20 MICROSCOPIC ANATOMY OF THE TEETH 



Fig. 4 shows the left half of the mandible of a seventeen- 

 weeks' embryo, 18 cm. long. The enamel organs of the five 

 temporary teeth of that side are seen growing from the 

 tooth-band, which shows indications of absorption between 

 the germs. The prolongation of the tooth-band backwards, 

 which gives origin to the first permanent molar, is very 

 clearly seen. It is evidently not produced from the neck of 

 the enamel organ of the temporary tooth, as formerly taught, 

 but from a further backward growth of the original tooth- 

 band. 1 



FIG. 4. Model of left half of mandible of a seventeen weeks' old foetus, 

 c. lip furrow ; &. lip furrow band ; a. tooth-band ; a', prolongation of 

 tooth-band to form first permanent molar. 



From these models and the foregoing description, it can 

 be easily understood that in the process of breaking up and 

 absorption of the tooth-band between the forming enamel 

 germs, remnants of this epithelial lamina may persist, and 

 give rise to such irregularities as supernumerary teeth, 

 odontomes, &c., other portions becoming degenerated and 

 not absorbed, giving rise to such abnormal structures as 

 cysts and epithelial pearls. 



The question of the existence of pre-milk and post- 

 permanent teeth is closely connected with this mode of 

 development of the teeth. 



Pre-milk teeth would be produced by buds given off from 



1 To understand clearly the relations of the parts in this model the page 

 containing the illustration should be held above the head (the lower 

 margin of the page forward), and viewed from below and from the front. 



