VERTEBRATES. 9 



Again, the sebaceous glands serve to lubricate the skin, and are closely connected with 

 the hair follicles. Glands of this character are often largely developed in various groups 

 of mammals, for the purpose of enabling individuals of a species to trace each other's 

 whereabouts. Finally, the milk glands themselves of mammals are developed in the 

 same way as the sebaceous glands, and, in the lowest division — the monotremes — their 

 arrangement is such as to suggest that structures that at one time had a very different 

 function have gradually been diverted towards that of supplying nutriment to the 

 young. 



Epidekm.vl Appendages of the Skin. 

 With the change from an aquatic to a terrestrial life, which results, as above 

 remarked, in the hardening of the exposed layer of epidermal cells with a horny 

 layer, we find that certain additional protective or defensive structures are developed, 

 composed of epidermal cells, which have undergone the same horny transformation, but 

 which are often very specialized and characteristic of the different groups of the air- 

 breathing vertebrates. These structures are always formed by the activity of the 

 mucous layer of the epidermis, which generally retains the power of reproducing 

 them when lost. Their presence always affects the underlying corium, and in many 

 instances they may have important relations to underlying bones, whether these 

 belong to the skin, as in the case of the horny tortoise-shell, which covers the bony 

 carapax of the turtle ; or to the internal skeleton, as is the case of the horns of oxen ; 

 or of the nails, claws, and hoofs of air-breathing vertebrates. But again, they may be 

 confined to the skin, without entering into such relations, as is the case with most 

 scales. Feathers and hairs, one of the structures referred to, belong to the category of 

 epidermal appendages, as do also the horny pads or callosities met with in places 

 liable to pressure, throughout all the groups of air-breathing vertebrates. In many 

 cases the cellular structures of these is readily demonstrable, but in others (scales, 

 hairs, horns) the elements are so altered as to give a fibrillated appearance to the 

 tissue. Throughout the chapters which treat of the air-breathing vertelirates, many 

 references will be found as to the extreme variety of form which these protective and 

 defensive corneous structures may assume, here it is very necessary to call attention 

 to the similarity in their mode of development. Even structures so different as hair 

 and feathers have, in this respect, many points in common ; only that phase of the 

 de\-tlopraent of the feathers in which the papilla of the corium pushes out the over- 

 lying epidermis beyond the fi'ee surface, is not found in the hair, where the first stage 

 of development is an inward growth of the mucous layer of the epidermis, into 

 which the papilla of the corium afterwards grows. 



SKELETAL STRUCTURES. 



Under this heading are embraced all the hard parts which serve for the sujjport of 

 the body, and likewise for the protection of internal organs. Frequently the skin is 

 the seat of such skeletal structures ; when this is the case the structures arc termed 

 collectively the exoskeleton ; the term endoskeleton is being reserved for those struc- 

 tures which are developed in deeper layers of the mesoblast. 



Exoskeleton. 

 Structures of this category are certainly more frequently found in the lower than 

 the higher Vertebrata. The scales of fishes are all of this character, however sing- 



