What Goes on in a Chicken Egg

By Julie Gauthier, Rob Ludlow

A chicken egg is a complete package of nutrition and protection for the developing embryo chick. The yolk, egg white, and shell provide all the nutrients the embryo needs for the 21-day incubation period.

The embryo develops from the ovum, a small white dot on the surface of the yolk, which contains half the genes of a new chick. The other half of the new chick’s genes comes from one of a rooster’s sperm cells, which fertilized the ovum inside the hen’s oviduct.

By the time the egg is laid the day after fertilization, the embryo has already been busy, dividing from that single-cell ovum into a blob of several thousand cells (although the embryo is still too small for you to see without a microscope).

After the egg emerges from the nice warm hen into a cold world (below 68 degrees F/20 degrees C), the embryo stops developing, unless the egg is rewarmed by a sitting hen or an artificial incubator.

The ideal incubation temperature for chicken eggs is 99–100 degrees F (37–38 degrees C). At room temperature (actually, between 68 degrees F [13 degrees C] and ideal incubation temperature), the embryo can start to develop, but probably won’t survive to hatch. The best temperature range for storing hatching eggs is between 55–68 degrees F (13–20 degrees C); the embryo is in suspended animation at those temperatures.

During incubation, the healthy embryo develops in a predictable way. The developmental milestones of a developing chicken embryo are listed in the table.

What to Expect When You’re Incubating
Stage of Incubation Developmental Milestone
Day 0 Sperm fertilizes ovum in the oviduct of the hen.
Days 1–3 Development begins for head, eyes, ears, and backbone; heart
begins to beat.
Days 4–6 Development begins for eye pigmentation, reproductive organs,
and beak.
Days 7–9 Development begins for feathers and toes.
Day 12 Down feathers are visible.
Day 17 Chick tucks her head under her right wing.
Day 19 The yolk sac is drawn in to the abdomen.
Day 20 Chick chips a hole (pips) through the shell.
Day 21 Chick hatches.