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:

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.


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