80 W. H. F. ADDISON AND J. L. APPLETON, JR. 
new conditions at the erupting end of the tooth. Before detail- 
ing these changes, it may be advisable to state, in a general way, 
the changing circumstances attendant upon eruption. Up to 
this time the anterior end of the tooth has been nearly stationary, 
but there has been continued growth backward of the posterior 
extremity. At this time the rate of progression forward is greatly 
increased, and the rate of progression backward much reduced. 
As suggested before, the process of eruption may depend largely 
upon the fact of increasing calcification in the bones, rendering 
them more resistant to the backward growth of the develop- 
ing tooth. Whatever may be the causal factors, from now on 
the tooth continues to grow out at a regular rate, through the 
development of new cells at the basal end of the formative 
organs, these cells in turn giving rise to the hard parts of the 
tooth. Within a few days after eruption, the use of the tooth 
involves the process of attrition by which, in spite of the regular 
rate of growth, the exposed length is kept nearly constant for 
any age. 
It is generally agreed that, by reason of the protoplasmic 
processes which extend into enamel and dentine from amelo- 
blasts and odontoblasts respectively, these cells must be car- 
ried along with the tooth as it moves. Thus, as there is con- 
stantly a regeneration of these cells at the basal end of the tooth, 
there must be an opposite process of some nature by which 
these cells are eventually lost at the apical end, when carried 
thither by the outward progress of the tooth. First we may 
follow the history of the ameloblasts in this locality. Before 
eruption, the enamel-organ is continuous with the stratified 
epithelium forming the sheath around the gingival margin, 
and this relation continues at and after eruption. As the tooth 
moves forward during eruption the ameloblasts must move 
along with it and, when those at the anterior end approach 
the gingival margin, they must either be held there, or be car- 
ried out on the enamel until detached. On examining longi- 
tudinal sections at 12 days (fig. 22) it is seen that the amelo- 
blasts, as they approach the gingival margin, become shorter 
and shorter, until, beneath the thickened sheath of epithelium 
