20 COMPARATIVE ANATOMY. 



were much more abundant in Amphibians of former times ; those 

 of the Carboniferous and Trias (Stegocephala, Labyrinthodonta) 

 were richly provided with them. 



For a further reference to the ring-like scutes and scales found in Csecilians, 

 the reader is referred to the chapter on the dermal skeleton, and it is only 

 necessary to state here that the rings accurately correspond in number with 

 the vertebrae, a very rare occurrence. The same is true of certain Amphis- 

 bsenians (e.g. Blanus cinereus). 



Reptilia. In contrast to the skin of Amphibians, that 

 of Reptiles is very deficient in glands. In Lizards, a series 

 of "femoral glands" occur along the ventral side of the thigh, 

 the secretion from which, as it passes out of the apertures, hardens 

 so as to form a series of papilloe or warts, which appear to serve as 

 clasping organs during copulation. In Amphisbsenians integu- 

 mentary glands are also present : they lie anteriorly to the cloaca, 

 and open into the " pre-anal pores." 



The characteristic peculiarity of the skin of Reptiles 

 is its capacity of producing scales, warts, prickles, shields 

 (e.g. the " tortoiseshell" of Chelonians), claws, rattles (of Rattle- 

 snake), and suchlike structures. 



All these integumentary organs, as already mentioned (p. 16), 

 are to be included in the same category as the feathers of 

 Birds and the hairs of Mammals : that is, all arise at first by a 

 proliferation of the epidermic cells, a portion of the dermal tissue 

 taking part in their formation later on. Certain special differences 

 are, however, always to be observed in the development of these 

 different structures, as will be mentioned presently. 



As in Amphibia, calcifications or ossifications may occur in 

 the derma. The horny layer of the epidermis may be either 

 periodically cast off entire (Snakes), or in shreds from time to 

 time : it is renewed from the Malpighian layer. Pigment-cells 

 also occur, rendering possible in many cases a change of colour 

 (e.g. Chameleon). 



Birds. Birds possess a thinner derma than do any other 

 Vertebrates, and it is not very plentifully supplied with- blood- 

 vessels, although, as will be mentioned in another chapter, sensory 

 organs (tactile corpuscles) are abundant. In the deeper layers 

 there is a strongly developed network of muscle-fibres, which are 

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



The feathers, arranged in so-called " feather- tracts " (pterylse) 

 separated by naked regions (apteria), form the most marked 

 peculiarity of the body of Birds, and their development is very 

 instructive. In the region where a feather is to be formed, the 

 dermal tissue becomes raised up towards the ectoderm (Fig. 12, A, 

 Cu, Sc, SM 1 ^), and thus gives rise to a papilla (Pap). As this papilla 

 grows out to form an elongated cone with a pointed apex, the 

 feather-germ (Fig. 12, B, FK), its base sinks gradually deeper and 



