258 



KENJI NITTONO 



of attrition begins within a few days after eruption, so that by 

 19 or 21 days of age, the typical occlusal surface is formed." 



"At 21 and 23 days the first two molars erupt in both upper 

 and lower jaws, and from now on the animal is able to secure 

 food for itself, and if necessary can maintain an independent 

 existence. The third molars are delayed until two weeks later 

 and do not appear until about the 35th day." 



In table 17 the first two columns are supplementary to the 

 table given by Addison and Appleton. 



Chart 5 Based on table 17 and giving in millimeters on age the total and the 

 extra alveolar lengths of the lower and upper incisors. 



Chart 5 shows graphically the relations of the data given in 

 table 17. The ordinate values represent total length and the 

 extra alveolar length, respectively, of the upper and the lower 

 incisors on age. In the graphs we can distinguish clearly three 

 phases of growth, namely, 1) the period of the rapid elongation 

 of the teeth, which extends from birth to 23 days; 2) the period 

 of the slow elongation which extends from 23 to 105 days, and, 

 3) the period of still slower elongation which extends from 105 

 days to 10 months after birth. 



We have noticed already that the myelination of the axis 

 is distinct at twelve days and about this time (eight to ten days) 



