Avian embryo development

Embryonic development  - Dr Stephan WARIN


The purpose of this supplement is to show the reader how to differentiate good incubating eggs from poor incubating eggs on the one hand and, when opening an egg, to determine its developmental stage and the timing of the occurrence of a potential problem, on the other hand.

Thorough knowledge of embryonic development helps us to understand the requirements for the good development of the future chick and the reasons why some incubation phases are crucial. Such knowledge is also the basis for embryo-diagnosis, which helps to determine the age and cause of death of the embryo with the aim to remedy this phenomenon and improve hatcheries’ performances.

The different stages of embryonic development

Total embryonic development times are 21 days for chickens, 27-28 days for ducks, turkeys and guinea fowls, 29-30 days for geese, 31-32 days for mulard ducks and 34-35 days for muscovy ducks. The following data applies to the hen species. Once the embryo has been fertilised (first day), embryogenesis begins and will last for five days, after which the embryo essentially grows until hatching. During the last three days, organs develop and the chick enters its maturation phase. 

A. Embryo Differentiation (1-6 days) 

The encounter between the ovum and sperms takes place in the portion of the oviduct called the magnum approximately three hours after ovulation. The egg cools down when in contact with the outer environment and embryonic development stops as long as the egg’s temperature is below 25°C (the physiological zero of incubation). Development will only resume in optimal conditions after raising temperature to 37.8°C.

1. From fertilisation to oviposition

The first cell division occurs when the egg is in the isthmus (it starts approximately three hours after ovulation) and continues throughout its descent toward the reproductive tract.
Six to eight hours prior to oviposition, two distinct zones on the yolk surface can be seen with the naked eye: the area pellucida (translucent) in the centre and the surrounding area opaca (early blastula stage). At this stage, the symmetry axis of the future embryo is determined by the coiling of chalazae during egg shell formation.
The late blastula stage (50,000 cells delimiting two superimposed cavities) is reached shortly before oviposition. Embryonic development then remains at this stage as long as temperature is kept under 21-22°C.

2. Embryo formation

• After 5-6 hours of incubation, thickening of the rear portion of the area pellucida occurs.
• After 16 hours, thickening extends along the entire length of the blastoderm and forms the primitive streak.
• At 18 hours, the cephalic extension can be seen, gastrulation is completed and neurulation begins.
• After 20 hours, the primitive streak shrinks while differentiation begins including cephalic folding and individualisation of somites.
• After 40 hours, the brain and heart are formed, and the anterior intestine takes shape. The embryo lifts up over the yolk and lies on its left side. The first heart beats occur (40/min.) and permit blood circulation between the embryo and the egg yolk.

A. Embryo Differentiation (1-6 days)

table 1

B. Organ Development (7-17 days)

table 2

C. Maturation and Preparation for Hatching (18-21 days)

table 3

D. Critical Development Phases

The embryo is particularly fragile at the following phases of incubation:

• First two days: risk of early mortality due to blastoderm fragility when embryonic development resumes, and structuring of vascular network. The transformation of the blastoderm into a basic embryo occurs during the first two days of incubation and during this time the embryo is at its most vulnerable to disturbances. Many of the abnormalities seen in late embryos or hatchling have their origins in mishaps at these early stages.

Around 5-6 days: the yolk vesicle completes its development, the vitelline membrane disappears and the allantois starts to function as a respiratory organ and as such is particularly susceptible to shocks.

At the end of incubation (18-19 days): The kidney comes into service and the organisation of aerial respiration occurs.
• At hatching: pipping difficulties resulting in mortality due to exhaustion – at hatching or shortly thereafter – respiratory failure, inadequate resorption of the yolk vesicle (due to excessive temperature causing an increase in organ volume, thereby hampering yolk retraction).

Risks of malformation are high, especially during the first six days of incubation, when organs differentiate. Afterwards, organs grow in volume: any abnormality during this phase results rather in a change (extension or shortening) in embryonic development duration.

In the chicken, the association of embryonic mortality with embryo age is approximately as follows:
0 - 4 days: 25%
4 - 16 days: 10%
16 - 21 days: 65%

Embryonic development day by day

Unfertilized egg: The embryonic disc of a sterile egg bears an accumulation of white material at its center 


Fertilized egg: The fertilized embryonic disc looks like a ring: it has a central area, lighter in color, which is to house the embryo. 


Day 1: The germinal disc is at the blastodermal stage. The segmentation cavity, under the area pellucida, takes on the shape of a dark ring. 


Day 2: Appearance of the first groove at the center of the blastoderm. Among extraembryonic annexes, appearance of the vitelline membrane which is going to play a major role in embryo nutrition. 


Day 3: The embryo is lying on its left side. Onset of blood circulation. The vitelline membrane spreads over the yolk surface. The head and trunk can be discerned, as well as the brain. Appearance of the cardiac structures which begin to beat. 


Day 4: Development of the amniotic cavity, which will surround the embryo: filled with amniotic fluid, it protects the embryo and allows it to move. Appearance of the allantoic vesicle: it plays a major role in calcium resorption, respiration and waste storage.


Day 5: Sensible increase in the embryo’s size; the embryo takes a C shape: the head moves closer to the tail. Extension of limbs. Differentiation of the fingers of the inferior limbs. 


Day 6: The vitelline membrane continues to grow and now surrounds more than half the yolk. Fissura between the first, second and third fingers of the upper limbs, and between the second and third fingers of the lower limbs. The second finger is longer than the others. 


Day 7: Thinning of the neck which now clearly separates the head from the body. Formation of the beak. The brain progressively enters the cephalic region: it progressively grows smaller proportionally to the embryo’s size. 


Day 8: The vitelline membrane covers almost the whole yolk. Eye pigmentation is readily visible. The beak’s upper and lower parts are differentiated, as well as the wings and legs. The neck stretches and the brain is completely settled in its cavity. Opening of the external auditory canal. 


Day 9: Appearance of claws. Budding of the first feather follicles. Growth of the allantois and increased vascularization of the vitellus. 


Day 10: The nostrils are present as narrow apertures. Growth of eyelids. Extension of the distal portion of the limbs. The vitelline membrane completely surrounds the yolk. Feather follicles now cover the inferior part of the limbs. Appearance of the egg-tooth. 


Day 11: The palpebral aperture has an elliptic shape that tends to become thinner. The allantois reaches its maximum size while the vitellus begins to shrink. The embryo now has the aspect of a chick.


Day 12: Feather follicles surround the external auditory meatus and cover the upper eyelid. The lower eyelid covers two thirds, or even three quarters, of the cornea. 


Day 13: The allantois shrinks to become the chorioallantoic membrane. Appearance of claws and leg scales. 


Day 14: Down covers almost the whole body and grows rapidly. 


Day 15 & 16: Few morphological changes: chick and down continue to grow. Vitellus shrinking accelerates. Progressive disappearance of the egg white. The head moves toward pipping position, under the right wing.


Day 17: The embryo’s renal system produces urates. The beak, which is under the right wing, points to the air cell. The egg white is fully resorbed.



Day 18: Onset of vitellus internalization. Reduction in the amount of amniotic fluid. This is the time for transfer from incubator to hatcher, and also in ovo vaccination


Day 19: Acceleration of vitellus resorption. The beak is against the inner shell membrane, ready to pierce it. 


Day 20: Vitellus fully resorbed; closing of the umbilicus. The chick pierces the inner shell membrane and breathes in the air cell. Gas exchanges occur through the shell, which is porous. The chick is ready to hatch. Piercing of the shell begins. 


Day 21: The chick uses its wing as a guide and its legs to turn around and pierce the shell in a circular way by means of its egg-tooth. 


It extricates itself from the shell in 12 to 18 hours and lets its down dry off.


Watch this video created by AXS Biomedical Animation Studio Inc.©  - 2013 Poultry CRC Ltd.

More information about in ovo vaccination with Egginject® here