80 Enamel in the Teeth of an Embryo Edentate 



developing sent prolongations between them which roughly followed the 

 contour of the teeth. The tooth-buds, however, were close together 

 and complete septa had not as yet been formed between them. In all 

 the back teeth except the eighth, a thin layer of dentine had been 

 deposited and in a few of them it was calcified. On the whole the 

 development of the teeth in the lower jaw was in advance of that of 

 the upper. In the embryo of 9 cm. the development was still further 

 advanced, and calcified dentine was found in most of the teeth. Plate 

 II, Fig. 3, shows the first back tooth with a well developed layer of 

 enamel appearing under the low powder as a much darker band than the 

 dentine, and broken at frequent intervals in the direction of the enamel 

 rods in the process of sectioning. As in the first three rudimentary 

 teeth, the columnar cells of the inner layer of the enamel organ have 

 become flattened over the thicker portion of the enamel layer, while 

 they still retain their shape over the thinner portions (Plate II, Pigs. 

 2 and 3, ce). The stellate reticulum and outer layer of the enamel 

 organ were still present over the sides of the dental papilla as shown 

 in Fig. 3. The portion marked eo, which has been torn in sectioning 

 from the body of the enamel organ, shows the epithelial ring (er) 

 in process of being constricted off. This has been described in full by 

 Ballowitz and has also been observed by Pouchet and Chabry in the 

 embryo of the sloths. I found this portion of the enamel organ in 

 many sections of both rudimentary and back teeth. In some sections 

 it has been separated from the enamel organ. Plate II, Fig. 4, shows 

 a high power drawing of the enamel and dentine from the same section 

 as Plate II, Fig. 3. The uncalcified dentine is easily distinguished from 

 the darker calcified dentine, being cut off from the latter by a sharp 

 line of demarcation, a condition which was not found in any 

 of the rudimentary teeth. This may indicate that in these teeth the 

 dentine has been completely deposited. If such proves to be the case in 

 later embryos, Ballowitz's theory concerning the epithelial ring would 

 have no weight. 



In the fifth and sixth tooth-buds, in the longitudinal sections of the 

 smaller embryo, the buds for the permanent teeth could be seen coming 

 off from the outer epithelial layer of the enamel organ. Plate II, Fig. 5, 

 which shows this, shows also a portion of the enamel organ with inner 

 and outer epithelium, stratum intermedium, stellate reticulum, and 

 Tomes' processes. A thin layer of uncalcified dentine has been deposited. 

 I do not find that the outer layer of the enamel organ is broken through 

 until after the enamel has begun to be laid down. Eose and Ballowitz 

 both describe the breaking up of this layer as taking place shortly after 



