Observation of living and plastic-embedded chick embryos
The development of chick embryos has been studied since Aristotle. It is one of the most intensely studied organisms. One reason for this is that there are great similarities between avian and mammalian embryology. Another advantage is that the embryo is relatively accessible and can be operated on or treated with teratogens (substances that perturb development) while still in ovo(in the egg). Finally, fertilized chicken eggs are available year round and can be "held" at 10-14ºC for several weeks before being set, producing embryos upon demand.
1. Examine plastic embedded specimens of "13-hour" or "16-hour", "24-hour", "33-hour","48-hour" and "72-hour" chick embryos. These terms refer to classic studies of chick development. In reality, chick embryos will develop at different rates for a number of reasons, including incubation temperature. In modern times, chicks always take longer to reach these stages. Two more accurate ways of describing the relative developmental age of a chick embryo are (1) the number of somites and (2) comparison to a staging series, such as the one devised by Hamburger and Hamilton (H&H; hamburger.pdf). The somites are the blocks of tissue on either side of the neural tube. Try to determine the number of somites and the H&H stage number of your embryos.
2. Try to identify the structures indicated in the diagrams. Pay particular attention to the developing circulatory system. Try to determine which germ layer forms each structure. Highlights include:
13- to 16-hour.Identify the area opaca and area pellucida. Locate the primitive streak. Hensen's node is located at the anterior end of the streak.
24-hour.Locate the neural folds and Hensen's node. Identify the anterior end of the embryo.
33-hour.The embryo is lying along the center of the blastodisc, dorsal side up. The heart is to the right, along the side of the hindbrain. It is a simple, looped tube;connected to the vitelline vein and the ventral aorta. What is the function of the vitelline vein? Identify the forebrain, midbrain and hindbrain. The optic vesicles are outpockets on either side of the forebrain. The neural plate runs along the dorsal midline and is not yet closed. You may also be able to see the notochord running along the midline.
48-hour.The head has bent forward and the body has begun to twist. The optic cup(eye) and the otic vesicle (ear) have formed. The neural tube has closed, except in the region of the hindbrain. A series of aortic arches run between the pouches