BILL AND HAIRS OF ORNITHORHYNCHUS PARADOXUS. 173 
in birds are merely secondary thickenings round the base of 
the feather. 
He compares the hair rather with the definitive feather, which 
arises from the root of the down papilla, and he emphasises the 
fact that in this case, as in the hair, the feather forecast has 
to make its way up through the neck of a solid follicle of de- 
generating cells (cf. his figs. 30—39), as on the appearance of 
the definitive feathers there has been an active downgrowth of 
the primary feather follicle. 
The spine of the hedgehog is, of course, derived from a more 
simple hair, and is not to be compared with the quill of a 
feather. 
On p. 629 he summarises his ideas on the mode of origin of 
the feather :—1. A simple thickening of the skin. 2. A radially 
symmetrical knob. 3. A backwardly directed papilla whose 
horny layers become thickened round the apex. 4. A back- 
wardly directed papilla whose point ended in a short, thick, 
hair-like process. 5. A longer hair-like structure, which con- 
sisted of a firmer cortical layer and a looser axial tissue, and 
whose base became sunken with the cutis papilla into the skin. 
6. By the bursting of the wall of the freely projecting part of 
this structure the enclosed tissue became free, and, separating 
into distinct filaments, gave rise to the primitive “ down.” 
But even admitting the general similarity in the mode of 
their development, one of the earliest to draw attention to the 
striking difference between hairs and feathers was Gegenbaur, 
who contrasted the early outgrowth of the feather forecast, in 
which the greatest share is taken by the corium papilla, with 
the early downgrowth of the hair forecast, due to active proli- 
feration of the epidermic constituents. 
This doubt as to the strict homology of feather and hair is 
accentuated by the statements made by other observers— 
Kolliker, Romer (12), Maurer (9), &c.—that the corium papilla 
is not the first to become defined, but that the epidermis is for 
some time the only representative in the hair forecast. This 
statement, which appears from Maurer’s work (1892) to hold true 
for a variety of forms, may be explained in either of two ways: 
