CHARACTERS OF AVES. 48 I 



All birds are oviparous, none bringing forth their young 

 alive, or being even ovo-viviparous. All birds are, lastly, pro- 

 vided with an epidermic covering, so modified as to constitute 

 what are known as feathers. 



Professor Huxley's account of the method in which feathers 

 are produced is so remarkably clear, that no apology is neces- 

 sary for quoting it in its entirety. Feathers "are evolved 

 within sacs from the surface of conical papillae of the dermis. 

 The external surface of the dermal papilla, whence a feather is 

 to be developed, is provided upon its dorsal surface with a 

 median groove, which becomes shallower towards the apex 

 of the papilla. From this median groove lateral furrows pro- 

 ceed at an open angle, and passing round upon the under 

 surface of the papilla, become shallower, until, in the middle 

 line, opposite the dorsal median groove, they become obso- 

 lete. Minor grooves run at right angles to the lateral furrows. 

 Hence the surface of the papilla has the character of a kind of 

 mould, and if it were repeatedly dipped in such a substance as 

 a solution of gelatine, and withdrawn to cool until its whole 

 surface was covered with an even coat of that substance, it is 

 clear that the gelatinous coat would be thickest at the basal 

 or anterior end of the median groove, at the median ends of 

 the lateral furrows, and at those ends of the minor grooves 

 which open into them ; whilst it would be very thin at the 

 apices of the median and lateral grooves, and between the 

 ends of the minor grooves. If, therefore, the hollow cone of 

 gelatine, removed from its mould, were stretched from within, 

 or if its thinnest parts became weak by drying, it would tend 

 to give way, along the inferior median line, opposite the rod- 

 like cast of the median groove, and between the ends of the 

 casts of the lateral furrows, as well as between each of the 

 minor grooves, and the hollow cone would expand into a flat 

 feather-like structure, with a median shaft, as a ' vane ' formed 

 of ' barbs ' and ' barbules.' In point of fact, in the develop- 

 ment of a feather, such a cast of the dermal papilla is formed, 

 though not in gelatine, but in the horny epidermic layer de- 

 veloped upon the mould, and as this is thrust outwards, it 

 opens out in the manner just described. After a certain 

 period of growth the papilla of the feather ceases to be 

 grooved, and a continuous horny cylinder is formed, which 

 constitutes the ' quill.' " 



A typical feather (fig. 210) consists of the following parts : 

 i. The '-'quill" or "barrel" (a), which forms the basal 

 portion of the feather, by which it is inserted in the skin on its 

 own dermal papilla. It is the latest-formed portion of the fea- 



2 H 



