544 



Now if the last deciduous premolar of a Dog, about three days 

 old, be examined in serial sections, we find the condition shown iu 

 fig. 4. 



Lingual side A 



Labial side 



Fig. 4. Section through Lower First Premolar of fa'tal Pig. 



There are three outgrowths of the dental lamina as in other re- 

 gions of the jaw, and it is from the central one that the tooth is 

 developed. In accordance with the views expressed above, this tooth 

 will be seen to belong to the same series as the true molars and not 

 to the same series as the other deciduous teeth. This would furnish 

 an explanation of the very curious fact of its similarity in characters 

 to the so-called first true molar. The labial outgrowth of the dental 

 lamina is seen to have a slightly enlarged extremity with a sac-like 

 arrangement of the connective tissue around it. But more noticeable 

 is the lingual outgrowth with its greater enlargement and its invagin- 

 ation. This downgrowth I take to represent the post-permanent tooth, 

 and it is in a far more advanced stage than in any other situation 

 in which I have seen it. This ultimately disappears and does not 

 give rise to the upper carnassial. This last deciduous premolar would 

 thus appear to belong to the same series as pm^. 



Turning now to the penultimate deciduous premolar, we see that 

 this has well-marked antero- and postero-external cusps. If serial 

 sections through this region be examined, we find a lingual down- 

 growth of dental lamina only, none on the labial side of the tooth. 

 This hngual downgrowth gives rise to the permanent tooth, which, at 



