentially similar. 
Fic. 23.—x nearly 200 diameters. A longitudinal vertical section through 
the bulb and neck of a large hair from the middle of the back of the same 
animal. The immense size and upward extension of the papilla is the most 
remarkable feature in the section. The tip, to which the reference letter p. 
points, is well above the bulb of the hair, and has reached a horizon at which 
the hair-cells have become distinctly fusiform. The papilla itself contains 
capillaries and branched cells. The upper surface of the hair is on the left 
side, and is at once distinguished by its thick cuticle (c.) and pigmentless 
cells. The root-sheaths are thin, and their structure indistinct. The section 
being exactly along the middle line of the hair there was nothing to keep the 
two halves together, consequently they have fallen outwards so that the struc- 
ture appears to be wider above the papilla. If this had not happened it would 
have been much narrower than the bulb, and the chink in which the letter p. 
is placed would have been obliterated, although in estimating the diameter we 
must allow for the medullary cells which have been accidentally removed in 
the manipulation. 
Fic. 24.— x nearly 200 diameters. A longitudinal section of a large hair 
on the head of the same animal. The section was somewhat oblique, passing 
through the middle line of the hair at the point where the latter is represented 
in diagrammatic transverse section. At this point and just below it the 
deeply staining medullary cells are seen surrounded by the pigmented hair- 
cells, and these by the cuticle (¢c.). The inner root-sheath (¢. 7. s.) is seen 
