BILL AND HAIRS OF ORNITHORHYNCHUS PARADOXUS. 168 
only the tips of the larger hairs had appeared above the surface, 
while the smaller ones were still at some distance below it (see 
Pl. 15a). 
The parts of the large hairs which had been formed evidently 
corresponded to the terminal shields of the adult, from which 
they differed in their greater thickness, although the differen- 
tiation of an upper surface with a greatly thickened cuticle 
from a lower surface in which the pigment was concentrated, 
‘was equally marked (figs. 19, 20, and 23). 
The gland, which was quite short in the young animal, opened 
in front of the large hair, as in the adult. 
The small hairs differ from those of the adult and in such a 
manner as to indicate the probable origin of the four bundles 
emerging from common mouths. 
When longitudinal sections or successive transverse sections 
of the young skin are examined, each large hair is found to be 
accompanied by four tubes, exactly like the ducts of glands, open- 
ing on the surface. These tubes correspond in position to the 
four bundles of small hairs in the adult. Tracing them down 
wards, each tube gives rise to a bundle containing a much smaller 
number of hairs than in the adult. Each bundle nearly always 
contains a single hair which is specially prominent, and it is 
this latter which occupies the lumen of the tube itself, the 
apex being however, in animals of this stage of growth, a con- 
siderable distance below the surface of the skin. In some 
cases, however, the four bundles of a group appear to contain 
only three chief hairs between them, in others as many as five 
or six. Four tubes appear to be always present, evidently 
representing the four common follicular necks and mouths of 
the adult. There is great disparity in size between the chief 
hair in a bundle and the smaller ones which are grouped 
around it. The hairs of the four sheaves terminate in bulbs, 
placed, as in the adult, at very different depths below the 
surface. 
From this structure it may be inferred that each of the four 
bundles is, in the course of development, at first represented 
by a single hair (the chief one), formed in a follicular 
