26 



COMPARATIVE ANATOMY 



vascular base gradually sinks deeper and deeper into the derm, 

 and thus becomes surrounded by a sort of pocket- -the feather- 

 follicle. The horny, as well as the Malpighian layer of the 

 epiderm extends into the base of the follicle, and thence on to the 

 feather-germ, the interior of which is throughout filled by cells of 

 the derm which give rise to the pulp. 



As the feather-germ keeps on growing, the cells of the 

 Malpighian layer begin to proliferate rapidly, giving rise to a 

 series of radial folds arranged along a central axis and extending 

 inward towards the pulp (Fig. 21, A). These folds, between 

 which the nutritive pulp extends, then become cornified and 

 separated from above downwards from the surrounding cells (B) ; 

 and, on a gradual drying of the central pulp-substance, give rise 

 to a tuft of horny rays (C), which are, however, at first bound 

 together by the enclosing stratum corneum, which forms a sheath 

 around them. Most Birds are hatched when the feathers are in 





IB P c 



FIG. 20. Two EARLY STAGES IN THE DEVELOPMENT OF THE FEATHEK 



(SEMIDIAGRAMMATIC). 



B, blood-vessel ; C, derm ; E, proliferating epiderm ; F, rudiment of follicle ; 

 h, horny, and m, Malpighian layer of epiderm ; P, pulp of the papilla. 



this stage of development, and they thus appear as if covered with 

 brush-like hairs. 



By the shedding of the investing horny sheath, the rays or 

 barbs on which smaller secondary rays or barbulcs become 

 developed become free, and thus an embryonic down-feather 

 (pluma) is formed. The whole feather-germ, however, does not 

 become divided up in this manner : its lower portion, embedded in 

 the skin, forms the quill (calamus), the interior of which contains 

 a peculiar flaky and air-containing horny substance, the dried 

 remains of the pulp. 



Thus the earlier stages of development of the feather and 

 reptilian scale are very similar, but during later stages the 

 feather becomes adaptively specialised. The warm-blooded but 

 still flightless ancestors of Birds probably possessed a covering of 

 down-feathers which served as a protection against the cold, and 

 which only later become adapted in connection with flight. 



In many Birds the feathers retain throughout life the essential 

 characters of down, with more or less differentiation (e.g. Ratitas, 

 and more especially the Cassowaries) ; but in most cases the 



