Feathers 23 
in sharks, which are among the most primitive forms of 
fishes, the skin is covered with tiny denticles or spines, 
which consist of enamel and dentine, and which rest on 
small bony plates. This form of scale is the most ancient 
known, and the hint of teeth which the description con- 
veys is not misleading; for we find that in some of these 
voracious fishes the spines in the skin become enlarged 
near the edge of the mouth, merging imperceptibly into 
the rows of cruel teeth which, to a certain extent, are 
homologous with the teeth of all higher animals. In 
other fishes the denticles become flattened scales, and 
many of these fish have teeth of corresponding plate- 
like form. So it is interesting to know that the scales of 
fishes and reptiles, the feathers of birds, and the teeth 
of animals have all evolved from skin structures which 
at an early stage of growth bear considerable resemblance 
to each other. 
But, in our young bird, the slender finger of cells 
which reaches upward, and whose base at the same time 
sinks deeply into the dermis, does not broaden out, but 
splits longitudinally into a number of folds, which grad- 
ually dry apart and harden into the slender, silky fila- 
ments which we know collectively as down. 
At the base of, and in fact attached to, the little pro- 
jection which gives rise to the nestling down is a small 
circular body of cells, which grows but little while the 
down plumage is serving its use; but when the bird is 
ready for a coat of true feathers this lower cellular mass 
begins to grow upward into a second finger, or column, 
of cells, pushing the base of the down feather out of its 
