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BILL AND HAIRS OF ORNITHORHYNCHUS PARADOXUS. 197 
sending out processes which pass between the adjacent epithelial cells. The 
younger hair, on the other hand, passes down to a great depth and ends in 
a bulb. The small hairs in the four bundles are seen to have separated, each . 
being enclosed in an epithelial tube corresponding to an outer root-sheath, but 
the walls of the tubes are at this level continuous peripherally in each bundle. 
Below, the pigmented younger hairs are continued to a far deeper level, their 
outer root-sheaths expanding into bulbs, while the unpigmented bases of fully- 
formed hairs terminate without bulbs very slightly bélow this horizon. Indeed 
two of them (one in each of the right-hand bundles) are already terminating, 
and have ceased to be corneous. 
Fic. 19.—x rather over 400 diameters. Transverse section of one large 
and many smaller hairs from the middle of the back of the young Ornithorhyn- 
chus. ‘The sections are of the first-formed hairs, and as these are by no means 
fully formed, only the tips of the large ones having appeared above the skin, 
no traces of the successional hairs are to be seen. The upper part of the large 
hair in the figure represents its anterior and upper side on emergence, and it 
is seen that the cuticle (c.) is here thicker, while the pigment is contained in 
the hair-cells which are undermost in the natural position. The medulla is repre- 
sented in section by four protoplasmic cells, and the nuclei of the hair-cells 
between this and the thickest cuticle stained with logwood. Outside the 
cuticle is a layer (¢. 7. s.), which is very characteristic of Ornithorhynchus, 
surrounding the hair above the neck of the bulb for a considerable ‘part of its 
passage through the dermal tissues. It consists of anastomosing, apparently 
corneous fibres, representing the inner root-sheath to which the homogeneous 
deeply staining layer outside it probably also belongs. The outer root-sheath 
(o. vr. s.) is at this horizon extremely delicate and thin, and is succeeded by 
alymph space. The entire sections of four small hairs are seen, as well as 
parts of four others. Four small hairs are at this age usually found beneath 
a large one, and each of them appears to represent one of the four groups of 
small hairs found beneath each large one in the mature animal. The four 
entire sections probably represent those which attend the-large hair in the 
section. The two inner ones are cut through at tie level of their bulbs, and 
centrally the papille are seen in section. The two outer indicate the struc- 
ture at a higher level as seen in section, the shaft being composed of six 
radially arranged pigmented hair-cells surrounded by a cuticle followed by a 
thick inner and irregular outer root-sheath and lymph space. It must be 
borne in mind that the delicate tissues of the young Ornithorhynchus had not 
been prepared for histological investigation, and that it was therefore extremely 
difficult to form-an opinion upon some of the layers. This caution applies 
especially to the layers interpreted as outer root-sheath and the outer part of 
inner root-sheath in Figs. 19 and 20 and the former in Fig. 23. 
Fic. 20.— xX rather over 400 diameters. A section of one of the large hairs 
from the same part of the same animal as Fig. 19, taken at a rather deeper level, 
