656 BERTHA E. MARTIN 



backward continuation of the dental lamina which gives rise to 

 an eighth back tooth. As I shall show later in this paper there 

 also occur in the premaxilla from three to five epithelial cysts 

 which probably represent front teeth in the last stages of degen- 

 eration. 



D. DEVELOPMENT OF THE FUNCTIONAL BACK TEETH 



1 . History and functions of the enamel organ 



A review of the literature on the development and fate of the 

 enamel organ makes it evident that the following problems remain 

 to be solved: 



1. Does the enamel organ form any secretion or is its only 

 function that of giving form to the developing dental papilla? 



2. If a secretion be formed, what is its nature? Is it a thin 

 structureless membrane, as Rose would have us believe, or is 

 there a formation of t-rue enamel, as Spurgin claims? 



3. If the enamel organ forms any kind of a secretion, what is 

 the time relation between the formation of this secretion and 

 the disappearance of the various parts of the enamel organ? 



The history of the enamel in the upper jaw is exactly similar 

 to that in the lower, except that, as Spurgin has already pointed 

 out, the development in the lower jaw is usually ahead of that in 

 the upper. For this reason, I have confined the following ac- 

 count to the events in the lower jaw. For the sake of uniformity, 

 I have, with one or two exceptions, used the sixth back tooth as 

 the basis for my figures. This is an arbitrary choice, and any 

 one of the functional back teeth in the upper or lower jaw could 

 just as well have been used. For figures showing the different 

 stages in the development of the functional back teeth I refer the 

 reader to figures 1, 2, 3, 4, 5, 6, 7 and 8, and text figure 4. 



My investigations show that the origin of the enamel organ in 

 D. novemcinctus is similar to that in other mammals — all parts 

 developing (fig. 3) although there is an early disappearance of 

 the enamel pulp. These results are in agreement with those of 

 Rose, Ballowitz, and Leche. Figure 3 shows all parts of the 

 enamel organ present in a 78 mm. embryo, while figure 4 shows 



