TOOTH DEVELOPMENT DASYPUS NOVEMCINCTUS 651 



C. NUMBER OF TOOTH GERMS PRESENT IN THE EMBRYO 



1. Number of tooth germs in the embryonic lower taw 



The literature on dentition in the armadillo reveals the fact 

 that previous investigators have given varying accounts of the 

 number of tooth germs present in the embryo. They seem 

 fairly unanimous in agreeing that the germs of 'eight back teeth' 

 are present, but the number of 'rudimentary incisors' reported 

 varies from none at all to seven (Leche). There also exists much 

 confusion regarding the classification of the armadillos and in 

 many cases it is impossible to discover whether investigators 

 are describing the same or different species. Many of the con- 

 flicting results are doubtless due therefore to the fact that dif- 

 ferent species were examined. 



My work was done entirely upon D. novemcinctus and in com- 

 parison with those of previous investigators my results are strik- 

 ingly uniform. As soon as tooth buds are definitel}^ formed, there 

 are found to be thirteen as a maximum and twelve as a minimum 

 number. This number, however, does not include the last back 

 tooth, which up to near the time of birth is represented only by 

 a backward continuation of the dental lamina beyond the last 

 definite tooth bud. At birth this backward continuation of the 

 dental lamina shows a definite enlargement which corresponds 

 to the future tooth germ. In all these early stages, whether 

 twelve or thirteen tooth germs occur, there are present in front 

 of the first tooth germ, scattered groups of epithelial cells which 

 plainly represent a degenerating dental lamina which extended 

 into this region before the formation of definite tooth germs. 

 This would indicate that the ancestors of Dasypus possessed 

 teeth in this region, and that, as observed by Leche, a reduc- 

 tion of teeth is now going on. That this reduction is now in 

 progress in also evidenced by the fact that when twelve tooth 

 germs instead of thirteen are present, it can be shown that it is 

 the most anterior tooth germ which has failed to develop in the 

 former case. It seems to be a matter of chance, whether or not 

 this front tooth develops. I found it in approximately 50 per 

 cent of the embryos which I examined. The chances are about 



