BILL AND HAIRS OF ORNITHORHYNCHUS PARADOXUS. 167 
while still nearer the bulb in fig. 20 the same inner layer shows 
distinct traces of cells with their nuclei. In both cases this 
inner zone probably represents Huxley’s layer, but it does not, 
at least in Ornithorhynchus, imply any differentiation of the 
sheath into layers, and when we consider the immense develop- 
ment of the structure in this animal, it seems probable that 
the distinction can hardly be sustained throughout mammals 
generally. The appearance which has led to the separation of - 
the sheath into two layers is, I believe, due to the fact that the 
inner and outer zones do not arise from the cells of the bulb 
at one and the same horizon, but that the inner zone rises at a 
higher level than the outer. It therefore follows that, in sec- 
tions at a certain horizon, the cells of the bulb will have 
undergone complete transformation into the corneous substance 
of the sheath in the outer zone, while they will still remain 
unchanged, and at a rather higher level only partially changed, 
in the inner zone. At any rate, I am convinced that this is 
the explanation of the two layers in figs. 20 and 22. 
In certain sections the inner part of the sheath appears to be 
made up of fibres and the outer part homogeneous (fig. 18), but 
in the thickest and best developed sheaths (fig. 21) the fibrous 
structure is distinct in the outer as well as the inner part (with 
the exception of the thin ‘‘cuticle’’). 
5. Mode of Succession of the Hairs. 
The succession of the hairs already alluded to presents 
some features of great interest. The appearance of two hairs 
in one follicle is spoken of as an occasional appearance in other 
mammals. In the large hairs of Ornithorhynchus it is the in- 
variable rule, and the succession follows the bilateral symmetry 
of the hair itself, the new hair always overlapping the anterior 
surface of the old. Hence a section of the neck of the: follicle 
commonly shows the terminal shield of the emerging young hair 
anteriorly, and the circular or oval base of the old hair opposite 
the middle of its posterior surface (fig. 17). We do not find 
two shafts in one neck, because the old hair is doubtless shed 
by the time that the new shield has risen above the surface, 
