Page 588 - My FlipBook

598 DENTAL EMBRYOLOGY AND HISTOLOGY

can be readily detached one from another, and each may be seen to be
marked by striations. By making ground-sections of the shell per-
pendicularly to its surface we obtain a view of the prisms cut in the
direction of their length (Fig. 334), and they are frequently seen to be
marked by delicate transverse strise closely resembling those observ-
able on the prisms of the enamel of teeth, to which this kind of shell-
structure may be considered as bearing a very close resemblance except
as rciiards the mineral izing; ingredients. If a similar section be decalci-
fied by dilute acids, the membranous residuum " (the interprismatic
cement-substance, which by its insolubility in acids shows it to be per-
haps identical with ]Mr. Rainey's calcoglobulin) " will exhibit the same
resemblance to the walls of the prismatic cells viewed longitudinally,
and will be seen to be more or less regularly marked by the transverse
stria just alluded to." (See Fig. 335.)
Fig. 335.

Fig. 334.

Sectiim of Shell of Pinnn in the direction of Oblique Section of Prismatic Shell-
its prisms. substance.
'' These appearances seem best accounted for by supposing that each
(})risni) is lengthened by successive additions at its base.
" This ' })rismatic ' arrangement of the carbonate of lime in the shells
of Pinna and its allies has been long familiar to conchologists and
regarded by them as the result of crystallization. When it was first
more minutely invested by Mr. Bowesbank and the author, and was
shown to be connected with a similar arrangement in the membranous
residuum left after the decalcification of the shell-substance by acid,
microscopists generally agreed to regard it as a calcified epidermis, the
long prismatic cells being sujiposed to be formed by the coalescence of
the epidermic cells in files and giving their shape to the deposit of car-
bonate of lime formed witliin them. The progress of inquiry, however,
has led to an important modification of this interpretation, the author
being now disposed to agree witii Prof Huxley in the belief that the
entire thickness of the shell is formed as an excretion from the surface