as indicative of the Age of the Animal. 
301 
separate stages, the papillary, the follicular, the saccular, and 
the eruptive. 
Fig. 12* 
In the prececUng diagram, which represents sections made 
across the jaw in the several stages of the process, the first of 
these stages is shown by the numerals 2 and 3 ; the second by 4 
and 5 ; the third by G, 7, and 8 ; and the fourth by 9. 
It is during the early existence of fcEtal life that the formation 
of the teeth, in common with the other parts of the body, com- 
mences. Provision is also made at this time for the permanent 
teeth which are to succeed the temporary. According to the 
observations of Professor Goodsir, it is as early as the sixth week 
that the process begins in the human foetus. At that time a 
groove appears along the border of the future jaws which has 
been called the primitive dental groove. This groove is lined by 
the membrane of the mouth — a circumstance to be kept in mind 
for the better understanding of the subject ; and also that this 
membrane, as an " internal skin," is composed of two layers in 
chief, analogous to the dermis and epidermis of the external or true 
skin. Fig. 12 — 1 shows the groove as it appears when the jaw 
is cut across. 
At the bottom of " the dental groove," projections, /ja/)z7Z«, spring 
* Diagram representing the successive stages of the development of the teeth ; 
altered from Goodsir. 
VOL. XV. 
X 
