THE TEETH. 407 



when once completed, no further change in the shape of the crown 

 can occur. 



The mode of formation of the dentine is as follows. The 

 cells at the surface of the dental papilla form a single layer 

 of finely granular nucleated cells, closely arranged side by side, 

 and spoken of as odontoblasts. The most superficial parts of 

 the odontoblasts become converted into, or else form by ex- 

 cretion, a gelatinous matrix in which calcification soon occurs, 

 forming the dentine. The deeper parts of the odontoblasts, 

 containing the nuclei, remain soft and unaltered ; they give off 

 fine processes towards the surface of the tooth, which lie in 

 channels in the dentine, these channels being the dentinal 

 tubules of the adult. By a continuance of this process the 

 dentine increases in thickness ; the odontoblasts, which are the 

 active agents in the process, forming a layer on the inner 

 surface of the dentine, and sending out fine radial prolongations 

 into the dentinal matrix. 



The follicle, or tooth-sac, is formed by a condensation of the 

 vascular mesoblast around the tooth. The cement is a thin 

 layer of bone formed round the tooth by the wall of the follicle, 

 which acts as the periosteal membrane. 



The permanent teeth are formed in the same way as the 

 deciduous teeth ; their enamel organs arising as outgrowths from 

 the necks of those of the deciduous teeth (Fig. 156). 



2. The Thyroid Body. 



The thyroid body of the rabbit arises early in the tenth day, 

 as a median thickening of the epithelium of the floor of the 

 pharynx, which grows downwards into the connective tissue 

 immediately in front of the pericardial cavity. The stalk of 

 connection with the pharyngeal floor narrows, and during the 

 eleventh day disappears, leaving the thyroid as a solid epithelial 

 body (Fig. 150, th) embedded in the mesoblast of the floor 

 of the pharynx, immediately in front of the truncus arteriosus, 

 and between the roots of the carotid arches. 



In the later stages the thyroid body widens transversely, 

 giving off two lateral lobes which rapidly increase in size. A 

 cavity appears in the median portion, and soon extends into the 

 lateral lobes, from which outgrowths, some hollow and some 

 solid, soon arise. As the heart shifts backwards into the thorax, 



