b. Oviduct. At the time of embryonic development, the oviduct is a paired
structure. As the chick matures, the right ovary and oviduct fail to develop, and diminish
in size until they disappear by the time the hen reaches maturity. The left oviduct
functions to complete the development of the egg and provides a channel to convey the
yolk to the exterior. It also stores sperm cells prior to fertilization of the germ spot. The
oviduct can be divided into five areas:
(1) Infundibulum. This is the funnel-shaped structure that receives the yolk
after it is released from the ovary. Although the main function of the infundibulum is to
pick up the yolk, it also serves as a reservoir for male sperm that, if present, fertilize the
germ and set up embryonic growth.
(2) Magnum. When the yolk is released, peristalsis (wavelike movements)
of the infundibulum conveys the yolk into the magnum, which is commonly referred to
as the front, or anterior, portion of the oviduct. The front part of the magnum secretes a
dense, plastic-like gel made up of threadlike strands. This gel is the chalaziferous layer.
As the egg travels along the oviduct, it rotates, causing a rope-like twisting of the
threads, which results in the chalaza in the completed egg. The chalaza holds the yolk
in the center of the egg and ensures that the germ spot (germinal disc) remains up
when the egg is being incubated. The dense, thick white (albumen) is secreted in the
last part of the magnum, and the chalaza and thick white are interwoven to provide a
firm support for the yolk.
(3) Isthmus. The isthmus is a constricted area of the oviduct through which
the developing egg passes. In the isthmus, the inner and outer shell membranes are
developed around the dense white. The outer shell membrane is the thickest and
conforms to the shape of the egg. The shell membranes are formed before the thin
white is secreted.
(4) Uterus. The uterus is a heavy-walled part of the oviduct and supplies
the final complement of white and minerals (which pass through the shell membranes
by osmotic pressure); then the shell, shell pigment, and cuticles are added.
(5) Vagina. This portion of the oviduct holds the completely formed egg,
allowing the exterior shell to dry and become hard before the egg is released. The
vagina is joined to the cloaca, the opening through which the egg is laid.
PHYSICAL STRUCTURE OF THE EGG
The physical structure of the egg includes yolk, white, shell membranes, and the