TOOTH-GENESIS IN THE CAVIIDA. 263 
the deciduous molar, and the molar immediately behind it, the 
two latter being usually interpreted as dpm.1 and m.1, but which, 
as will be seen subsequently, I believe must be regarded in a 
somewhat different manner. The enamel-germ of the upper 
incisor has assumed the characteristic bell-shape. There is a 
decided labial downgrowth of the dental lamina (PI. 26. fig. 5), 
which is, I believe, to be regarded as the vestigial remains of the 
deciduous incisor. That it cannot be looked upon as affording 
evidence of a pre-milk dentition is clear from an examination of 
the later stages, the bell-shaped germ of this stage growing 
continuously to form the permanent incisor of the adult. This 
labial downgrowth is also extremely well marked in the case of 
the lower incisors. I was not able to detect any trace of a tooth 
anteriorly or posteriorly to this in the incisor region in the upper 
jaw. Throughout the several succeeding sections the dental 
lamina can be traced definitely running through the diastema, 
but without any indications of enamel-germs. It then dips 
deeply into the substance of the jaws both upper and lower, and 
gives rise to a well-marked enamel-germ. This germ has slight 
traces of both labial and lingual downgrowths in the upper jaw; 
while in the lower jaw the latter only is present, but more dis- 
tinct than is the case with the corresponding process in the 
upper jaw. From a consideration of the reconstructed diagrams 
of this and the subsequent stages, I think it will be evident that 
this is the germ of the deciduous tooth. From the wax model 
it is seen to consist of a cone surrounded by a cingulum. 
The latter structure projects to form a very definite upwardly- 
directed and somewhat pointed process arising from the inner 
side of the main cone. There is a corresponding, though less 
pronounced projection on the outer side. Some little distance 
behind this tooth the dental lamina again becomes distinctly 
enlarged at its deeper extremity, forming a flask-shaped mass, as 
yet not invaginated by any appearance of the dental papilla. No 
further trace of any germ is visible at this stage. 
With regard to the labial downgrowth of the dental lamina in 
connection with the functional incisor, which I have represented 
(Pl. 26. fig. 5), it is interesting to compare it with thec ondition 
found in other Rodents. Huxley was the first [12] to note the 
existence of minute milk predecessors to the large functional 
incisors in the rabbit. This discovery has been verified by 
Pouchet & Chabry [14], Freund [10], and Woodward [27]. 
“19% 
