Germinal spot in a fertile egg
The newly formed single cell begins to divide into 2, then 4, 8, 16, 32 and so on. At the time of laying, hundreds of cells are grouped in a small, whitish spot (the blastoderm or germinal disc) that is easily seen on the supper surface of the yolk. This spot in a fertilized, freshly laid egg in the beginning of the chick.
When the egg is laid and cools, division of the cells ceases. Cooling the egg at ordinary temperature does not result in the death of the embryo. It may resume its development after several days of rest if it is again heated by the hen or in an incubator.
Development During Incubation
As soon as the egg is heated again, the cluster of cells in the blastoderm begins to multiple by successive divisions. The first cells formed are all alike. Then, as the division of cells progresses, some differences begin to appear.
These differences become more and more pronounced. Gradually the various cells acquire specific characteristics of structure and cell grouping or layer. These cell groupings are called the ectoderm, mesoderm and endoderm. These three layers of cells constitute the materials out of which the various organs and systems of the body are to be developed.
From the ectoderm, the skin, the feathers, beak, claws, nervous system, lens and retina of the eye, and linings of the mouth and vent are developed. The mesoderm develops into the bone, muscle, blood, and the reproductive and excretory organs. The endoderm produces the linings of the digestive tract and the secretory and respiratory organs.
Development from a single cell to a pipping chick is a continuous, orderly process. It involves many changes from apparently simple to new, complex structures. From the structures arise all the organs and tissues of the living chick.