Egg formation

• The hen’s reproductive system is a very complex system that can produce an egg in 24 hours.
• An egg consists of the yolk (30 – 33%), albumen (~ 60%), and shell (9 – 12%).
• The formation of an egg occurs in the ovary and oviduct. Although two sets of ovaries and oviducts are present during embryonic development, only the left set fully develop in chickens.
• The mature ovary will have several follicles in different development stages at any one time .
• The largest follicle is the one to be ovulated to produce an egg. The ovary will normally produce one mature yolk on a 24h light/dark cycle.
• The follicular membrane ruptures and releases the follicle at the time of ovulation. This usually occurs about 30 minutes after the hen has laid an egg.
• After ovulation, the yolk is captured by the infundibulum. The formation of the chalazae and perivitelline membrane occurs in this section of the oviduct.
• The activated enzyme, in conjunction with the rotation of the egg as it moves down the oviduct, causes the albumen to separate and twist at the poles, forming the chalazae.
• It will take approximately 15 minutes for the yolk to pass through the infundibulum. Fertilization occurs in this region of the oviduct in breeding birds.
• The albumen, or egg white protein, is deposited when the yolk is in the magnum.
• The magnum is the longest part of the oviduct and the yolk will take 2-3 hours to pass through the magnum.
• Next, the developing egg passes into the isthmus, where the inner and outer shell membranes are formed. This process takes approximately 1½ hours.
• The egg then enters the shell gland or uterus, where it will spend 18 to 21 hours.
• During that time the albumen takes up electrolytes and water through a process called “plumping”.
• The shell, which consists of roughly 95% calcium carbonate and 5% organic material, is also formed in the shell gland.
• The hen’s calcium requirements are highest at this stage of egg formation. Once the shell is completely formed, a protective coating called the cuticle is laid down over the shell.
• The laying of the egg, or oviposition, is induced by hormonal contractions of the uterus.
• The egg passes through the vagina and exits via the cloaca.