208 oliKUX OF FEATHEHS FROM SCALl^S. 



tlii()u<^li the f^radual transformation of reptilian scales by a 

 complicated s])litting of horny upgrowths. In the present chaotic 

 state of evohitionary theory we are only justified in presenting 

 the two issues involved. 



EXPLANATION OF PLATES. 



Plate IV. 



Fig. L — Portion of skin from fore-leg of ostrich chick about the time of 

 liatching. The skin is here covered with small, flat polygonal scales, and 

 from the lower border of each arises a short upgrowth or papilla, which 

 represents an early stage in the development of a feather. 



Fig. 2. — Portion of skin from a little higher up the leg than that shown 

 in Fig. 1. The scales, each with a feather papilla, are present in the 

 lower part, but disappear above, where longer feather filaments occur, which, 

 on opening out, give rise to the down which covers the upper part of the 

 leg of the ostrich chick. Two of the longer filaments have been plucked out 

 to show more clearly the incipient plumulpe (filoplumes) around their base. 



Plate V. 



Fig. 3. — Scales from skin of ostrich chick, about two weeks after 

 hatching. Four of the larger feather filaments have opened out into small, 

 tuft-like feathers, composed of barbs with barbules, resembling the down 

 which covers the young chick. The feather papillae on the other scales 

 have shrunken, and soon completely atrophy; the scale-feathers also fall 

 away. 



Fig. 4. — Section of skin of ostrich chick, in about the same condition as 

 that shown in surface view in Fig. 1. The section passes through a immbev 

 of scales (•'-c.), and in three of tiiem a feather papilla (f^fp.) is included. 

 The feather at this early stage appears as a definite upgrowth from the 

 hind part of a scale, the epidermis {rp.) being followed by the underlying 

 dermis (der.). 



Fig. 5. — Section through a feather filament showing it to be an out- 

 growth from a scale. The epidermis of the scale is made up of several 

 layers of cells. The lowermost is the formative layer or ^Nlalpighian layer 

 (//.) ; its cells are active and continually dividing, adding others to the 

 layers above. The second is the intermediate cell-layer (inf.L), while 

 the third is the sheath or horny layer (.«/;. /.), in which cells are formed into 

 horny material. The epitrichium (cpt-) is the outermost layer of the 

 epidei'mis. The same layers are contiimed from the scale into the feather 

 and form the various parts of the feather, such as barbs, barbules and 

 feather-sheath. The middle pith or pulp of the feather (/>'.), which nourishes 

 the feather during its growth, is a continuation of the dermis (der.). the 

 under-layer of the skin. The lower part of the feather grows downwards 

 into the dermis, forming a follicle or socket, and thus gives the appearance 

 of the feather growing through the scale. 



