TOOTH DEVELOPMENT — DASYPUS NOVEMCINCTUS 653 



which do develop into functional back teeth. For these reasons 

 it is not strange that all previous investigators have considered 

 this to be one of the functional back teeth. Thus Rose describes 

 eight back teeth and two rudimentary incisors in a 6 cm. and 

 7 cm. embryo. A comparison between Rose's figure 10 and my 

 figure will show the striking resemblance between Rose's 'first 

 back tooth,' and the last front tooth as I have described it. 

 Kiikenthal describes eleven back teeth of which three are rudi- 

 mentary. Leche also mentions eight well-formed tooth anlagen 

 which he considers to represent functional back teeth ; and Spur- 

 gin claims to have found thirteen teeth in each half jaw, of which 

 eight are back teeth and five rudimentary incisors. Spurgin 

 also says that the eighth back tooth, which has no predecessor 

 in the milk dentition, has a well-developed enamel organ. As I 

 shall show later the eighth back tooth is indeed not represented 

 in the milk dentition, but, since Spurgin mistook the last front 

 tooth for the first functional back tooth, his 'eighth back tooth' 

 is in reality the seventh. That his 'eighth back tooth' could 

 not possibly have been the true eighth is proven by the fact that 

 during the entire embryonic life I have found the eighth back 

 tooth to be represented only by a backward continuation of the 

 dental lamina, which does not begin to take on the appearance 

 of a tooth germ until about the time of birth (100 to 108 mm.) 

 which is much later than Spurgin's oldest embryo (90 mm.). 

 Spurgin evidently overlooked this backward continuation of the 

 dental lamina entirely. 



My reason for not considering this tooth to be a functional 

 back tooth is that it is not ordinarily found in the jaw after tooth 

 eruption; at least in the jaws of animals four or five months old, 

 and the functional teeth of these animals can, as I have said be- 

 fore, be homologized with the seven large teeth in the posterior 

 part of the embryonic jaw. The ultimate fate of the last front 

 tooth can be ascertained only by a study of stages of develop- 

 ment between the time of birth and the youngest post-embryonic 

 stages that I have been able to procure. I have actually found 

 this tooth in two of the twenty-nine post-embryonic skulls in 

 my possession. In one case the tooth was not erupted, but its 



JOURNAL OF MORPHOLOGY, VOL. 27, NO. 3 



