420 COMPARATIVE ANATOMY. 



Various changes affect the morphological characters of the feather, 

 after the development of its shaft and vexillum ; but these are 

 beyond the scope of this book. 



A follicle is not formed till somewhat late in the course of the 

 development of the feather ; it then contains that portion of the 

 rachis which is known as the " quill," and the vascular papilla which is 

 continued into it ; the hair, however, is characterised by the early 

 appearance of the follicle. In this case then the papilliform thicken- 

 ing of the epidermis is a very early stage, and one of short duration ; 

 for the hair is not formed in this first elevation, but in a follicle 

 which dips down from the epidermis into the cutis (Fig. 216, D E), 

 and at the base of this follicle the cutis-papilla [F] grows up. The 

 shaft of the hair {F ivs) is differentiated from the invaginated epi- 

 dermis by the cornification of its cells, while other cellular parts of 

 the follicle form the root-sheaths. 



The various forms of hairs, whether woolly or contour hairs, sette, 

 or spines, are merely modifications of one and the same early 

 condition. 



§ 322. 



The glands differentiated from the epidermis are, when simplest, 

 modifications of single cells, the protoplasm of which is differentiated 

 into fine granules, which are passed out to the exterior. These 

 mucous cells (goblet-cells), which are placed among the other 

 epidermal cells, form unicellular glands (Fishes). They are found 

 also in the Amphibia, but in them there are, in addition, more com- 

 plicated glandular organs. These latter have the form of flask- 

 shaped tubes, which are scattered over the integument; several 

 forms of these maybe distinguished. In many cases they get to be 

 very large, and form knob-like projections, which give the integu- 

 ment a roughened or wart-like appearance (Toads, Salamander). 

 Sometimes a large number of integumentary glands are collected 

 together, and characterise certain regions of the body (parotid). 



The integumentary glands are less widely distributed in the 

 Reptilia. In the Saurii the so-called " crural pores " lead into 

 glands, which look like compound tubes, and which secrete cells 

 which harden and fill up the lumen of the glands. In Birds the 

 number of integumentary glands is still less. A number of glands 

 unite to form the anal glands (glandula uropygii), which are especially 

 large in the Natatores ; their secretion serves to oil the feathers. In 

 the Mammalia they are divided into two distinct groups ; sweat glands 

 and sebaceous glands, which are frequently connected with the hair- 

 follicles. These two sets of g-lands are more easily distinguished by 

 their anatomical characters than by the quality of their secretion, 

 which is exactly known in a few cases only, although indeed the 

 same form of gland may have a different function in different 

 regions. The simpler tubes, which are coiled at their ends, are called 

 sweat-glands, while the sebaceous glands are generally lobate. A 

 number of them are often connected with one hair-follicle, and may 

 become so greatly developed in relation to it, that the hair-follicle 



