calcium carbonate, a chemical derived from food containing oyster shells, mineral
supplements, and limestone, to form the hard, outer shell. This shell is quite porous,
containing 3000 to 8000 pores, with most of them in the large end. The cuticle, a waxy,
mucous-like substance secreted by the uterus, forms a protective covering for the egg
and prevents or inhibits the loss of moisture and gases. The developing egg spends
about 21 hours in the uterus.
(5) Moving finally into the vagina, the fully formed egg enters the cloaca and
the vent, and is laid. The entire time from ovulation to laying is usually slightly more
than 24 hours. As the contents of the egg cool, immediately following laying, an air cell
forms between the two shell membranes, usually at the large end of the egg. About 1/2
hour after a hen has laid an egg, she releases another yolk (ovulation), and it will
likewise travel the length of the oviduct.
FORMATION OF THE SHELL MEMBRANES
The shell membranes (see figure 1-3) are added as the partly formed egg enters
the isthmus. The shell membranes are tough and fibrous and are composed chiefly of
protein, similar in nature to that in hair and feathers. The inner membrane is thinner
than the outer and together they are only about twenty-four ten-thousandths of an inch
FORMATION OF THE SHELL
a. The shell (see figure 1-3) is formed in the uterus. It constitutes approximately
11 percent of the egg and is composed of three layers:
(1) Mammillary or inner layer consisting of calcite crystals over the surface
of the outer shell membrane in knoblike formations set perpendicular to the surface of
(2) Spongy layer consisting of small calcite crystals that are not arranged in
any order except in the outer portion of the layer where crystals are set at right angles to
the shell surface.
(3) Cuticle which is sometimes erroneously referred to as "bloom" and
which is of a chemical composition similar to the shell membrane.
b. Calcium carbonate comprises about 94 percent of the dry shell. A hen may
use as much as 47 percent of her skeletal calcium for egg shell formation.
c. Pores are formed through the spongy layer connecting some of the space
between the knoblike mammilla with the surface. When the egg is laid, the pores are
filled by the matrix material and covered by the cuticle.