The next stage in the formation of the egg is
the laying down of two shell membranes, an
inner one and an outer one, around the
albumen. The membranes are formed in the
thin, tubular isthmus. When the membranes
are first laid down, they cover the albumen
tightly, but they soon stretch. By the time the
egg enters the shell gland they fit quite
loosely.
The egg passes the next five hours in the
process known as "plumping." This entails the
entry of water and salts through the
membranes until the egg is swollen. The
plumping period appears to be an essential
preliminary to the main process of shell
calcification, which occupies the next 15 to 16
hours.
The shell is composed of calcite, which is
one of the crystalline forms of calcium
carbonate. A sparse matrix of protein runs
through the crystals of the shell. The final
stage in the formation of the egg is the
deposition of a cuticle on the fully calcified
shell; this is accomplished just before the egg
is laid.
Let us now look at the structure of the
eggshell in rather more detail. The shell is
attached to the outer membrane by
hemispherical structures known as mam
millary knobs. Histochemical studies have
shown that the cores of the knobs consist of a
protein-mucopolysaccharide complex rich in
acid groups, and that anchoring fibers run
from the outer membrane into the knobs.
The cores of the mammillary knobs are laid
down as the membrane-covered egg passes
through the part of the oviduct called the
isthmo-uterine junction; it is between the
isthmus and the shell gland. It seems probable
that the knobs are calcified soon after they are
formed, before the egg enters the shell gland,
and that they subsequently act as nuclei for the
growth of the calcite crystals comprising the
shell. Modern ideas on the mechanism of
biological calcification—whether in bones,
teeth, eggshells or any of the other places
where calcium is deposited in animal bodies—
emphasize the importance of crystal growth.
Earlier theories seeking to explain the
mechanism laid much stress on the role of
precipitation of calcium salts from
supersaturated solutions, but in the light of
more recent evidence this concept no longer
seems valid.
The mechanism whereby the mammillary
knobs are calcified is not well understood. It is
thought to involve the binding of calcium ions
to the organic cores of the knobs by means of
the sul-fonic acid groups on the acidmucopoly-
saccharide-protein material of
which the cores are composed. It is suggested
that the spatial arrangement of the bound
calcium ions is the same as it is in the lattice of
the calcite crystal, so that these oriented
calcium ions act as seeds or nuclei for the
growth of calcite crystals forming the shell.
Some years ago my colleagues and I found
that the isthmus contains extremely high
concentrations of both calcium and citric acid,
the former reaching a maximum of about 90
milligrams per 100 grams of fresh tissue and
the latter about 360 milligrams. We concluded
that the high level of calcium in this region
may be of significance in the calcification of
the mammillary knobs.
The main part of the shell was once known as
the spongy layer but has more recently come
to be called the palisade layer. It is composed
of columns of tightly packed calcite crystals;
the columns extend from the mammillary
knobs to the cuticle. Occasional pores run up
between the crystals from spaces formed
where groups of knobs come together. The
pores reach the surface in small depressions
that are just visible to the unaided eye on the
outside of the shell. It is through these pores
that the embryo takes in oxygen and gives out
carbon dioxide during the incubation of the
egg.