HISTOLOGY OF THE TEETH. 189 



which time only a tenacious soft mass will be found. Remove 

 some of this with a small pointed piece of wood, place on slide, 

 cover, and examine. 



Study of Teeth in situ. — This is a good, instructive way to see 

 the many features in one specimen. Take a rat's jaw, remove the 

 fiesh, soften with picric acid in the way above described ; then 

 immerse in spirit, embed, cut and stain with carmine — which is 

 good for double stain with the picric acid — or, better, stain with 

 picro-carmine and logwood and mount in glycerine. Besides show- 

 ing teeth, the bone is well seen. At the lower part the constantly 

 growing incisor, which extends in the rat below the molars to the 

 back part of the jaw, exhibits the large, elongated odontoblasts of 

 a developing tooth, with their well-marked dentinal processes 

 (fibres of Lent), which in some parts project like harp-strings across 

 a small space which intervenes between the cells and the dentinal 

 substance. It will be remarked, also, that in these teeth the most 

 newly-formed layer of dentine becomes, especially near its junc- 

 tion with the older parts, very intensely stained by the logwood. 

 This is the case with all teeth which are still in process of deve- 

 lopment. Carmine does not exhibit the same action. 



Development of Teeth.— Perhaps the most convenient animals 

 to choose are new-born rats, since sections of their jaws exhibit 

 not only the mode of development of the teeth, but also the hair, 

 inferior maxilla (which ossifies in the connective tissue around 

 Meckel's cartilage), the tongue, and many other parts. The foe- 

 tuses are decapitated and the heads dropped into a large beaker of 

 one-sixth per cent, chromic acid. After a week's time, during 

 which the liquid is now and then stirred, they are transferred to 

 weak spirit, and in twenty-four hours to strong spirit. Leave in 

 this for a day or two when they will be ready for cutting. Embed 

 either the lower jaw separately, or the whole head may be placed in 

 the mould, and both jaws cut simultaneously. Stain the sections 

 with logwood, some in carmine (made by dissolving 2 grms. of 

 carmine in a few drops of ammonia, and diluting with water to 100 

 cub. cent.) The earlier stages in the development of the teeth may 

 be perhaps seen in the molar region ; the later stages comprising 

 the development of the dental tissues, especially the dentine and 



