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596 DESTAL EMBRYOLOGY AND HISTOLOGY.

332) to be themselves composed of a very homogeneous substance, but
to be separated by definite and strongly-marked lines of division. When
such a lamina is 'submitted to the action of dilute acid, so as to dissolve
awav the carbonate of lime, a tolerably firm and consistent membrane

Fig. 332.

'J

Section of Shell oi Finnn, taken transversely Membranous Basis of Shell of Pinna.
to the direction of its prisms.
is left, wliich exhibits the prismatic structure just as perfectly as did the
original shell (Fig. 333), its hexagonal division bearing a strong resem-
blance to the walls of the cells of the pith or bark of a plant." The
shell of the Pinna and other species of the mollusk family is a
structure analogous to enamel. Both are calcified products of the epi-
blast. We see variations in the form of the products of the connective-
tissue grou])—as, for instance, in bone—and we also find variations in
calcified epithelial structures. We have seen, in the calcification of
bone, that cells do not hcemnc calcified, but simply superintend calcifica-
tion ; and I think we shall be able to show that the same rule holds
good for the calcified products of the epiblast.
As we have already said, salts of calcium enter into chemical com-
bination with proteids, and thus form a new group of products called by
j\Ir. Rainey c((fcor/Iohu/in. Tliis modified form of albumen is insoluble
in acids ; therefi)re wc liave an organic matrix left behind after decalci-
fving the several varieties of bone. The form of the b(me is seen in the
decalcified material. This . of sliells, but not in enamel. There must, then, be some reason for its
existence in the one case, and not in the other.
I hold, with Drs. Carpenter and Huxley, that all calcified products
are excreted, and that there is not an actual conversion of living cells
into calcified ti.-^sues. In some instances the cells become encapsuled,
l)ut lime salts are not deposited in the body of the cell ; in other words,
calcification by conversion cannot be demoui^trated. In the formation
of bone variations occur due to the jiosition and matrix in which the
dc])osition takes j^lace ; this is also the case with the products of the
epiblast. The shell of the Pinna is excreted, or shed xmt, upon the sur-