Day 4: Setting Up Camp

Extra-embryonic Membranes

Not only do the early embryonic germ layers (ectoderm, mesoderm, and endoderm) differentiate into specialized tissues of the body, but also they form membranes outside the body which help protect and nourish the developing chick embryo. Three extra-embryonic membranes are formed from the primitive germ layers:

The Yolk Sac

Endoderm and splanchnic mesoderm grow from the area opaca to form the yolk sac. The yolk sac surrounds the yolk and draws nourishment from it. Blood islands, which eventually form channels followed by vessels, are derived from the splanchnic mesoderm. These blood vessels, the vitelline vessels, extend to connect with the omphalomesenteric vessels coming off the heart of the embryo. The endodermal cells lying next to the yolk become more and more specialized to digest the yolk and pass its nutrients on to the neighboring vitelline circulation and thus to the embryo.

The Amnion

Ectoderm and somatic mesoderm surrounding the embryo, form a protective covering over the embryo called the amnion. The inner layer of cells secretes amniotic fluid in which the embryo floats. This fluid keeps the embryo from drying out and helps protect it when the egg is jarred.

The Chorio-Allantoic Membrane

Ectoderm and somatic mesoderm form the chorion which lines the eggshell and performs gas exchange and waste elimination. The allantois is the embryo's connection to the chorion. It appears as a balloon-like structure coming off the hindgut at about day 4 or 5. It is made up of endoderm and splanchnic mesoderm and carries with it the allantoic vessels coming off the heart. The allantois grows larger as the chick grows, wraps around above the chick, and fuses with the chorion. It works together with the chorion to exchange gasses between the embryo and the outside environment. The egg shell is porous and thus allows oxygen and carbon dioxide to pass freely back and forth from the environment to the inside of the egg. The chorio-allantoic membrane is also responsible for drawing calcium from the egg shell and carrying it back to the chick. Calcium is needed to carry on general metabolism and make bone. For the first ten days the chick can absorb a sufficient amount of calcium from the yolk. But at around day 10, the formation of bones begins to accelerate and the embryo must draw calcium from the egg shell. The allantois also stores wastes (urine) once the embryonic kidneys begin to function. When the chick hatches, the chorio- allantoic membrane remains attached to the egg shell and is therefore discarded with it.

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