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Appendix 9. Notes on the anatomy of the embryonated egg

Shell and shell membrane

The membrane is closely attached to the shell. Together they function as an exchange system and gaseous and liquid molecules pass in both directions. This is why eggs must be incubated in humid conditions. Eggs incubated in low humidity will lose moisture and eventually the embryo will die.

Air sac

Eggs have a rounded and a pointed end. The air sac is the space at the rounded end and has a function in respiration and pressure adjustments.

Chorioallantoic membrane and allantoic cavity

The membrane is attached to the embryo and functions to remove soluble, insoluble and gaseous waste products. As the embryo develops, the sac increases in size. The sac contains allantoic fluid into which Newcastle disease virus is shed after inoculation of the allantoic cavity. The fluid is harvested for vaccine production.

Some viruses are propagated by inoculation of the chorioallantoic membrane. This involves placing inoculum on the membrane.

Yolk sac

This is also attached to the embryo and contains the nutrient-rich yolk. As the embryo develops, the yolk sac decreases in size to approximately 1 cm diameter 3 days before the embryo hatches.

Amniotic Sac

This sac surrounds the embryo. It is filled with liquid and serves to protect the embryo against physical damage as well as functioning as an area of exchange of molecules. As the embryo develops, the membrane stretches and the amniotic sac is barely visible in the fully developed embryo.


The egg white, which consists mainly of protein.

To examine the structure of embryonated eggs at various stages of development, remove the entire contents of the egg into a Petri dish.

Figure 28: The anatomy of a ten-day old embryonated egg

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