THE KERATINIZED TISSUES 59 



skins of land forms where it serves to lubricate and improve the water- 

 proofing of the skin. It is difficult to suggest a reason for the extraordinary 

 variety of compounds found in these secretions. 



Birds and reptiles have fewer cutaneous glands than mammals. The 

 heavily-keratinized epidermis of reptiles does not favour their development 

 nor for perhaps the opposite reason does the thin skin of birds. Birds 

 usually possess a uropygial gland opening in front of the tail, the secretion 

 of which, spread over the feathers during preening, helps to waterproof 

 the layer of feathers. Vitamin D is produced from its secretion by the 

 action of sunlight and plays a part in nutrition (Hotta, 1928 and 1929) 

 which seems to demonstrate a special reason for the presence of some lipid 

 molecules. 



The mammalian skin is rich in number and has more varied types of 

 glands. The types generally present are : (1) mammary glands which give 

 the phylum its name, (2) sebaceous glands, and (3) sweat glands. The 

 sebaceous glands are usually associated with hair follicles and produce an 

 oily secretion which softens the skin and helps to lubricate the hair. 

 Mammary glands and sweat glands are histologically similar. The small 

 sweat glands or eccrine glands derive directly from the embryonic 

 epithelium, the apocrine (partly sebaceous, larger sweat glands) and the 

 sebaceous glands indirectly via the follicular epithelium of the outer root 

 sheath (p. 96). The latter glands are found usually in association with 

 hairs; the eccrine (sweat) on hair-free surfaces where, as in man, they 

 may play an important role in temperature control. Sebaceous and 

 apocrine glands possibly are related to similar glands in the hairless skins 

 of earlier vertebrates. The very active cutaneous mucinogenic glands of 

 cyclostomes are said to be partly holocrine and may be remotely related 

 (Rothman, 1954). Some cutaneous holocrine glands of reptiles, undoubtedly 

 derived from similar phylogenetic ancestors, are sac-like invaginations of 

 the epidermis producing fatty materials. Certain mandibular cloacal 

 glands of alligators are reported (quoted by Rothman) to produce lipids 

 and keratin simultaneously. The cell peripheries keratinize and the centre 

 produces lipids ; the whole is ultimately shed. This is a demonstration of 

 bifunctionality of epidermal cells. It would seem that epidermal cells can 

 still produce lipids, in addition to the structural phospholipid of cell 

 membranes (Table 4, p. 486, Rothman, 1954) indicating that the 

 potentiality is still present (see next section). 



There is a sense in which the entire epidermal system may be regarded, 

 as Montagna has put it, as an immense holocrine gland. Most of the 

 " secretion," i.e. the keratinized material, is shed and lost, but certain 

 other constituents may be absorbed either by their producer, its young or 

 associates, and serve further physiological ends. This is most obviously 

 so among the mammals whose milk glands are elaborated sweat glands, 



