The Microscope. 99 



barbicels, of two forms called hamuli or booklets, and cilia. 



By these barbules of the anterior series overlying those of the 

 posterior series of the next preceding barb, and attaching them- 

 selves by the booklets, a web is formed of the vane of the feather. 

 The barbicels of the posterior series of barbs seldom bear hook- 

 lets, but cilia only. 



This description of a feather is a sort of base-line from which 

 to make surveys into unexplored fields of investigation. There 

 are three groups of feathers, the one given representing the pen- 

 naca' or feathery, the second the plumulacse or down, embracing 

 those having the stem short and weak, with soft barbs and shaft, 

 long slender thread-like barbules with knotty dilations instead 

 of barbicels without hamuli ; the third the filoplumacae or hairy, 

 containing those with a thin, stiff calamus with no pith in the 

 rhachis, fine, cylindrical, stiff" barbs and barbules, with no barbi- 

 cels, knots or hamuli. All feathers so far as known are con- 

 structed upon one of these three plans in general, but in their 

 various modification in difi'erent species of birds, as well as in 

 the different parts of the same bird, the group-characteristics are 

 frequently found present in one feather, and some of the more 

 striking variations arise by the partial, or total absence, or the 

 excessive development of one or more of the elements of a typi- 

 cal feather. 



Having outlined a typical feather as it appears to the unaided 

 eye, the way is prepared to examine some of its modifications, 

 found in its minuter structures which are only to be seen by 

 employing the microscope. We will pluck off' a single barb of 

 the anterior vane of a primary in the wing of a domestic pigeon. 

 Fig. 1 is a diagram of such a barb in lateral view. The shaft of 

 the barb may be seen represented by the dark line of the upper 

 border, immediately below which are the remaining stubs of the 

 excised barbules and under which are the pith-cells of the ex- 

 panded, appressed shaft. 



These cells disappear some distance above the transparent in- 

 ferior border that consists of the united, horny envelope of the 

 pith, terminating in this barb with an exceedingly minute frill 

 of fimbreaj. Fig. 2 represents, also a side view, a barb from the 

 primary of another species under the same magnification. In 

 this the shaft is inconspicuous, the anterior row of barbules 

 being inserted at its extreme upper angle, and the thin expansion" 



