BILL AND HAIRS OF ORNITHORHYNCHUS PARADOXUS. 165 
part of the outer root-sheath often contains cystic growths like 
those described in other mammals. 
The bulb is entered by a dermal papilla which is, at any 
rate in the large hairs of the young animal, of enormous 
size and length, extending beyond the bulb far into the base 
of the hair proper, where the cells are fusiform, pigment 
abundant, and the cuticle well defined (see fig. 23, in which, 
however, the two halves have fallen apart, so that the hair 
appears to be thicker than the bulb). The bulbs and papille 
are especially large in the tail. 
A very long, well-marked papilla also penetrates the long 
narrow bulbs of the smaller hairs. The relations of the adult 
bulb and papilla appeared to be very similar, but were made out 
with greater difficulty than in the young animal, which was 
chiefly employed for the histological side of this inquiry. 
From the tip of the papilla, at any rate in the larger hairs, 
an axial rod of soft protoplasmic cells, deeply staining in 
reagents, is continued (fig. 18, mature; figs. 19 and 24, 
young). This, when dried and shrivelled, admits the air and 
forms the characteristic medulla. 
Around the papilla the inner zone of cells of the bulb forms 
the hair proper with its cuticle, the structure and mode of 
formation being typical except for the bilateral symmetry of 
the larger hairs indicated by their shape, thickened upper 
cuticle, and predominant pigment on the under surface (fig. 
18, mature; figs. 19, 20, and 23, young). 
External to this zone the cells form the inner root-sheath, 
while the outer root-sheath appears to be continuous with 
the lower part of the bulb. The latter sheath is probably 
always typical, but it was extremely hard to make out in cer- 
tain sections of the young animal (figs. 19, 20, 23), although 
in others it was perfectly distinct and of normal appearance 
(figs. 21, 22, 24). This discrepancy is due to the facts that the 
thickness of the sheath varies greatly at different levels, and 
that the animal was not prepared for histological investigation. 
The inner root-sheath is always present in the developing 
hair, and is a structure of great importance, throwing much 
