What's Happening Inside the Egg — A Day-by-Day Look at Embryo Development

The Science of Incubation — Part 5

One of the most remarkable transformations in nature happens silently inside a chicken egg over the course of just three weeks. A single cell divides and multiplies, organising itself into billions of cells that form bones, organs, blood vessels, and feathers. Understanding what happens day by day inside that egg isn't just fascinating—it's essential knowledge for anyone serious about incubation success.

In this fifth instalment of our Science of Incubation series, we're opening up the mystery of embryo development. We'll follow the chick's journey from the moment the egg is laid through to the moment it breaks free from its shell. You'll discover why certain days matter more than others, when you should be watching for problems, and how to support the process through proper incubator management.

Days 1–4: The Foundation

The first four days of incubation are when the true magic begins. Before you even place an egg in your incubator, development has already started—but it slows dramatically once the egg cools after being laid. Returning the egg to incubation temperature of 37.5°C reactivates the process, and the embryo begins its incredible transformation.

Day 1: The First Divisions

Within hours of returning to incubation temperature, the cell that will become the chick begins dividing rapidly. The blastodisc—a small disc of cells on the yolk surface that you can see with the naked eye—starts forming layers. These layers will eventually become every structure in the chick's body: the nervous system, organs, muscles, and bones.

At this stage, the embryo is microscopic and invisible to the naked eye without magnification. There's no visible movement, no heartbeat. Yet the foundation for everything to follow is being laid down at the cellular level.

Day 2: Blood Islands Appear

By day two, something extraordinary is starting to happen: blood islands are forming. These are the first structures that will become the circulatory system. The heart tube is also forming—a small channel of cells that will soon begin to beat and pump blood throughout the growing embryo.

Blood vessels begin spreading outward from these blood islands, creating networks that extend across the yolk. This vascular system is essential—it will transport oxygen and nutrients from the yolk to the developing embryo, and return waste products to be managed by the egg's environment.

Day 3: The Heart Starts Beating

One of the most exciting milestones happens on day three: the heart begins to beat. If you were to open an egg on this day and look under magnification, you'd see a tiny, flickering heartbeat. This is the moment the embryo truly becomes alive in a functional sense.

As the heart starts working, head and brain structures begin forming rapidly. The neural tube—which will become the nervous system—is developing. You might be able to see dark pigmentation forming where the eye will be, though it's still incredibly small.

Day 4: Limbs and Protection Begin

By day four, the embryo is becoming distinctly recognisable as a living creature. Limb buds—the beginnings of wings and legs—are starting to protrude from the body. Eye pigmentation becomes more visible. The amnion, a protective sac of fluid surrounding the embryo, begins to form. This sac will cushion the chick throughout development and during the hatching process.

Still, the embryo remains invisible to the naked eye. You cannot candle an egg on day four and see these details, though experienced breeders sometimes notice a faint blood network if they look very carefully.

Days 5–10: Rapid Growth

During this phase, the embryo grows at an astounding rate. The structures that began forming in days 1–4 now develop rapidly into recognisable features. The chick is no longer microscopic—by day seven, it's roughly the size of a small marble.

Days 5–6: Building a Bird-Like Form

The beak begins to harden and take shape. Wing and leg structures are developing clearly now, with distinct segments forming. The embryo is growing so quickly that you'd notice a significant size difference if you compared day five to day six.

The allantois—a membrane that functions like a lung for the developing chick—is expanding. This is crucial, as it allows the embryo to exchange gases with the air cell inside the egg. Remember from our earlier discussion on air cells in Part 2 of this series—this structure is vital for the embryo's respiration.

Day 7: The First Candling Checkpoint

Day seven is a significant milestone for breeders: it's the first reliable point at which you can candle an egg and see clear evidence of development. When you hold a candling light up to the egg, you'll see a well-developed network of blood vessels spreading across the interior. These vessels appear as dark, branching lines radiating from a darker mass—the embryo itself. You should also see the dark eye spot, which is the pigmented eye developing.

Candling on day seven allows you to identify infertile eggs or those with early developmental problems. Eggs that show no blood network by this point are unlikely to hatch and can be removed from the incubator, allowing you to focus your efforts on viable eggs.

Days 8–10: Feathering Begins

Feather follicles are now forming all over the embryo's body. The beak continues to harden. Toes and claws are developing distinctly, with individual digits becoming visible. By day ten, the embryo is unmistakably bird-shaped: you can see the head, wings, legs, and body in their proper proportions.

The yolk is being absorbed gradually, providing nutrition. The embryo is pressing against the inside of the shell membrane as it grows larger. Blood vessels are now distributed across almost the entire inner surface of the egg, creating a vast network for nutrient and gas exchange.

Days 11–17: Maturation

The rapid growth phase is slowing, but this doesn't mean development is less important. During these days, the chick is maturing and positioning itself for the journey ahead.

Days 11–14: Growing Down Feathers

Down feathers are now growing across the chick's body. These soft, fluffy feathers will keep the newly hatched chick warm. Scales are forming on the legs, and toenails are hardening. The embryo is taking up more and more space inside the egg—what was a relatively small form in a large space is now filling the shell noticeably.

The albumen (egg white) is being steadily absorbed. Remember, the albumen served two purposes during early incubation: it provided water and minerals, and it acted as a cushion. As the chick grows, it no longer needs this cushioning in the same way, and the nutrients within are being gradually incorporated into the chick's body.

Days 15–17: The Critical Turn

During these three days, something crucial happens: the chick begins to position itself for hatching. The yolk sac is beginning to be drawn into the body cavity—a process that will continue through day 20. This is an important moment, because it means the chick's nutrition is shifting from external yolk consumption to internalised reserves.

Interestingly, these are the days when the developing chick's oxygen demand is at its absolute highest. The allantois has expanded as much as it can, and the chick is consuming oxygen at maximum capacity. This is why maintaining proper incubator ventilation is absolutely critical during this period—something we explored in detail in Part 1 of this series. Even a slight drop in oxygen availability during days 15–17 can result in developmental problems or failed hatches.

The chick is also growing its beak and eye tissues to their functional state. The eye, which seemed impossibly small just days ago, is now developed and the chick will soon be able to use it.

Days 18–21: Lockdown and Hatching

We've reached the final phase of incubation. This is when your role as an incubator operator changes significantly, and the chick begins the process of emerging from its shell.

Day 18: Lockdown

Day eighteen is when you stop turning eggs—if you haven't already—and increase humidity to hatching levels. This is called "lockdown," and it marks the beginning of the end for incubation. The chick no longer needs to be rotated, because it's now positioning itself for hatching and will remain in that position until it breaks free.

The yolk sac continues to be drawn into the body cavity, and the remaining yolk provides the chick with concentrated nutrition for the final push. The air cell, which we discussed in Part 2, is now at its maximum size. This air cell will be the chick's first source of oxygen when it begins to breathe with its lungs rather than through the allantois.

Days 19–20: Internal Pip

Around day nineteen or twenty, the chick does something remarkable: it breaks through the membrane separating the air cell from the rest of the egg. This is called the internal pip. As soon as this happens, the chick's beak enters the air cell, and it takes its first breath of air using its lungs.

This moment is significant. The chick has been "breathing" through the allantois for the entire incubation period, but now it must transition to proper lung respiration. The air cell provides the oxygen it needs during the final days. You won't see any external sign of the internal pip—the chick is still inside the shell—but if you candle the egg during this time, you might see the chick's beak in the air cell.

The chick begins to pip the shell itself, making tiny holes. Each pip is a small break in the shell, made with the chick's egg tooth—a temporary point on the tip of the beak used only for this purpose. The chick rests between pips, a process that might take 12–24 hours.

Days 20–21: External Pip and Hatching

When the external pip occurs, you'll see a small hole in the shell. This hole might be barely visible at first—just a tiny star-shaped crack. Over the next 12–24 hours, the chick continues to enlarge this hole by breaking more bits of shell.

Then comes the zipping process. The chick rotates inside the shell, using its egg tooth and neck muscles to cut around the shell in a circular motion. It's creating a cap that will eventually come away, allowing the chick to push its way out. This rotating, cutting action is where the term "zipping" comes from—the chick is literally zipping itself out of its shell.

Finally, the chick pushes hard against the cap, breaking it free, and emerges wet and exhausted into the world. The entire hatching process—from the first internal pip to the moment of emergence—typically takes 12–24 hours. The newly hatched chick will spend 12–24 more hours drying and fluffing up before it's ready to leave the incubator.

Why This Knowledge Matters

Understanding these developmental stages transforms you from a passive bystander into an informed manager of your incubation process. When you know what's supposed to be happening each day, you know when something has gone wrong. You understand that:

• Candling on day seven isn't optional—it's your checkpoint for identifying problems early.
• Days 15–17 demand your absolute best ventilation management because oxygen demand is peaking.
• The transition from early growth to maturation to lockdown requires different management approaches.
• Humidity changes are timed to specific developmental phases, not arbitrary calendar points.
• A chick that hasn't internally pipped by day 20 is genuinely behind schedule and might need intervention.

This knowledge also builds respect for the process. Incubation isn't just about setting the right temperature and humidity—it's about understanding the living system developing inside the egg and supporting it through each critical phase.

Looking Ahead: Candling Techniques

Now that you understand what's happening inside the egg at each stage, you're ready to learn how to actually see these developments yourself. In Part 6 of our Science of Incubation series, we'll explore candling in detail: the best techniques, what you should see at each checkpoint, and how to identify problems before they become failures.

Candling is one of the most valuable tools in your incubation toolkit, and understanding the embryology makes candling far more meaningful. You'll know exactly what you're looking for because you'll understand what should be developing.

Questions? Let's Chat

The science of embryo development fascinates us, and we love sharing it with other poultry enthusiasts. If you have questions about any stage of embryo development, or if you're curious about how this applies to your own incubation setups, please leave a comment below. We're here to help you succeed.

Happy incubating!