DEVELOPMENT OF THE DIGESTIVE ORGANS. 971 



milk tooth, and, when this is absorbed, against the fang itself. A 

 peculiar, thick, pulpy, vascular areolar tissue intervenes between them, 

 and appears to accomplish a gradual absorption, first of the crusta 

 petrosa, afterwards of the dentine, and even of the lower part of the 

 enamel of the milk tooth. Sometimes this process of absorption, which 

 is not due merely to pressure, is temporarily interrupted, a renewed 

 deposition of dentine taking place. At length, the side of the socket, 

 the fang, neck, and even part of the crown of the milk tooth, having 

 been absorbed, the tooth is loosened, hangs for a time by the gum, and 

 finally drops out. The permanent tooth rises in its place; the crown 

 appears above the gum, whilst the fang, closely followed by a consen- 

 taneous development and modelling of the osseous tissue of the jaw, 

 becomes firmly fixed in its socket. The giving way and disappearance 

 of the bone in one place, and its growth in other directions, by inter- 

 stitial absorption and deposition, are remarkable examples of the plastic 

 endowments of the osseous tissue. 



The development of the teeth in the Mammalia conforms generally 

 to the process above described. 



The dentition of the Marsupialia, however, has lately been shown to be 

 very peculiar. The only tooth which is successional is the last premolar, on 

 each side of both jaws ; it alone is preceded by a temporary tooth, which has 

 a molar shape. The other teeth of the Marsupial jaw, not having any prede- 

 cessors, might perhaps be regarded as themselves belonging to the first set, 

 though not falling out ; but this view seems contrary to the homologies of 

 these teeth, which show them to be permanent. Perhaps in a very early 

 stage of development, transitory rudiments of other temporary teeth may be 

 met with in the forepart of the jaw of the Marsupials. (Flower.) The milk 

 premolars of the guinea-pig are shed even before birth ; the milk teeth of the 

 Bats, Insectivora, and Seals are very simple and temporary ; the first incisors 

 of the elephant are very small, and the large incisors of the Eodents do not 

 appear to have temporary predecessors. 



The teeth are always parts of the exo- or dermo-skeleton. The replacement 

 of the deciduous, by permanent teeth, occurs only once. But in the carniv- 

 orous Cetacea, the Edentata, and the Monotremata. there is no succession of 

 teeth. Such animals are called Monophyodonts, the ordinary Mammalia 

 being named Diphyodonts. The teeth of Keptiles and Fishes are also devel- 

 oped upon papillae, and are mostly inclosed in sacs, or follicles, in certain 

 cases, provided with an enamel organ ; in the parrot fish, the rudiments of 

 the teeth sink into alveoli ; in the pike, they sink only into sacs or follicles ; 

 in the sharks and rays, they remain as papillae on the surface, thus represent- 

 ing the papillary, follicular, saccular, and alveolar stages of development of 

 the teeth of Mammalia. 



The Digestive Grlands, the Lungs, and the Spleen. 



The mucous glands of the mouth, the salivary glands, the mucous 

 glands and special tubular glands of the stomach and intestine, the 

 liver and pancreas, and even the lungs, are developed from the mid- 

 dle and innermost germinal layers, the latter forming the epithelial 

 lining of the several glandular organs. They usually commence by 

 processes of epithelium, which sink into the corium, penetrate into 

 numerous bud-like projections, corresponding with the future lobes 

 and lobules of the gland, and are subsequently hollowed out, to form 

 the ducts. 



