Before Egg Laying

• Fertilization
• Division and growth of living cells
• Segregation of cells into groups of special function (gastrulation)

Between Laying and Incubation

Virtually no growth. Stage of inactive embryonic life

During Incubation

Day 1

Development of area pellucida and area opaca of blastoderm.

Major developments visible under microscope:

• 18 hours, appearance of alimentary tract
• 19 hours, beginning of brain crease
• 20 hours, appearance of vertebral column
• 21 hours, beginning of formation of brain and nervous system
• 22 hours, beginning of formation of head
• 23 hours, appearance of blood islands
• 24 hours, beginning of formation of eyes

Day 2

Embryo begins to turn on left side. Blood vessels appear in the yolk sac. Major developments visible under microscope:

• 25 hours, beginning of formation of veins and heart
• 30 hours, second, third and fourth vesicles of brain clearly defined, as is heart, which now starts to beat
• 35 hours, beginning of formation of ear pits
• 36 hours, first sign of amnion
• 46 hours, formation of throat

Day 3

Formation of nose, wings, legs, and allantois begins. Amnion completely surrounds embryo.

Day 4

Formation of tongue begins. Embryo completely separate from yolk sac and turned on left side. Allantois breaks through amnion.

Day 5

Proventriculus and gizzard formed. Formation of reproductive organs and sex determined.

Day 6

Formation of beak and egg-tooth begins. Main division of legs and wings. Voluntary movement begins.

Day 7

Indications of digits in legs and wings are visible. Abdomen more prominent due to development of viscera.

Day 8

Formation of feathers begins.

Day 9

Embryo begins to look bird-like. Mouth opening appears.

Day 10

Beak starts to harden. Skin pores visible to naked eye. Digits completely separated.

Day 12

Toes fully formed. First feathers appear.

Day 13

Scales and claws appear. Body fairly well covered with feathers.

Day 14

Embryo turns its head towards blunt end of egg.

Day 15

Small intestines taken into body.

Day 16

Scales, claws and beak becoming firm and horny. Embryo fully covered with feathers. Albumen nearly gone and yolk increasingly important as nutrient.

Day 17

Beak turns toward air cell, amniotic fluid decreases, and embryo begins preparation for hatching.

Day 18

Growth of embryo nearly complete.

Day 19

Yolk sac draws into body cavity through umbilicus. Embryo occupies most of space within egg except air cell.

Day 20

Yolk sac completely drawn into body cavity. Embryo becomes chick, breaks amnion, starts breathing air in air cell Allantois ceases to function and starts to dry up.

Day 21

Chick hatches!