TOOTH DEVELOPMENT DASYPUS NOVEMCINCTUS 667 



blasts at the base of the tooth are becoming enclosed within 

 their secretions. In the 78 mm. embryo, some calcified dentine 

 is present, and here again odontoblasts are found imprisoned in 

 the dentine at the base of the tooth. This process continues, so 

 that at birth normal dentine occurs only at the apex of the tooth. 

 In these later stages the pulp cavity is narrow and at birth its 

 entrance is almost closed. The pulp always contains many 

 blood vessels (text fig. 10, b). 



In the beginning of its development the fourth tooth shows no 

 change from preceding stages. It has the same poorly developed 

 and little differentiated enamel organ. It deposits less enamel 

 than either of the preceding stages. It does not pass through 

 the changes in shape that characterize the fifth and sixth teeth. 

 It always has a rather high narrow papilla ; also there is a greater 

 tendency on the part of the dentine-secreting cells to become 

 enclosed within their secretion. There are a few normal den- 

 tinal tubules near the apex of the tooth. As development pro- 

 ceeds, the entrance to the pulp cavity becomes narrower from 

 the deposition of cellular dentine at the base of the tooth. At 

 birth the entrance is still open, although it is apparently closed 

 in the 82 mm. embryo. (See text figs. 4 c, 10 c, and 11 which 

 show this tooth at birth. In 4 it will be seen that there is a much 

 greater difference in size between teeth 5 and 4 than there is in 

 text figure 10.) 



In the earlier stages the second and third teeth possess enamel 

 organs similar to those of the front teeth already described. In 

 the 92 and 108 mm. stages an almost imperceptible layer of 

 enamel is found over the cusps of these teeth (text figs. 12 and 13). 

 This layer is so thin that it was some time before I was able to 

 prove to my own satisfaction that any enamel was present. It 

 will be recalled that in an 85 mm. embryo, Spurgin described 

 a much thicker deposit of enamel on these teeth. 



In the third tooth, from the 78 mm. embryo on, there is an 

 increasing enclosure of cells within the dentine and an increasing 

 tendency for the entrance of the pulp cavity to be closed by the 

 deposition of dentine. In the 82 mm. embryo, the tooth germ 

 has become an almost solid mass of dentine. In the 92 mm. 



