60 W. H. F. ADDISON AND J. L. APPLETON, JR. 
The structures to be described here, as in the development 
of the crowns of all teeth, are the enamel-organ with the amelo- 
blasts, and the dental papilla (which becomes the pulp-sub- 
stance) with the odontoblasts. There are two factors, however, 
which alter the usual history of the development of these struc- 
tures, and especially of the enamel-organ. First, in permanently 
growing teeth of which these are examples, all these struc- 
tures continue functional throughout life, so that the enamel- 
organ is also a persistent structure. The other factor and one 
correlated to some extent with the first, is that the enamel is 
formed on one side of the tooth only, and here only does the 
enamel-organ develop to its most highly differentiated functional 
condition. 
The history of the development and growth of the tooth 
may be conveniently considered in two stages (1) pre-eruptive, 
and (2) post-eruptive. The pre-eruptive stage extends from 
the 14th or 15th day of fetal life until eruption of the tooth 
takes place between the 8th and 10th post-natal days. Until 
near the time of birth there is no formation of enamel and den- 
tine, but from birth onwards these substances are laid down 
rapidly, so that at eruption, the teeth have their characteristic 
elongated narrow form. This pre-eruptive stage is characterized 
by the rapid elongation of the tooth-forming organs, and by the 
teeth attaining very similar relations to the other structures of 
the jaw which the imbedded portions of the erupted teeth pos- 
sess. Thus, the anlage of the lower incisor appears under the 
oral epithelium in the anterior region of the mandible, and 
grows continually backwards, until its growing end reaches 
the region beneath the developing molars. At this time the 
growing end presumably reaches a region which, by reason of 
its increasing calcification, offers resistance to further progress. 
The result of the ever-continuing mitotic division and cell growth 
at the basal end, is the pushing of the whole tooth and its forma- 
tive organs, in the opposite direction, and the consequent erup- 
tion of the tooth. During the latter half of this pre-eruptive 
stage, the anterior tip of the developing tooth structure is im- 
mediately beneath the oral epithelium, and remains at a fixed 
