as indicative of the Age of the Animal. 
305 
separation of the substance of the papilla to produce the form of 
the future tooth, this membrane, as its original covering, is not 
cleft, but merely adapted to the altered shape of the pulpy mass. 
The process of development goes on, and after a time the two 
caps of dentine unite at the bottom of the hollow, and thus 
form a cup, in which enamel, is afterwards produced by its mem- 
brane, lining the cavity throughout. Compare figs. 13 and 14. 
The preceding illustrations are taken from the molar teeth of a 
calf during their development. Tiieyare both vertical sections made 
across the teeth, at their opposite ends. The same letters apply 
to each figure ; a, the pulp cavity ; b, the enamel cup, which is 
still unclosed at the bottom in fig. 13, but closed in fig. 14 by a 
coalescence of the forming plates of Dentine ; n, the dentine, and 
E, the enamel. The depth to which these cups extend, and also 
the peculiar arrangement of the enamel in these compound teeth, 
are correctly depicted in fig. 19, which represents a vertical sec- 
tion of the posterior half of a sheep's molar tooth. 
A question of some importance arises at this stage of our in- 
quiry on the proper solution of which depends the correctness of 
all which has further to be explained with regard to the formation 
and arrangement of the other structures of the teeth, — enamel and 
crusta. It is whether the Dentine, described as being formed 
originally on the apex of the papilla, is in reality produced beneath 
the membrane which covers the papilla ? or otherwise, whether 
this membrane is or is not the nucleus of the formation ? Under 
the supposition that the membrane gives rise to the first produc- 
tion of the Dentine, and is itself obliterated by the process, it has 
been called the pe/foi'inative membrane of the dental papilla.* 
* Without presuming to decide this vexed question, I must say that, after 
repeated investigations, I agree in opinion with those who state that the membrane 
<\oes not become obliterated, but that the dentine is formed altogether beneath it. 
The mcinhrnnc )-eiiuiiiis upon the cup of dentine, but in consequence of the altered 
circumstances under which it is now placed, and especially with regard to the 
natui'e of the structure immediately beneath it, and having also an important part 
to play in the production of the other constituents of the tooth, its original 
■character is greatly changed. 
To the circumstance of this membrane, as an ordiuaiy inflection of the mucous 
membrane of the mouth, being covered with an ejiithelitim, which is analogous to 
the epidermis of the skin, special attention has been already called. A reference 
to figs. 4, 5, and (j in the diagram, will show that when the young tooth becomes 
sacculated, the^ac itself is merely lined with a continuation of the same membrane 
which is reflected upon the papilla. Under these circumstances the epithelium of 
the lining of the sac, and likewise that of the coveriiuj of the papilla, is changed 
into a pulpy mass, which has been called the outer or enamel jm/p, from the belief 
that the enamel was directly produced from it. These several things are rendered 
very clear in fig. G of the diagi'ani, where a represents the papilla ; n, the mem- 
brane which covers it, or rather that part of the membrane which answers to the 
true skin deprived of its epidermis : c, the altered epithelium of the membranous 
lining of the sac and the covering of the papilla, called now the outer pulp ; and 
D, the " true skin " or vascular portion of the membrane of the sac. e is the germ 
of the permanent tooth, and f the gum. These several symbols also apply to the 
