1. Why Scientists Divide the 21 Days of Egg Development by Shape, Not Just by Days
A chicken egg hatches after being warmed for 21 days under good conditions. If you include the time from when the egg begins to form until it is laid (about 1 day inside the oviduct), the total time to hatching is about 22 days [1].
Development does not progress strictly by the number of days. It can be a little faster or slower depending on temperature and the specific egg. Because of this, researchers use a system of 46 stages based on physical features of the body, established by Hamburger and Hamilton in 1951 [2]. The final stage, number 46, is the newly hatched chick.
This system allows researchers to compare different studies by asking "what shape is it?" rather than just "what day is it?". First, let us look at the heart, which is the first thing to start moving.
2. When the Chick Embryo's Heart Starts Beating and Its Organs Form (Days 2–4)
The heart begins to beat around the second day of incubation. Sources describe this differently—as 42 hours, 44 hours, or day 3—so we cannot state an exact time [3][1].
By the end of day 4, it is said that all the organs needed for the chick to live after birth are in place [3]. The remaining 17 days of the 21-day period are spent growing these organs larger and preparing them to work.
So, what is the most surprising thing that happens during those remaining days? The next section is about the egg shell.
3. How the Eggshell Lets Air In and Supplies Calcium for Bones
The eggshell has about 17,000 tiny holes called pores. The embryo receives oxygen through these pores. The number of pores varies by species and egg. Gas exchange for the embryo happens through diffusion across the shell and through a membrane rich in blood vessels. This membrane is a combination of the allantois and chorion, called the CAM [4].
Another function of the shell is supplying calcium. The shell acts as a source of calcium. Around day 11 of incubation, calcium and other minerals begin to dissolve from the inside of the shell. The blood vessels of the CAM carry this dissolved calcium to the embryo, where it is used to form bones [5].
To use a familiar example, it is like shaving off parts of a house wall to use as building materials for your own body. However, not all the calcium comes from the shell; some calcium from the yolk is also used [5].
4. How the Chick Embryo Changes Position on Day 14 and Tucks In the Yolk Around Day 19
Around day 14, the embryo turns its head toward the rounded end of the egg (the blunt end) [1]. This means it has entered the position needed to break out of the shell.
Around day 19, the yolk sac (the bag that holds the yolk) begins to be pulled into the body, specifically into the inside of the abdomen. In the UNSW development stage chart, stage 45 (days 19–20) corresponds to the state where "the yolk sac is about half inside the body" [2]. The yolk is the nutrient source for the embryo.
It is like packing a lunch box inside your body before leaving on a trip. The preparations that need to be done before coming out proceed step by step inside the shell. Next is the story of breathing and the shell.
5. How the Chick Begins to Breathe With Its Lungs and Breaks Out Around Day 20
Around day 20, the chick's beak reaches the air space (air cell) at the rounded end of the egg [3]. Then, breathing with the lungs begins. The membrane rich in blood vessels (the CAM), which had supported the embryo until then, dries out and stops working, so the chick begins to breathe using only its lungs [4].
Next, the chick uses the egg tooth at the tip of its beak to tap the inside of the shell. Cracks appear in the shell, followed by small holes. The chick uses its legs to slowly turn its body and break the shell open. Around day 21, the chick comes out [3].
The switch from getting oxygen through the eggshell to breathing with the lungs overlaps with the time when the shell is broken. This may have been a very active time for the chick.
6. How Researchers Raised Chicks Without a Shell to Watch Development From Outside
In October 2024, research from the University of Tsukuba and others was published. It reported that several normal chicks hatched on day 21 of incubation using a method where shell-less culture containers were shaken and rotated on a shaker [6].
This method is introduced as a way to observe the entire process from egg laying to hatching in real time [6].
As seen in the earlier section, the shell is also a source of calcium. How this role is compensated for when raising chicks without a shell is not discussed in detail on the research introduction page.
7. A Simple Way to Investigate a Hard-Boiled Egg Yourself
Make a hard-boiled egg (with adult supervision because you use a stove), let it cool, and peel off the shell. You may find a thin membrane and an air space (air cell) on the inside of the shell at the rounded end. Compare the air cell where the chick first breathes with the air cell in a hard-boiled egg.
Another idea: make your own timeline of the days and events. Line up days 2, 4, 10, 11, 14, 19, 20, and 21. Write what happens on each day and look for the periods with the most changes.