46 W. H. F. ADDISON AND J. L. APPLETON, JR. 
MATERIAL AND METHODS 
The albino rat is a variety of Mus norvegicus, the common 
gray rat (Donaldson 712). This has been shown by similarity 
of skull measurements (Hatai ’07) and of hemoglobin crystals 
(Reichert and Brown 710) and also by the fact that the two 
interbreed freely. 
The material used was obtained from the rat colony of The 
Wistar Institute. Serial sections in paraffin or in_ paraffin- 
celloidin were made of decalcified heads of fetuses taken at 
daily intervals from the 16th day onwards until birth, and of 
jaws of animals newly-born and at short intervals until one month, 
and of several older stages. Serial sections of fetuses younger 
than 16 days were examined in the collection of The Wistar 
Institute. Ground sections were made of the isolated teeth, 
and the petrifaction method of imbedding in Canada balsam 
was used to prepare the teeth and adjacent soft parts in silu. 
Also a series of prepared crania, some entire and some disarticu- 
lated, was made at selected ages, varying from birth to old age. 
The ‘gold dust’ method of Davison, as tested out for different 
ages at The Wistar Institute was used for the preparation of 
the former, and maceration in tap water for the latter. Schultze’s 
clearing method was found useful in studying the early periods 
of calcification. 
DENTITION OF ADULT ANIMAL 
The dental formula of the albino rat is I - Cc 1 P >) M =. 
There is only one set of teeth, and hence the dentition is mono- 
phyodont. The time of eruption of the various teeth extends 
over a period of 33 weeks. The incisors are the first to appear, 
viz., at 8 to 10 days after birth. The first and second molars 
erupt at about the 19th and 21st days respectively, and it is 
after this period that the young animals may be weaned and 
are able to maintain an independent existence, as far as food is 
concerned. The third molars are delayed until 2 weeks later 
and do not appear until about the 35th day. 
