TOOTH DEVELOPMENT — DASYPUS NOVEMCINCTUS 655 



apex could be felt just under the surface of the gum. In the 

 other case the tooth was erupted (lower jaw of this animal in 

 text fig. 3.) A glance at text figures 2 and 3 will show that this 

 tooth is much more superficially placed in the jaw than are any 

 other of the back teeth. This tooth is evidently either resorbed 

 or else erupted and shed very soon after birth. Reinhardt says 

 that the last front tooth is sometimes retained in the half grown 

 animal. 



The other front teeth become successively smaller toward the 

 anterior end of the jaw. The first two are always very small, 

 and in fact, the first three actually decrease in size between a 78 

 mm. stage and a 108 mm. stage. Since these teeth are so small 

 and inconspicuous it is easy to overlook them with an unsatis- 

 factory stain unless very careful observations are made. This 

 probably accounts for the fact that previous investigators have 

 usually failed to describe more than three or four of the small 

 front teeth. It is, however, difficult to understand why in a 78 

 mm. embryo, Leche failed to find any of them with the excep- 

 tion of the last one which he considers to be the first back tooth. 

 It is, of course, quite possible that in an animal in which there is 

 occurring a progressive reduction of these front teeth, fewer teeth 

 than usual may sometimes be formed, so that these investiga- 

 tors may have observed all of the tooth germs that were present 

 in their material. However, this explanation would scarcely 

 account for an entire absence of the first five front teeth in a 78 

 mm. embryo such as Leche describes. 



2. Number of tooth germs in the embryonic upper jaw 



So far as I am aware no account has been given of the number 

 of tooth germs present in the embryonic upper jaw with the 

 exception of Spurgin's statement that he cound find no trace of 

 rudimentary incisors in either an 8.5 or 9 cm. embryo. From 

 my own investigations, I can say that there are present in the 

 embryonic upper jaw seven well-formed tooth buds which can 

 be homologized with the seven back teeth of the post-embryonic 

 upper jaw a few months after birth. There is also present the 



