The anatomy of the hatching egg Is the yolk sac the remnant of the yolk?
It is important to know how a fertile egg is composed to understand the incubation process and to be able to assess a hatching egg.
It is not the case that the original yolk is partly used for energy and development of the embryo and that the yolk sac remains as ‘leftover’ of these processes. During the incubation process the albumen and the original yolk become more fluid with embryonic membranes and fluid compartments developing. This starts at day 3 and is visible as a ‘blood ring’ until day 11-12 when the chorion-allantoic membrane covers the whole inner eggshell membrane. In the meantime there is a continuous exchange of fluids between the various compartments. Towards the end of incubation, water is extracted and the yolk sac becomes more viscous, but by that time its content has changed a lot. The fact that it has a similar colour and is in a similar position within the egg, does not automatically mean that it is the same thing!
Air cell. The space between the shell membrane and the egg membrane. Provides oxygen for the chick’s first breaths after internal pipping. Albumen (egg white). Protects the egg yolk (buffer and anti-bacterial effect) and provides additional nutrients to the embryo.
Latebra. The column of white yolk that connects the blastodisc to the centre of the yolk (pale in fertilised eggs on an MRI-image).
ec
te
d
Cuticle. A layer of protein is deposited on the shell just before laying. It protects the contents of the egg and the shell by closing the pores.
pr ot
Germinal disc. The area of the egg from which the embryo begins to develop in the hen’s body after fertilisation.
rig py
Eggshell. The outer protective layer of the egg. The shell regulates the exchange of heat, gases (CO2 and O2) and water vapour (H2O) through the pores.
co
Chalazae. These cords attach the yolk in the centre of the unincubated egg.
remnants of vitelline membrane
shell membranes
chorion Embryo. The first stage of development starts in the hen’s body when the egg is produced. sero-amniotic connection Amniotic fluid. Serves to cushion the embryo and prevents dehydration.
Avascular mesoderm Vascular mesoderm Ectoderm Endoderm
yolk sac membrane
albumen
Chorion (outer membrane). Regulates the exchange of gas and protects against harmful bacteria.
White yolk. Provides nutrients for the first development phase of the embryo and has a slightly different composition than the yellow yolk. Vitelline membrane. Functions as a barrier that allows for diffusion of water and selective nutrients between the albumen and the yolk.
ht
Eggshell membranes. Protect the embryo from losing too much moisture and prevent entry of pathogens.
Yellow egg yolk. Acts as a source of nutrients for the embryo.
yolk Yolk sac. Supplies the embryo with nutrients.
extraembryonic coelom amnion embryo
allanto amnion
chorio allantois
Allantois. Helps with the exchange of gases at a later stage.
The various membranes and compartments in the egg containing an embryo after three days of incubation. There are many interconnections between the compartments that allow the exchange of fluids facilitated by egg turning.
2. Th e i n c u b a t i o n p ro c e s s
19
