Amniote Egg Dissection Key

Pre-Lab Research:

• This website has a video of an egg hatching.
• Review this website about the Avian Embryo.
• Study the instructions, diagrams, and photos on this lab key.

The Egg Lab:

Note: the term egg is best reserved for the shelled structure laid by the hen, while the egg cell, that structure released from the ovary, is called an ovum.

Carefully crack the egg and drop its contents into the large half of a plastic petri dish. Place the shell in the small half of the dish. Does the rounded end of the shell have an air space between the shell and the membrane? Locate all the parts shown on the drawing below.
(Hint: the blastoderm of your egg will be a pale white color.)

The chicken egg consists of the ovum (egg cell), commonly called the yolk, and its accessory structures.

The ovum consists of the yellow yolk mass plus a small area of yolk-free cytoplasm, the germinal disc, which is visible as a white spot on the surface of the yolk. Because of large amounts of yolk, cleavage (cell division) occurs only in the germinal disc and the embryo grows by absorbing raw materials for its metabolism from the yolk. The yolk of the ovum contains proteins, carbohydrates, lipids and vitamins that sustain the metabolic needs of the rapidly growing young chick during its normal 21-day incubation. The yolk is surrounded by a delicate vitelline membrane.

During its passage through the oviduct, several accessory structures are added to the ovum:

• In the first section of the oviduct, a viscous stringy albumen is secreted and adheres to the ovum. As the ovum moves along, rotation twists the stringy albumen into a pair of strands which surround and project beyond the ovum. In the fully formed egg, this chalazae extends from the yolk to each end of the shell centering the yolk for protection.
   
• In the next oviduct region, a more watery albumen is added. This albumen serves as a water reservoir for the early embryo and a source of food for the later embryo; it also contains substances that retard the growth of bacteria.
   
• Two shell membranes form next. The amnion forms around the embryo and albumen. The allantosis forms to line the shell.
   
• Finally, a calcium shell forms enclosing the entire structure. This hard protective shell is porous, permitting gas exchange with the environment. It is the semi-permeable shell membranes that regulate things passing in and out of the egg.

24 Hour Chick Embryo: After cleavage of the blastoderm, it separates from the underlying cavity, the blastocoel. In gastrulation, the blastoderm becomes two-layered, the endoderm lying ventrally and the ectoderm on the surface. Note primitive streak, somites and neural groove.
48 Hour Chick Embryo: Some of the prominent features include primitive streak, somites, heart bud and neural groove. Note that the notochord lies immediately beneath the neural groove.
72 Hour Chick Embryo: The embryo has a distinct C shape at this stage. It is covered by a thin- walled sac, the amnion. Note the development of the somites. The embryo has well-developed circulation at this stage. Observe the heart and its chambers. The smaller, slightly more anterior portion is the atrium and the larger, more ventral portion is the ventricle
96 Hour (4 day) Chick Embryo: Extending out from the embryo is a thin delicate membrane with blood vessels at its surface. This membrane, called the allantois, expands as the embryo develops until it completely lines the shell. The transparent sac surrounding the embryo is the amnion. The material in the amnion is principally water from the egg white. The embryo is actually developing in the amniotic fluid. Limb buds are just visible in this picture. The eyes are large when compared with the size of the embryo. The large eyes are a characteristic that separates bird embryos from all other vertebrate embryos at the same stage.

Lab Clean-up:

• When the dissection is finished, ALL parts of the egg - and the plastic petri dish - go into the provided trash can.
• Clean and dry the tray with paper towels - DO NOT WASH.
• Clean and dry all utensils with paper towels - DO NOT WASH.
• Return tray and utensils to lab table.