94 W. H. F. ADDISON AND J. L. APPLETON, JR. 
The length of the teeth varies with the size of the cranium 
(table 1) so that the persistent growth is not only sufficient to 
offset the continual attrition, but also serves to keep the length 
of the teeth in a definite relation to the length of the skull, as 
the latter increases in size. 
The lower tooth is always longer than the upper, and this 
difference manifests itself even in the anlagen of these structures 
in the 19-day fetus (figs. 11 and 12). 
The contour of the enamel, as seen in cross-sections, is char- 
acteristically different in the upper and lower teeth (fig. 4). 
The enamel of the tooth is composed of two layers which are 
different in appearance. The enamel rods run in two sets which 
decussate with each other in the inner or plexiform layer, but 
they change their direction as they continue into the outer layer, 
so that in it they are all parallel. In longitudinal sections, the 
general direction of the rods (fig. 26), is to incline towards the 
apex of the tooth, as they run from the enamel-dentine boundary 
to the outer surface of the enamel. 
In conclusion, we wish to thank Professor Piersol for generous 
assistance in many ways, and Professor Donaldson for his con- 
stant interest in the study. We also wish to acknowledge the 
kind assistance of Mr. E. F. Faber in the preparation of the 
drawings. 
