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Birth and Development

The egg cell of a bird is really what we call the yolk. In the chicken's egg this yellow ball of yolk is about 20 mm in diameter. The yolk is the food supply for the developing embryo. The nucleus of the bird's egg can be found in the middle of a white spot which is found on top of the yolk. You need a microscope to see the nucleus itself but the white spot is visible to the eye.

The egg that a hen (any female bird, not just chickens) lays is covered in layers of albumen (a material like jelly), two membranes and a hard shell. The shell and the membranes protect and support the egg. The albumen feeds the developing embryo with water and protein. The albumen, membranes and shell are added to the yolk layer-by-layer as it passes down a tube called the oviduct inside the hen's body.

The oviduct carries the egg from the ovary out of the hen's body. The last layer to be added is the shell. If a bird's egg is to be fertilized, mating must take place so that the egg will meet a sperm cell in the oviduct before these layers are added.

A chart Showing the Development of a Chick Inside the Egg of a Hen

Time line of chick development
0 hours 20 hours 24 hours 33 hours 40 hours 48 hours 5 days 7 days 20 days 21 days
FERTILIZATION The nervous system begins to develop THE EGG IS LAID The brain starts to form. The heart begins to beat. hours A blood supply grows out of the embryo to cover the yolk so that the embryo can feed and breathe. The wings and legs start to develop. The embryo is complete but only 2 cm long. The chick breaks into the air space at the end of the egg.it starts to breathes. The chick calls to its mother from inside the egg. The chick uses its egg tooth to make a hole in the shell. The chick will now be cared for and protected by its parents until it is old enough to care for itself.