160 EDWARD B. POULTON. 
resembling those of the bill of the same animal or the sweat- 
glands of mammals generally. On the head and back, the duct 
invariably opens just in front of the large hair, so that it is the 
most anterior member of the whole group. 
Such a group is distinctly represented in transverse section 
immediately beneath the skin in fig. 17, where the young 
successional hairs, both large and small, are at once distin- 
guished by the presence of pigment from the colourless bases 
of the shafts of older hairs. At this level the gland-duct (d.) 
is seen in section in front of the large hair. At a lower 
level, as shown in fig. 18, the duct is replaced by a secretory 
tube of typical structure (g.), which is often placed, as in this 
figure, between the large hair and one of the bundles of smaller 
hairs. The latter, at this level, have become separate, but their 
outer root-sheaths are continuous peripherally, forming a single 
epithelial mass. Furthermore, the two masses on each side 
have approached and tend to coalesce (compare figs. 17 and 18), 
soon doing so completely. At this level, and just below it, 
the small unpigmented bases of the older hairs come to an 
end (one is thus ending in each of the right-hand bundles in 
fig. 18). The younger growing shafts, with their outer root- 
sheaths no longer continuous, descend much deeper, gradually 
converging to form a single bundle, which lies in the middle 
line under the follicle of the large hair. The growing small 
hairs then end in bulbs at various levels, but the most deeply 
placed do not, as a rule, descend so far as the bulb of the large 
hair. 
The large hairs terminate superiorly in large, although 
narrow, flattened leaf-shaped expansions borne by a shaft long 
enough to carry them just beyond the ends of the small hairs. 
As the neck of this shaft is comparatively thin, it is probable 
that in the living state the terminal shields tend to fall over, 
and lie flat on the finer hair beneath, their tips pointing back- 
wards and overlapping the basal part of the shields behind 
them. 
The tip of the shield is beautifully formed and free from 
pigment. Looked at from the side or in section, the pigment 
