662 



BERTHA E. MARTIN 



labial cusp in the 108 mm. embryo is an exception to the usual 

 condition. It is evidently an exaggeration of the tendency 

 towards reduction of the labial cusp, which is shown in the young 

 animals after birth. 



Fig. 5 Diagrammatic sections through second back tooth showing change 

 from a bicuspid to a one-cusped condition. 1, section, through tooth in 73 mm. 

 embryo; 2, section through tooth in 83 mm. embryo; 3, 78 mm. embryo; 4, 82 mm. 

 embryo. X 88 (reduced J). 



Fig. 6 Diagrammatic sections through eighth back tooth showing change 

 from a bicuspid condition. X 15 (reduced |). 



Text figure 6 shows the conditions in the eighth back tooth, of 

 which I have no history from the time when it is represented by 

 a thickening of the dental lamina at birth (fig. 8) until it is a well- 

 developed tooth in the process of eruption. After eruption, it 

 is always one-cusped, but unerupted teeth show evidence of two 

 cusps, a higher lingual and a lower labial. The younger the 



