BILL AND HAIRS OF ORNITHORHYNCHUS PARADOXUS. 171 
Emery regards hairs as “substitution-derivatives” of certain 
elements of the placoid scales of fishes; whilst (c), finally, 
there are authors who have contented themselves with con- 
testing the hair-feather-scale theory, but have not attempted 
to homologise hairs with anything else, appearing to regard 
them as structures sui generis. 
All authorities appear to be in agreement that scales and 
feathers are homologous structures. The mode of origin is 
closely similar in the two cases—the first forecast being an up- 
growth of the corium giving rise to a papilliform projection 
carrying the epidermis outwards ; the cells of the epidermis 
then proliferate to a slight extent, and later become horny. 
In the scale the papilla is more or less flattened, with its free 
edge (apex) directed backwards, and the cornification becomes 
more marked on the upper (outer) surface than on the lower 
(inner) surface of the papilla.!_ In the case of the feather, the 
papilla becomes more or less cylindrical and more upright; 
further, its base sinks down into the corium at an early age, so 
that the root of the feather comes to lie in a follicle. The axis 
of the papilla (corium) gives rise to the pith of the feather 
axis ; the enveloping cornified epidermis to the barbs. But the 
most superficial coat of the papilla (the original outermost 
layer of epidermis) forms a sheath which closely surrounds the 
feather forecast. As the latter grows it breaks through the 
apex of the sheath, which then dwindles and is ultimately cast 
off. This feather-sheath consists of two layers of cells—a more 
superficial cornified layer, and a deeper layer of granular cells. 
The feather, then, may be derived from a scale by supposing 
that the original papilla, after growing outwards for a time, has 
sunk downwards into the corium so as to give rise to a follicle 
and a root-sheath, for the purpose of better support and nutri- 
tion. 
Those authors who take the view that the hair is homologous 
with the feather, believe that this process of sinking has gone 
1 Tt is noteworthy that this statement is in every detail an exact description 
of the formation of the large hairs of Ornithorhynchus, save that they are 
developed as an upgrowth at the bottom of an open pit.—H. B. P. 
