668 BERTHA E. MARTIN 



embryo, the pulp cavity is not closed, but it is practically closed 

 in the 108 mm. embryo, the inside of the mass being occupied 

 by blood corpuscles and dentine secreting cells (text figs. 12, 10 d, 

 and 4 b). This same tendency appears in the first and second 

 teeth but to a still greater degree. (See text figures 13 and 14 

 which show these teeth at birth. Also see text figure 9 which 

 shows the second tooth in a 78 mm. embryo.) 



In the 92 mm. embryo the second tooth may or may not have 

 an entrance to the pulp cavity, but at birth I have always found 

 this tooth represented by a closed mass of dentine, with cells 



Fig. 9 Section through the second front tooth of a 78 nun. embryo. Notice 

 the enclosure of odontoblasts within their secretion. EO, enamel organ; D, 

 dentine; PC, pulp cavity; BV, blood-vessel. X 560 (reduced |). 



imbedded in it. The center of the mass may not yet be entirely 

 filled with dentine, although dentine deposition is still going on 

 (text fig. 13). 



In these later stages it can be seen that the third tooth has a 

 high narrow form while the second tooth is lower and rounded in 

 shape (see text figs. 12, 13 and 14, 10, d, e, and b, also test fig. 

 4 a and b) . After the 78 mm. stage there seems to be a slight 

 decrease in size in the third and second teeth and at birth they 

 are more deeply imbedded in the substance of the jaw than in 

 earlier stages. 



As I have said before, the first tooth may or may not be 

 formed. In the 53 mm. stage an enamel organ for this tooth is 

 evidently forming, and in a 55 mm. stage an enamel organ is 

 present which is as well developed as in the case of the other 

 teeth. In the 61 mm. embryo, I found no trace of a first tooth; 



