172 H. SPENCEli HARRISON. 



this divergence from the rule, as this particular tooth is 

 formed before the dental lamina, and may easily have 

 reached its present relative position through the growth of 

 the epidermis. The palatine dental lamina of each side has 

 four enamel organs, of which the first two only have partially 

 calcified teeth (fig. 4, p. e. o. ; this section passes through the 

 anterior portion of the first enamel organ). The second is 

 the only one having an epidermal strand lingual to the 

 euamel organ. Between the teeth the dental lamina is con- 

 tinued as a well-marked strand of cells (fig. 5, d. I.) in both 

 palatine and maxilla : in the maxilla it terminates without 

 becoming separated from the epidermis ; in the palatine 

 the dental lamina and the lingual epidermal ingrowth run 

 together behind (as they arise together in front), and form a 

 roundish knob of cells attached to the epidermis, which, 

 again, is continuous posteriorly with a groove in the roof of 

 the mouth. 



In the mandible of the last embryo described we saw 

 that of the first three teeth on each side the second was more 

 advanced than the other two, and this is the case in the 

 present specimen. It has the same claim to be considered 

 a member of an earlier series than the others as has the 

 second premaxillary. There are now ten pairs of calcified 

 cheek teeth, and a pair of enamel organs without calcification 

 on the posterior free prolongations of the dental laminae. A 

 reference to fig. 11 will show that the mandibular teeth 

 exhibit an alternation in size similar to, though not so con- 

 spicuous as, that seen in the maxilla. I have been unable, 

 however, to convince myself that there is a corresponding 

 difference in their period of development ; and as the mandible 

 is in a more advanced stage than the maxilla as regards the 

 epidermal ingrowths which are so conspicuous at a later 

 stage, the evidence of a possible residual dental lamina 

 cannot be relied upon. As a matter of fact, each of these 

 mandibular teeth has a well-marked epidermal ingrowth on 

 its lingual side (fig. 5, man. lin.), but the interpretation 

 of this structure as a residual dental lamina is hardly justi- 



