Day 1
Appearance of the primitive streak, primitive gut, primordial germs, vertebral column, somites blood islands, vitelline circulation and eye (6 somites).
Day 2
Appearance of heart, veins, amnion, ear, cranial flexure and torsion; heart beats; beginning of yolk sac; formation of throat; embryo begins to turn on left side.
Day 3
Appearance of nasal pits, posterior and anterior limb buds, and allantois; growth of amnion completely around embryo.
Day 4
Appearance of tongue and esophagus, vascularization of yolk sac; growth of allantois outside embryo; face and nasal parts begin to take shape; limbs bend to form elbows and knees; embryo separates from yolk sac; rudiments of body organs formed.
Day 5
Appearance of proventriculus, gizzard and reproductive organs differentiation of sex.
Day 6
Appearance of beak, egg tooth and duodenal loop.
Day 7
Allantois and chorion (serosa) anastomose and lie against the shell; Ceca and digits are formed.
Day 8
Appearance of tracts and feather germs.
Day 9
Appearance of toes on feet and scales on legs, embryo assumes bird-like appearance; a chalk-like deposit indicates the beginning of horny beak; mouth opening appears, growth of allantois almost completed.
Day 10
Beak begins to harden; skin pores are visible; digits are completely separated;
Days 11 & 12
Abdominal walls are established, loops of intestine are seen hanging in yolk sac; feathers are visible.
Day 13
Appearance of scales, claws, down feathers and muscle; completion of cartilaginous skeleton; ossification in progress.
Day 14
Embryo oriented to lie parallel to the long axis of the egg; the head is towards the broad end and near the air cell.
Day 15
Small intestine formed.
Day 16
Beaks, claws and scales cornified; albumen is practically used up leaving yolk, chick boy is covered with down feathers.
Days 17 & 18
Neck acquires a double bend so that the head is under the right wing and beak towards the lower part of the air cell; intestines are retracted completely within the body walls, amniotic fluid decreases.
Day 19
Yolk sac begins to enter body cavity.
Day 20
Yolk is completely ingested and umbilicus starts to close over; the chick pierces the shell and full pulmonary respiration is established; allantois circulation and respiration cease and allantois dries up; the chick may be heard to peck at the shell.
Day 21
Chick has its head under its wings and legs towards the head; chick pecks through the shell; the shell is brittle because some of its content (calcium) has been used to form bones.
