Feathers 



23 



in sharks, which are among the most primitive forms of 

 fishes, the skin is covered with tin}^ 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 

 liomologous 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 dr\^ 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 



