Feathers. 



115 



hooked at their extremities, and so interlock with neighbouring 

 feathers. Thus the firm nature of a feather is arrived at. 



Down feathers, which are, as their name impHes, the 

 softer feathers of the body, frequently do not possess the 

 terminal hooks — ihe hamuli, as they are sometimes called — and 

 therefore do not interlock, hence their softness. Moreover, in 

 down feathers, the barbs frequently arise in a clump from the 

 calamus, the rhachis being absent. 



The quill, of course, is formed by being moulded upon the 

 feather papilla ; but in the natural posi- 

 tion the rhachis, with its branches, is 

 folded so that the extremities of the 

 barbs meet, thus forming a cylinder 

 above : the umbilicus forms the com- 

 munication between this and the cavity 

 of the quill. The feather itself is, there- 

 fore, simply a continuation of the quill 

 cylinder, with an irregular cornification, 

 incomplete in the middle dorsal line, 

 the end, therefore, becoming free di- 

 rectly the feather is fully formed. When 

 an after-shaft is present, it is formed on 

 the opposite side of the feather papilla from the main rhachis. 



While the scales of lizards and of birds are purely epidermic 

 structures, those of fishes are either mesodermic or are formed 

 by both epidermic and mesodermic elements. The minute 

 scales of the dogfish, which, together with the intervening 

 skin, form the substance known as shagreen, consist of a base 

 of dentine — formed by the mesoderm, and similar in its 

 characters to the dentine of the teeth of the same anilnal — 

 and of a cap formed by epidermis, which is, in its turn, like the 

 enamel of the teeth. The identity of structure between these 

 body scales and teeth has led to the inference that they are 

 identical, homologous, structures. At first sight it may appear 

 difficult to compare structures lying on the outside of the body 

 with the teeth lying in the interior of the mouth; but 

 remembering the hairs, purely skin structures, which line the 

 cheeks of the rabbit, it will not be difficult to see that in the 



Fig. 58. — A down feather. 

 (FVom Gadow.) 



Sh, sheath. 



