INTEGUMENT 21 



In the deeper layers there is a strongly developed network of 

 muscle-fibres showing traces of transverse striation : these are 

 inserted into the feather-sacs, and serve to erect the feathers. 



Apart from a gland present in the neighbom-hood of the 

 auditory passage amongst Gallinacese, there is only a single gland 

 situated at the base of the rudimentary tail (uropygium) : this 

 uropygial gland is present in nearly all Birds, and its secretion 

 serves to oil the feathers. Dermal bones are characteristically 

 absent, while epidermic structures, such as feathers, claws, spurs, 

 foot-scales, and becck-shcaths, are strongly developed. 



One of the most marked characteristics of Birds is the pos- 

 session of feathers. In the majority of Birds they are of two 

 kinds — down-feathers and conto^ir-feathcrs, and are usually 

 arranged in so-called feather-tracts (jpterylce) separated by naked 

 regions [apteria). The base of each feather is embedded in 

 an epidermic sac or follicle. Their mode of development corre- 

 sponds essentially with that of the epidermic scales of Reptiles. 



In the region where a feather is to be formed, the dermal tissue becomes 

 raised up towards the ectoderm (Fig. 14, A), and thus gives rise to a vas- 

 cular papilla. Aw this papilla grows out to form an elongated cone with a 

 pointed apex, the fenther-ijerm (B), its base sinks gradually deeper and deeper 

 into the dermis, and thus becomes surrounded by a sort of ])(jcket — the 

 faathei'-follicle. The horny, as well as tlie Malpighian layer of the epidermis 

 extends into the base of the follicle, and tlience into the feather-germ, the 

 interior of which is throughout filled by cells of the dermis, which give rise to 

 the pii}p- -■^N the feather-germ keeps on growing, tlie cells of the Malpighian 

 layer begin to proliferate rapidly, giving rise to a series of radial folds 

 arranged along a central axis, which extend inward tcjwards the pulp, and 

 are immediately bounded by the horny layer (C). These folds, between 

 which the nutritive pulp extends, then become cornified and separated from 

 above downwards from the surrounding cells ; and, by a gradual drying of 

 the central pulp-substance, give rise to a tuft of horny rays, which are, 

 however, at first bcjund together by tlie enclosing stratum corneum. ilost 

 Birds are hatched wlien tlie feathers are in this stage of development, and 

 they thus appear as if covered with brush-like hairs. 



By the shedding of the surrounding horny layer the rays or barbs become 

 free ?D), and if these are all similar to one another, an embryonic diitru- 

 feather is formed. The whole feather-germ, however, does not become 

 divided up into barbs in this manner : its lower portion, embedded in the 

 skin, retains a more uniform character and forms the <fiill (ckIkiiiiis). 



The embryonic down-feathers (E), on the individual barbs of which 

 smaller secondary rays or barhttli's become developed, may retain their char- 

 acter as such throughout life or may be replaced by definitive feathers. In 

 this case a second, larger, follicle early arises from the base of the follicle of 

 the down-feather, the pulp of the two being in connection (D). The papilla 

 developing witlim the interior of this new follicle grows rapidly, gradually 

 pushes the base of the down-feather out nf its follicle, and ccjmes to the 

 surface. 



Each contour feather (penna) at first closely resembles a down- 

 feather (pluma) in structure, and consists of a tuft of similar rays 

 or barhs provided with secondary rays or harlules. In the course 

 of further growth, however, one of the rays becomes rapidly 



