Chick embryo development — day by day
In just 21 days a fertilised chicken egg becomes a chick. We explain what happens inside the egg each day, when and how to candle eggs (checking them with a bright light in a dark room) to remove non-viable eggs, and what you can see through the shell at each stage. In other poultry species incubation takes longer, but the sequence of stages is similar.
verifiedFrom the team that has organised work on poultry farms for years.
A chicken egg placed in an incubator will — if fertilised — become a chick in 21 days. The process starts from a microscopic structure called the primitive streak and ends with a fully formed bird that breaks through the shell on its own. Understanding the stages helps the farmer know what to expect, when to act, and why candling eggs at specific moments makes sense.
How long does incubation take in different species?
For chickens approximately 21 days, for turkeys about 28 days, for ducks about 28 days (Muscovy about 35 days), for geese about 30 days. The sequence of stages — primitive streak, heartbeat, eye and limb formation, feathering, positioning for hatch — is similar across all poultry species. This guide uses chicken days as the reference. Details on incubation are in the guide on poultry egg incubation, and pre-incubation storage in storing hatching eggs.
Where do the day numbers come from?
The days given are indicative values based on poultry embryology descriptions (the Hamburger-Hamilton staging series for the chicken is the standard reference). In practice the rate may vary slightly depending on the genetic line, temperature and humidity in the incubator. Treat them as a guide, not a rigid schedule.
Chick embryo development — day by day
- 1
Days 1–3 — primitive streak, heart and first blood vessels
On the first day after setting the egg the primitive streak appears — the first outline of the embryo, visible only under a microscope. By the end of day 2–3 the heart starts beating and the first blood vessels form. Candling at this stage reveals nothing characteristic to the naked eye — it is too early to assess fertilisation. The egg looks empty.
- 2
Days 4–5 — eye and limb buds
Eye buds appear (a dark dot visible when candling) along with limb buds — the future wings and legs. The blood vessels spread and when candling you can see the characteristic "spider" — a fine network of vessels around a small dark dot. This is the first point at which you can begin to distinguish live eggs from unfertilised ones (which look like clear, transparent "blank" eggs with no spider).
- 3
Days 6–7 — first candling and removal of non-viable eggs
The embryo is growing, the dark eye dot is more prominent and the vessel network is more extensive. This is the optimal time for the first candling: in a dark room you hold a bright torch or dedicated candler against the egg and check for a visible vessel network and live embryo. Unfertilised eggs (clear, no vessels) and eggs with a dead embryo (vessels gone, the egg darkens unevenly or shows a "blood ring" — a dark circle) are removed from the incubator. More on post-incubation egg analysis in the guide on hatch analysis (breakout analysis).
- 4
Days 8–14 — feather, beak and claw formation
The embryo grows rapidly: feathers (first feather primordia appear around days 8–10), beak, claws and internal organs all form. When candling the egg becomes progressively darker — the embryo is taking up more space and the air cell (the air pocket at the blunt end of the egg, which enlarges as the egg loses moisture) becomes clearly visible. By this stage the embryo already looks recognisably like a bird.
- 5
Days 15–18 — positioning for hatch and second candling
The embryo fills most of the egg and positions itself with its head towards the air cell (blunt end). Day 18 is the time for the second (final) candling — you check that the embryo is alive and how large the air cell is (it should occupy roughly one third of the egg, indicating correct moisture loss). Eggs with a dead embryo are removed. After this candling the eggs are transferred to the hatcher — a chamber with temperature and humidity set for hatching.
- 6
Days 19–21 — membrane penetration, pip and hatch
On day 19 the embryo breaks through the membrane separating it from the air cell and breathes with its lungs for the first time. On day 20 the pip occurs — the chick breaks the shell with its beak (the first crack, called the pip). On day 21 the chick rotates inside the egg and pushes off the cap of the shell — it hatches. Do not rush this process or help the chick by force: a chick that cannot exit the shell on its own is usually not ready and intervention typically does more harm than good.
Candling eggs — how, when and what to look for
Candling is shining a bright, focused light through the egg in a dark room. It allows you to monitor development and remove non-viable eggs that could rot and contaminate the incubator.
What is egg candling?
Candling means holding an egg in front of a strong, focused light source in a darkened room. Light passes through the shell and allows you to observe the contents. Historically a candle was used (hence the name candling); today a strong torch or a dedicated candler is used. The smaller and smoother the egg, the easier it is to candle. Brown and thick shells transmit less light.
First candling — day 6–7
In a dark room hold the candler against the blunt or side of the egg. You should see a network of red blood vessels radiating from a dark embryo dot — a good sign, the egg is alive. An unfertilised (clear) egg is bright, almost transparent, with no vessels. An egg with a dead embryo may show a "blood ring" — a dark circle without living vessels — or may be dark inside without a clear network. Remove those eggs — a rotten egg can explode and contaminate the others.
Second candling — day 18
Three days before the expected hatch you transfer eggs to the hatcher, but first you perform the second candling. At this stage the embryo fills almost the whole egg — you mainly see a large dark mass and a clear air cell at the blunt end. You check whether the air cell is large enough (too small = too little moisture loss = too high humidity in the incubator) and whether the embryo is alive (visible movement or a dark mass pressing against the membrane). Remove dead eggs and transfer the rest to the hatcher.
How to recognise a live embryo, an unfertilised egg and a dead embryo
At every candling you compare three types of egg. Below are the characteristic features of each.
Live embryo
Day 6–7: visible network of red vessels, dark embryo dot, vessels radiating like a spider. Day 18: large dark mass (embryo), clear air cell at the blunt end, no transparency in the rest of the egg. Possible faint movement of the embryo under strong light.
Unfertilised egg
Throughout incubation an unfertilised (clear) egg is bright, almost transparent, with no vessels at all. You can see the yolk as a darker shadow in the centre and nothing else. You can safely remove such an egg at any candling.
Egg with a dead embryo
If the embryo died at an early stage (before day 7): a visible blood ring — a dark circle without living vessels, the egg noticeably brighter than a live one. If it died later: the egg is dark but without a clear air cell or with an unusually small one; no movement; may give off an unpleasant smell. Remove such eggs — there is a risk they will rot and contaminate the remaining eggs.
Common mistakes when following embryo development
A few things farmers do incorrectly that can harm the hatch or give a false picture of development.
Candling too early (days 1–3)
In the first few days candling reveals nothing visible to the naked eye — the embryo is microscopic. Removing eggs too often and too early needlessly chills them and exposes them to knocks. Perform the first candling no earlier than day 5–6, optimally on day 6–7.
Helping the chick out of the shell
When pipping takes a long time or the chick looks exhausted, the temptation to help is strong. However, opening the shell for the chick usually does more harm than good: the blood vessels in the membrane are still active at this stage and the chick may bleed if the intervention is not careful. The exception is when the chick is clearly stuck for many hours — in that case consult an experienced farmer or a vet.
Ignoring humidity and air cell size
The air cell grows as the egg loses moisture through the shell. If it is too small, humidity in the incubator was too high — the chick may have difficulty hatching. Checking the air cell at the second candling (day 18) lets you assess whether humidity was correct throughout incubation. The structure of the egg, including the air cell, is described in the guide on the structure of a chicken egg.
Leaving dead eggs in the incubator
An egg with a dead embryo or an unfertilised egg can rot over time and explode inside the incubator, contaminating the remaining eggs with bacteria. At every candling remove all dead and unfertilised eggs. If in doubt — it is better to remove one live egg by mistake than to leave one dead one.
Frequently asked questions about embryo development in the egg
On which day does the heart start beating in a chicken embryo?add
The heart of a chicken embryo starts beating approximately between day 2 and day 3 of incubation. It is one of the first signs of life, though it is not visible to the naked eye through the shell. When candling at this point there is nothing to see yet — blood vessels and the embryo dot only become clearly visible around day 5–7.
When is the best time to candle eggs?add
Optimally twice: the first candling on day 6–7 (remove unfertilised eggs and those with dead embryos) and the second candling on day 18 (before transferring eggs to the hatcher, check the air cell and embryo viability). Candling earlier (days 1–3) gives no information because nothing is visible yet. Candling every few days is possible but unnecessary and exposes eggs to chilling.
What is the air cell in an egg and why should you monitor it?add
The air cell is an air pocket at the blunt end of the egg, visible when candling as a bright area. It grows during incubation as the egg gradually loses moisture through pores in the shell. By day 18 it should occupy roughly one third of the egg. If it is too small — humidity in the incubator was too high. If too large — too low. The chick breathes from the air cell in the last days before hatching, so its size affects the success of the hatch.
Why should you not help the chick out of the shell?add
The effort of breaking out of the shell is natural and necessary for the chick — it strengthens the muscles. More importantly, the membrane lining the shell is still filled with blood vessels at this stage. If the chick has not fully absorbed them (visible as still-red vessels) and you open the shell, the chick may bleed out. Intervention makes sense only in exceptional situations — after a very long lack of progress (many hours) and ideally after consulting an experienced farmer.
What is a blood ring when candling an egg?add
A blood ring is a dark circle visible when candling an egg, most often between days 6 and 14. It appears when the embryo has died at an early stage: blood vessels have receded or disappeared and their trace in the membrane forms a dark ring. An egg with a blood ring is dead and should be removed from the incubator.
Are the embryo development stages the same in ducks, turkeys and geese as in chickens?add
The sequence of stages is similar — primitive streak, heartbeat, eye and limb formation, feathering, positioning for hatch, pip and hatch. What differs is the incubation length: turkey and duck about 28 days, goose about 30 days, Muscovy duck about 35 days. Candling windows shift proportionally — first candling typically around day 7–10, second candling 3 days before the expected hatch.
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