TOOTH DEVELOPMENT — DASYPUS NOVEMCINCTUS 673 



of cells and not a tooth bud. Such a grouping of cells is not con- 

 stant at an}^ stage of development and, if it ever does represent a 

 secondary tooth bud, it remains aborti\'e, and does not produce 

 a permanent tooth bud. These groups of epithelial cells some- 

 times give rise to cysts which are apparently the exact homologues 

 of the epithelial cysts which always occur in the premaxilla (figs. 

 15, 16, 17 and 18). 



The homologies of these teeth are doubtful. From its shape, 

 position in the jaw, and the fact that a marked diastema occurs 

 between it and the first functional back tooth, the sixth and last 

 of the front teeth is probably a canine. If shape were the only 

 criterion, the fifth or even the fourth tooth (text fig. 4, c and d, 

 10, h and c) might be identified as the canine. But the position 

 of this tooth is such that it would undoubtedly articulate with 

 a tooth situated just behind the premaxillo-maxillary suture if 

 such a tooth were present. Tomes thus defines a canine tooth: 

 "The nearest approach to a good definition is that which de- 

 scribes the canine as the next tooth behind the premaxillo-maxil- 

 lary suture, provided it be not far behind it; and the lower canine 

 as the tooth which closes in front of the upper canine." The 

 other front teeth must then represent incisors, and, if we have 

 correctly homologized the back teeth, the dental formula of the 

 lower jaw of D. novemcinctus is M 1, Pm 7, C 1, I 6 or 5. 



F. DEVELOPMENT OF TOOTH VESTIGES IN THE PREMAXILLA 



The literature on tooth development in the anterior part of the 

 upper jaw seems to be limited to the following quotation from 

 Spurgin: ''Although I carefully examined the sections from the 

 upper jaw of both embyros (8.5 and 9 cm.), I failed to find any 

 trace of rudimentary incisors." I have made sections through 

 the upper jaw of embryos ranging from 35 mm. to 108 mm., and 

 have a complete history of the events up to birth. 



On each side of the jaw of a 35 mm. embryo, there is a con- 

 tinuous dental lamina which extends for a few sections into the 

 premaxillary anlage. In the posterior part of the jaw this 

 dental lamina is connected with the oral epithelium, but loses 



