TOOTH DEVELOPMENT — DASYPUS NOVEMCINCTUS 679 



2. Nowhere else in the jaw have I found similar structures. 



3. Their number is fairly constant, ranging from three to five. 



4. They are similar in structure to the so-called 'epithelial 

 pearls' whose relation to tooth formation has been recognized. 



5. Before transformation into horny material, their structure 

 is exactly like that of the 'nodules' and 'concentric epithelial 

 bodies' which occur in Perameles, Ornithorhynchus, and the 

 Gymnura, and which are evidently vestigial teeth. 



In none of the cases where epithelial pearls have been inter- 

 preted to represent vestigial teeth has their transformation into 

 horny material been described. If later stages of the embryos 

 of Ornithorhynchus, Perameles, and the Gymnura were studied, 

 it is very probable that horny cysts would be found. In Dasy- 

 pus, these pearls have so far lost their capacity to form tooth 

 germs that, inside the substance of the jaw, they behave just as 

 ihey would had they remained on the outside. Downward 

 growth into the underlying tissue seems to be the only action 

 which is reminiscent of their former behavior as enamel organs. 

 As I have mentioned above, Bland-Sutton has found all grada- 

 tions between epithelial pearls, horny cysts and enamel organs. 



Since these structures always occur in the premaxilla, they 

 may represent incisors. In this connection it is interesting to 

 note that their maximum number is five, and that this is also the 

 maximum number of vestigial incisors in the low^er jaw. This 

 raises the question as to where the missing canine is. I can only 

 state that with the exception of the dental lamina which extends 

 through this region in the early embryo, I have found no trace 

 of tooth germs between the first of the functional back teeth and 

 the last of the vestigial front teeth. For this reason, I think that 

 the last of these vestigial front teeth may represent the canine. 



G. SUMMARY 



1. In the embryonic lower jaw, the anlagen of either thirteen 

 or fourteen teeth arise. 



2. The last eight of these teeth germs become back teeth. 

 The anterior tooth germs do not develop into functional teeth. 



