Incubation and Embryology - University of Illinois

Candling Eggs

Eggs are candled to determine the condition of the air cell, yolk, and white. Candling detects bloody whites, blood spots, or meat spots, and enables observation of germ development. Candling is done in a darkened room with the egg held before a light. The light penetrates the egg and makes it possible to observe the inside of the egg.

The candler should be set on a box or table at a convenient height (about 38 to 44 inches from the floor), so the light will not shine directly into the eyes of the operator. In candling, the egg is held in a slanting position with the large end against the hole in the candler. The egg is grasped by the small end and, while held between the thumb and tips of the first two fingers, is turned quickly to the right or left. This moves the contents of the egg and throws the yolk nearer the shell. Because of the color of their shells, brown eggs are more difficult to candle than white eggs.

To do a reasonable job, an extensive knowledge of candling is not necessary, particularly if the eggs are all relatively fresh. One should be able to distinguish a fresh egg from a stale egg and detect such abnormalities as bloody whites, blood spots, meat spots, and cracked shells. In a fresh egg, the air space is plainly visible and moves freely. The white is thin and clear. In a stale egg, the air space is plainly visible and moves freely. The white is thin.

Most newly laid eggs are good quality. Eggs not over two or three days old, if held under good conditions, will meet the specifications for Grade A. The only eggs to be removed by candling are those with bloody whites, blood or meat spots, and cracked shells.

Candling Incubated Eggs

Incubated eggs are candled to determine whether they are fertile and, if fertile, to check the growth and development of the embryo. White eggs should be tested for fertility on the third day. Brown shelled eggs on the fifth or sixth day because it is difficult to see the embryo clearly before this time.

A small reddish area with blood vessels extending away from it will be visible in fertile eggs. This is the embryo floating around inside the egg, looking like a huge red spider. If the embryo dies, the blood draws away from the embryo and forms what is called a blood ring. All clear eggs and eggs showing blood rings or streaks should be removed from the incubator. If eggs are not candled during the early stages of incubation, it will be difficult to determine whether the egg was fertile; embryos that die early soon decompose and are not easily distinguished from rotten eggs.

Candle the eggs every few days to observe the growth and development of the embryo. Record findings on the Candling Chart in My 4-H Record.

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