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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.
Embedded embryos
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). 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. Capture digital images of at least one of
the embryos as described below.
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
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