Chicken Embryo Development
The chicken embryo has been intensively studied because embryos are available in large numbers. Even though the eggs of chicks and mammals contain very different amounts of yolk, the similarities of the post-blastula chick embryo to the mammalian embryo are striking, and thus the chick embryo is a reasonably good model for studying mammalian embryonic development.
Chick embryos that have been incubated for approximately 48, 72 and 96 hours after fertilization will be available for you to examine. However, specimens of similar ages post-fertilization may show a wide variation of maturation stages. The incubation times provide only an estimation of how far development of the embryo will have progressed. The temperature of incubation (which may be different among eggs depending on their locations within the incubator), as well as the rate of growth of the individual embryo itself will affect the overall developmental process. Not all embryos will look the same, so compare several to each other. If you see anything interesting, share your observations with your fellow students.
Stages in chick embryo development
The 48-hour chick embryo
These embryos will be set up as demonstrations under dissecting microscopes. Locate the embryo situated on top of the large yolk mass. It will be very pale in color. It may be necessary to use a vital stain solution (e.g., neutral red) to highlight the embryo structures. Compare the live embryos with the prepared slides of 33-hour embryos.
The 72-hour chick embryo
This embryo has reached a stage that allows some manipulation on your part. You should use the following procedure to remove the embryos from the shell and place them in a shell-free culture system.
Shell-free Cuture System Procedure (Eggs must be at 72-hours of incubation, or the yolk will be too fragile to be transferred without breaking)
1. Fill a plastic cup with an inch of water. Place a sheet of plastic wrap over the top of the cup, letting it drape down a bit. It must be secured with a rubber band to hold the plastic wrap in position.
2. Trim away excess plastic wrap around the rubber band. (If plastic is not trimmed, the egg albumen will wick out of the container creating a mess in the incubator and reducing the embryo survival)
3. Obtain a 72-hour egg. Wipe the shell with 70% ethanol and allow it to dry. Hold the egg horizontally for 1 minute to allow the embryo to rotate to the upper side of the shell.
4. Refer to image below - Carefully crack the egg as if for frying and let the contents drop onto the plastic wrap. Try to crack the egg gently but firmly so that a single large crack is produced. DO NOT BREAK THE YOLK. If the yolk breaks, there is little chance that the embryo will survive.
5. Look for the embryo (see image below). It may have ended up underneath the yolk. It usually will float to the top of the yolk within a few minutes.
72 hours of incubation - The leg buds are slightly larger than wing buds; 30-36 somites are present, which extend beyond leg buds; allantois is short, thick-walled pocket and not yet vesicular.
6. Cover the entire assembly with a 100mm-diameter plastic petri dish lid. Place the culture in the egg incubator (at 38oC). Observe the cultured embryos daily. As the embryo develops, note changes in the eye, limbs, and extra-embryonic membranes.