Miracle of Life:

Formation of the Egg

The egg is a delicate organism and it is assembled in a precise order. First, the yolk and ovum are produced. They develop in the ovary, a grape like cluster of ovules or miniature yolks located in the back of the hen. The yolk remains in the ovaries until it is ripe. When it is mature, the yolk drops into the funnel section (infundibulum) of the oviduct. Here the ovum is fertilized by the male sperm cell. As the egg passes to the magnum, a thin layer of albumen (egg white) is placed around the yolk in the form of a thin sack. At either end of the yolk, thin strands of albumen are twisted to form the chalaza. The function of the chalaza cord is to hold the yolk in place as it travels the length of the oviduct, and to prevent it from rising and bruising itself on the shell membranes. It was thought the chalaza affected the fertility of the egg, this is not so. Its sole purpose is to hold the yolk in the proper position. Before the yolk leaves the magnum the majority of the albumen is wrapped around the yolk. The egg then enters the isthmus where two shell membranes grow around the yolk and albumen. These membranes are loose fitting at this point. Final egg development occurs in the uterus. Actually, about 80% of the time it takes to form an egg is spent in the uterus. The uterus is often referred to as the shell gland for here the inner shell membrane is made tight and the outer, harder shell is produced. The egg moves through the vagina to the cloaca and is finally expelled. The whole egg formation process takes a little more than 24 hours.

Glossary:

• Albumen: The white of the egg.
• Amnion: A thin protective membrane forming a sac around the embryo.
  
• Chaloza: A spiral band of thickened albuminous substances in the white of the hen's egg, extending from the chalazifeous layer toward each end of the egg.
• Cloaca: The common cavity in birds into which the oviduct, the vasadeferentia, and the urinary and digestive tracts open.
• Embryo: An organism in the early stages of development; a bird prior to hatching.
• Fertilization: Union of the sperm and the egg (ovum).
  
• Gene: The basic unit of inheritance in the germ plasm; a hereditary factor.
  
• Hormone: A chemical secretion from a ductless or endocrine gland which stimulates or inhibits some organ of he body. Hormones are transported by the blood.
  
• Isthmus: The portion of the oviduct where the egg shell membranes are laid down.
  
• Magnum: The section of the oviduct where the egg shell membranes are laid down.
  
• Ovary: The primary reproductive organ of the female; n domestic fowl normally only the left ovary develops.
  
• Oviduct: The tube through which theyolk passes to receive additions of egg white, shell membranes, and shell.
  
• Ovipostion: The act of laying an egg.
  
• Ovulation: The release of the ovum from the ovary.
  
• Ovum: A nucleated cell formed in the ovary. In reference to the hen, the term usually applies to the cell product of the ovary along with a much larger mass of nutrients which comprise the yolk of the egg.
  
• Semen: Fluid produced in the male reproductive organs that contains the spermatozoa.
  
• Spermatozoa: Mature reproductive cells of the male; sperms.
  
• Uterus: The section of the oviduct where the egg shell is formed.
  
• Vagina: The section of the oviduct connecting the uterus and the cloaca.
  
• Vitelline Membrane: A very thin membrane enclosing the yolk.
  
• Yolk Sac: A more or less sperical sac attached to an embryo and enclosing the food yolk.
  

Events in Embryonic Development

Days	Growth and Development
Before egg laying:	Fertilization Division and growth of living cells Segregation of cells into groups of special function (tissues)
Between laying and incubation:	No growth; stage of inactive embryonic life
During Incubation: First Day:	First sign of resemblance to a chick embryo. Appearance of alimentary tract. Appearance of vertebral column.
Second Day:	Beginning of formation of nervous system. Beginning of formation of head, eyes, heart, ears. Heart begins to beat. Beginning of formation of nose, legs, wings.
Fourth Day:	Formation of reproductive organs and differentiation of sex.
Eighth Day:	Beginning formation of the beak and feathers.
Tenth Day:	Beginning hardening of the beak. Appearance of scales and claws
Eleventh Day:	Embryo gets postion suitable for breaking the shell. Beak becoming firm and horny.
Twelveth Day:	Beak turns toward air cell. Yolk sac begins to enter body cavity.
Thirteenth Day:	Yolk sac completely drawn into body cavity. Embryo occupies practically all the space within the egg except the air cell.
Fourteenth Day:	Hatching of the chick.

Back to Top of Page
To Article Page
To Home Page