After the hen lays the egg, the chick develops in 21 days. Some highlights:
Day 1: The embryo weighs 0.0002 g. The vascular system begins to form, but the heart does not beat until day 2.
Day 4: All vital organs are already formed. The beak, wings, legs and nostrils begin to appear.
Day 7: Digits appear on the wings and feet, and the heart goes inside the ribcage – it used to beat outside the body of the embryo.
10th day: The first feathers appear, and the beak begins to harden. The chick weighs just over 2 g.
Day 14: The nails are forming, and the embryo is already moving towards the hatching position (egg exit).
Day 17: With all the structures formed, the embryo already weighs almost 17 g, but it will still gain weight by another 13 g.
Day 20: It pierces the membrane and breathes through the lungs, sucking air from the cavity between the membrane and the shell.
Other curiosities of chicken reproduction
1. When the rooster charges at the hen, she lifts her tail. This facilitates contact between the cloacas – outlets from the digestive, urinary and genital systems of birds. It’s called a cloacal kiss. The male introduces his erectile organ (phallus) in the female and releases its spermatozoa, fertilizing the egg, which becomes “galado”.
2. The shell is oval because the egg spirals down the oviduct. The shape also helps when the egg comes out of the hen, with the tip opening the way. Another highlight of the design: the oval shape guides the chick to break the wider end of the egg. Thus, it hits an air pocket to breathe before exiting.
3. The fertilized egg comes out of the hen within 24 hours. During this period, the materials that form the egg are deposited. First, four layers of albumen (white), then a thin membrane – that skin that separates the shell from the white in hard-boiled eggs – and, finally, the shell and the pigments that give it its color.
4. At “hatching” time, the chick pushes its head towards the shell. The secret weapon for escape is a sharp keratin structure that grows on top of the beak and is called the “egg tooth” or “diamond”. It falls a few days after the chick hatches from the egg.
5. The chick is inside the amniotic sac, completely filled with liquid. His food comes from the yolk, rich in minerals and fats. Surrounding everything is the egg white, which provides protein to the embryo. Nutrient capture rolls through an umbilical opening that connects to the sac – not to be confused with the umbilical cord, unique to mammals.
6. The shell and the membrane have tiny pores, which protect the chick against the entry of external infectious agents, in addition to preventing excessive loss of water to the environment. It is through these microscopic holes that the embryo receives oxygen and eliminates water vapor and carbon dioxide.
7. For the embryo to develop, the internal temperature has to be constant – around 37.8°C. To do this, the hen lies on it, supporting the part of the belly where feathers do not grow: the incubation plate. At the time of hatching, blood circulation increases in the plate and the hen even becomes feverish.
Source: Guilherme Renzo Rocha Brito, postdoctoral fellow in Ornithology at the department of vertebrates at the National Museum of UFRJ