THE AYES. 71 



which differ in number and in development in different birds. 

 Lastly, the integument of birds is always provided with horny 

 appendages, which result from the conversion into horn of the 

 cells of the outer layer of the epidermis. But the majority of 

 these appendages, which : are termed " feathers," do not take 

 the form of mere plates developed upon the surface of the skin, 

 but are evolved within sacs from the surfaces of conical papillee 

 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 proceed 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 

 obsolete. 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 substance as a solution of gelatine, and withdrawn to cool until 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 sub- 

 stance, 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 ; while 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 

 developed 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 leather ceases to be grooved, and a 



