34 AMPHIBIA t^HAii. 



paratively few vertical septa Ly which the upper and denser 

 layers, the corium proper, are connected with the underlying 

 muscles. The spaces are filled with lymph, and into some of them 

 the abnormally expanded vocal sacs extend, notably in Pn] mUcola, 

 Leptodactylus, and other Cystignathidae, and in Bhiwidrrnm. 



The cutis frequently forms papillae and prominent folds, 

 sometimes regular longitudinal keels on the sides of the back ; but 

 dermal, more or less calcified or ossified scales are restricted to 

 the Stegocephali and to the Apoda, q.v., pp. 79, 87. We con- 

 clude that the Urodela and Anura have entirely lost these organs. 

 Dermal ossifications, besides those which now form an integral 

 part of the skeleton, like maiiy of the cranial membrane-bones, are 

 rare, and are restricted to the .Vnura. They are least infrequent 

 on the head, where the skin is more or less involved in the 

 ossification of the underlying membrane-bones, for instance in 

 Triprioii, C'alyptocejihalus, JHemijihractus and Pelohates. The 

 thick ossifications in the skin of the back of several species of 

 Ceratophrys are very exceptional. In Brachyceptliahis ephippium 

 these dermal bones enter into connection with the vertelsrae ; 

 small plates fuse with the dorsal processes of the first to third 

 vertebrae, while one large and thick plate fuses with the rest 

 of the dorsal vertebrae. Simple calcareous deposits in the 

 cutis are less uncommon, for instance, in old specimens of Biifo 

 vulgaris. We are scarcely justified in looking upon these various 

 calcifications and even ossifications as reminiscences of Stego- 

 eephalous conditions. 



The skin contains pigment. This is either difi'use or granular. 

 Diffuse pigment, mostly dark brown or yellow, occurs frequently 

 in the epidermis, even in the stratum corneum. The granular 

 pigment is stored up in cells, the chromatophores, which send 

 out amreboid processes, and are restricted to the cutis, mostly 

 to its upper stratum, where they make their first appearance. 

 Contraction of the chromatophores withdraws the pigment 

 from the surface, expansion distributes it more or less equally. 

 The usual colours of the pigment are black, brown, yellow, and 

 red. Green and blue are merely subjective colours, due to 

 interference. A peculiar kind of colouring matter is the white 

 pigment, which probably consists of guanine, and is likewise 

 deposited within cells ; cf the description of the white spots in 

 the skin of Hyla coerulea. 



