The Timing of Major Embryonic Developments
Before Egg Laying
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
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
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
Formation of nose, wings, legs, and allantois begins. Amnion completely surrounds embryo.
Formation of tongue begins. Embryo completely separate from yolk sac and turned on left side. Allantois breaks through amnion.
Proventriculus and gizzard formed. Formation of reproductive organs and sex determined.
Formation of beak and egg-tooth begins. Main division of legs and wings. Voluntary movement begins.
Indications of digits in legs and wings are visible. Abdomen more prominent due to development of viscera.
Formation of feathers begins.
Embryo begins to look bird-like. Mouth opening appears.
Beak starts to harden. Skin pores visible to naked eye. Digits completely separated.
Toes fully formed. First feathers appear.
Scales and claws appear. Body fairly well covered with feathers.
Embryo turns its head towards blunt end of egg.
Small intestines taken into body.
Scales, claws and beak becoming firm and horny. Embryo fully covered with feathers. Albumen nearly gone and yolk increasingly important as nutrient.
Beak turns toward air cell, amniotic fluid decreases, and embryo begins preparation for hatching.
Growth of embryo nearly complete.
Yolk sac draws into body cavity through umbilicus. Embryo occupies most of space within egg except air cell.
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.