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AMELIFICATION. 597
face ; the calcified product occupies the position of the stratum granulo-
sum, or what I term the older layer of cells. Upon the surface of the
prismatic layer we find a layer of hornified or corneous cells, and, under-
neath the calcified tissue, the formative layer—viz. the rete ]Malpighii, or
in/ant layer of cells.
It is well known that this infant layer, or rete jNIalpighii, consists
of nucleated structures which lie in a bed of protoplasm. This layer
of protoplasm suiTounds and bathes the cells of the older layer to a
certain extent, growing less and less in (piantity as we approach the
surface. This protoplasmic substance does not differ in character
from that which surrounds the cells of the connective-tissue grou]).
So far as we know, the two fluids are identical : salts of calcium
enter into chemical combination with protoplasm in the latter case,
and why not in the former?
The first-formed layer of the shell of the mollusk lies in the proto-
plasm which bathos the formative, or infant, layer of cells of the rete
JNIalpighii. These cells are the active agents in its deposit ; they have
become specialized and endowed with new functional power that they
may superintend the deposition of the salts of calcium \\hich enter into
the composition of the shell. Their office is identical with that of the
osteoblast.
The «dcium salts are shed out from the ends of the cell as from the
surface of a membrane—which, indeed, they form. They do not
become individually encapsuled, as do the osteoblasts, but the body
of the Pinna, covered externally ^vith epithelium which remains as
the lining membrane of the shell, becomes enclosed by the shell, and
thus the infant layer may be said to be encapsuled.
The thickening of the shell is at the expense of the size of the body
of the Pinna, just as the thickening of the wall of the calcospherule is
at the expense of the size of the osteoblast. If we decalcify a shell and
make sections, we find a matrix which differs only in form from that
found in bone. The prismatic layer of the rete Malpighii has laid
down the calcified products in the form of prisms, and cross-sections
of the decalcified product will reveal their form just as well as ground-
sections. Cross-sections will present the appearance of a honeycomb
from which the honey has been extracted : the extracted honey com-
pares to the salts of calcium, which before decalcification existed as
prisms, and occupied, as did the fluid honey, the cells of the.comb.
In the development of the shell the protoplasm shed out between the
prismatic cells of the infant layer is lin\ited in quantity. The secreted
salts of calcium which are thrown out bi/ the cells enter into chemi-
cal combination with the peripheral layer of protoplasm and form
calcoglobulin, which, as we have before shown, is insoluljle in acids.
The sheath which surrounds the prism, as the protoplasm does the
prismatic cell, may be compared to the wax in the cell of the honey-
comb. Regarding the sheath, and the material which enters into its
structure. Dr. Carpenter says
" It sometimes happens in recent, but still more commonly in fossil,
shells that the decay of the animal membrane leaves the contained prisms
without any connecting medium. As they are then quite isolated, they
AMELIFICATION. 597
face ; the calcified product occupies the position of the stratum granulo-
sum, or what I term the older layer of cells. Upon the surface of the
prismatic layer we find a layer of hornified or corneous cells, and, under-
neath the calcified tissue, the formative layer—viz. the rete ]Malpighii, or
in/ant layer of cells.
It is well known that this infant layer, or rete jNIalpighii, consists
of nucleated structures which lie in a bed of protoplasm. This layer
of protoplasm suiTounds and bathes the cells of the older layer to a
certain extent, growing less and less in (piantity as we approach the
surface. This protoplasmic substance does not differ in character
from that which surrounds the cells of the connective-tissue grou]).
So far as we know, the two fluids are identical : salts of calcium
enter into chemical combination with protoplasm in the latter case,
and why not in the former?
The first-formed layer of the shell of the mollusk lies in the proto-
plasm which bathos the formative, or infant, layer of cells of the rete
JNIalpighii. These cells are the active agents in its deposit ; they have
become specialized and endowed with new functional power that they
may superintend the deposition of the salts of calcium \\hich enter into
the composition of the shell. Their office is identical with that of the
osteoblast.
The «dcium salts are shed out from the ends of the cell as from the
surface of a membrane—which, indeed, they form. They do not
become individually encapsuled, as do the osteoblasts, but the body
of the Pinna, covered externally ^vith epithelium which remains as
the lining membrane of the shell, becomes enclosed by the shell, and
thus the infant layer may be said to be encapsuled.
The thickening of the shell is at the expense of the size of the body
of the Pinna, just as the thickening of the wall of the calcospherule is
at the expense of the size of the osteoblast. If we decalcify a shell and
make sections, we find a matrix which differs only in form from that
found in bone. The prismatic layer of the rete Malpighii has laid
down the calcified products in the form of prisms, and cross-sections
of the decalcified product will reveal their form just as well as ground-
sections. Cross-sections will present the appearance of a honeycomb
from which the honey has been extracted : the extracted honey com-
pares to the salts of calcium, which before decalcification existed as
prisms, and occupied, as did the fluid honey, the cells of the.comb.
In the development of the shell the protoplasm shed out between the
prismatic cells of the infant layer is lin\ited in quantity. The secreted
salts of calcium which are thrown out bi/ the cells enter into chemi-
cal combination with the peripheral layer of protoplasm and form
calcoglobulin, which, as we have before shown, is insoluljle in acids.
The sheath which surrounds the prism, as the protoplasm does the
prismatic cell, may be compared to the wax in the cell of the honey-
comb. Regarding the sheath, and the material which enters into its
structure. Dr. Carpenter says
" It sometimes happens in recent, but still more commonly in fossil,
shells that the decay of the animal membrane leaves the contained prisms
without any connecting medium. As they are then quite isolated, they
