44 W. H. F. ADDISON AND J. L. APPLETON, JR. 
cation of the gnawing animals. The observations here recorded 
are based on the study of the cellular processes involved in the 
formation, eruption and growth of the incisor teeth in a single 
rodent form—Mus norvegicus albinus. An additional interest 
was lent to the work by the fact of the increasing use of this 
animal for laboratory purposes, which makes it desirable to learn 
the time-relations of its life-processes, as a basis of comparison 
in various forms of experimental studies. Although the rodent 
incisors have been the object of much study, few observers have 
carried out their observations through the complete life-history, 
including developmental stages and adult structure, in one 
form of animal and this it has been our aim to do. 
HISTORICAL SURVEY 
Oudet (23) proved the phenomenon of permanent growth 
in the incisor teeth of rodents by cutting off the teeth at the 
gingival margin and observing that they were regenerated. Ret- 
zius (’37) and others noted the overgrowth of these teeth in cases 
of malocclusion. MacGillavry (76) observed the rate of growth 
of the incisors of a rabbit by making marks on the teeth and 
noting the gradual advance and disappearance of these marks, 
as the teeth grew out and were worn away. 
Questions which have called forth much study and contro- 
versy are (1) does the rodent incisor belong to the milk or to 
the permanent dentition; and (2) which of the three incisors 
of the typical mammalian dental formula does it represent. 
Without exception, all who have studied the first question agree 
that the large rodent incisor belongs to the second or permanent 
dentition. These same studies show that abortive milk incisors 
occur in a varying degree in the several families of the Rodentia; 
and that they are slightly, if at all, represented in the Muridae. 
As to the second question, Cope on palaeontological evidence 
decided that the large rodent incisor was I. Adloff (’98) on 
embryological evidence confirmed this view. Freund (’92), 
Woodward (’94) and Stach (’10) believed it to be I,. Weber 
(04) has given a resumé and extended bibliography of this 
work, up to the date of his writing. 
