the succeeding row, the papillæ are larger, the cone broader and
flatter, and its apex is covered with a small cap of dense and glistening
dental substance which is readily removed ; though not without
displacement of part of the pulp, the granules of which, adherent
to the cavity of the displaced dental cap, are always readily recognizable
under the microscope. The third series of papillæ, counting
from below in the lower jaw, have acquired the size and
shape of the future tooth, with the crenate edges well marked;
half the tooth is completed, and its removal from the fleshy base
of the pulp cannot be effected without evident laceration of the
pulp ; when this is done under the microscope, the torn processes of
the pulp continued into the medullary canals of the new formed
tooth are plainly visible. The fourth tooth is completely formed,
as also the fifth and sixth, in the ascending series ; these progressively
diminish in size. The last or highest, which is first exposed on reflecting
the thecal fold, and the first which was completed in the
order of development, consists of a simple cone, similar in form and
size to the apical third of the ordinary sized teeth below it ; yet
its growth is quite completed, and its base firmly attached to the
maxillary membrane.
In a foetus of a Carcharias, three inches long, which had not lost
its external branchiae, the membranous groove between the jaw and
thecal fold was much shallower, and only two rows of papillæ were
present on the maxillary membrane. The minute anterior teeth in
the more advanced foetus are doubtless developed from these primitive
papillæ, which must be succeeded by others of progressively
larger size till the normal form and dimensions of the adult teeth
are attained.(1 )
The unossified pulps, examined with a high power, consist of semiopaque
polyhedral granules or cells suspended in a clear matrix, and
the whole is inclosed in a tough transparent membrane which forms
the outer surface of the pulp. Beneath this membrane at the crenate
margins, the granules or cells are arranged in lines precisely correspond- 1
(1) The foetal shark is peculiarly favourable for such comparisons, as it presents numerous
pulps and teeth in every stage of formation, easily detached and without violence from their
exposed situation, and of a flattened form well adapted for microscopical observation.
ing with those of the subsequent calcigerous tubes. The formation of
I the tooth commences by the deposition of earthy particles in the tough
I external membrane of the pulp. I have been unable to recognize the
I distinct arrangement of the hardening salts in this layer. It is trans- I parent, extremely dense, and forms the enamel-like polished coating
I of the tooth ; in sections of fully formed teeth, the finest terminal
branches of the parallel peripheral calcigerous tubes are lost in the
above clear enamel-like substance. When the enamel-like outer
layer of the apex of the tooth is completed, it is so easily detached
from the subjacent pulp that it might be readily supposed that there
was no organic connection between them. If, however, the so exposed
pulp be now examined with the microscope, and compared with
an uncalcified pulp, it is seen to be no longer covered with the
smooth dense membrane observable in the latter ; but the apical edges,
■ from which the enamel-like cap has been detached, appear villous
lor flocular. It is obvious that the first shell of the tooth has been
■ neither transuded from the superficies of the external membrane of
■ the pulp nor has been deposited between that membrane and the
■ granular part of the pulp, but is due to a conversion of the external
■ membrane into a dense enamel-like bone. The formation of the body
lof the tooth by deposition of earthy particles in pre-existing and
jpre-arranged cavities is still more satisfactorily demonstrable. In
jproportion as the formation of the tooth has advanced, the difficulty
of separating the calcified from the uncalcified portion of the
jpulp is increased, and at the same time it becomes easier to detect
the continuation of the processes of the pulp into those medullary
■ canals which form so many separate centres of radiation of the plexi-
form calcigerous tubes.
The application of the principle of dental development by con-
Iversion and not by transudation, as illustrated in the dentition of
fthe shark, to the formation of the mammiferous tooth, is by no means
■ forced, but natural and obvious. In the ivory part of a simple mam-
jmiferous tooth there is a single medullary canal, the cavitas pulpi, and
a single system of radiated calcigerous tubes ; but the plan and prin-
jciple of formation is the same as in the shark.
In proportion to the quantity of earthy matter deposited in the