l894-] NEW-YORK MICROSCOPICAL SOCIETY. 49 



the centre of the germ in a series of ridges, thus forming folds 

 between them which run the length of the genu. Immediately 

 surrounding this is the outer, horny layer of the epidermis 

 (Fig. 3, c). Each separate ridge of these internal cells now 

 becomes horn-like, and the central pulp substance dries up. We 

 thus have a bundle of horny rays surrounded by an outer sheath, 

 and of these the pencil-like structures seen on the bodies of 

 newly-hatched birds consist. The outer, horny covering finally 

 breaks off, the rays or barbs become free, and a down-feather is 

 formed (Fig. 3, d). The lower portion remains in the skin as the 

 quill, and the entire structure may remain or be replaced by defi- 

 nite feathers. In such case a second germ forms at the base of 

 the first, the papilla grows rapidly, undergoes nearly the same 

 •changes as the other, the embryonic down-feather is pushed out 

 and may often be found attached to one of the barbs of the new 

 feather. At first the two kinds are much alike, but, in the second, 

 •one of the rays becomes rapidly thickened and forms a stem, to 

 which the barbs are attached on each side, with their barbules 

 (Fig. 3, /). This theory of feather development, as advanced, 

 with perhaps some slight differences of detail, by Studer (1873) 

 and Kerbert (1876), has been, and is to a great extent even to 

 this day, accepted as the simplest and most probable. 



In Birds we have no trace of true dermal bones, and also a 

 marked deficiency in glands, the only ones being the uropygial 

 glands at the base of the tail, whose secretions serve to oil the 

 feathers. Many important epidermal structures, in addition to 

 feathers, are found in this group ; such are : claws, spurs, foot 

 scales, and beak sheaths. 



E. Mammals. ' 



We now come to the highest class of the vertebrata, and, as in 

 Birds, we will consider first its most prominent feature — namely, 

 bair. Histologically this is quite distinct from the hair-like struc- 

 tures of Birds and Reptiles, which have no true hair. Its devel- 

 opment is very interesting. 



At the spot where a hair is to be formed an increase in the 

 number of cells of the inner epidermal or Malpighian layer takes 

 place, forming a dome-like mass directed toward the interior 

 -(Fig. 4, a, b, c, M). The cells of the derma now arrange them- 



