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in the pharynx and converge into the dorsal aorta. The heart has begun to differentiate into an atrium and a ventricle.

72-hour.The flexure of the embryo is now pronounced and the embryo is lying on
its left side. The brain has further subdivided and the olfactory pit (nose) is
present over the tip of the forebrain. The lens of the eye should be visible.
Lateral swellings representing the limb buds and a curved tail are also present.

Living embryos

1. Isolate a living 3- to 6-day chick embryo. Obtain an egg and clean scissors, forceps,
a plastic spoon. First prepare a small petri dish with
Howard Ringer's solution.
Gently rotate egg, clean with 70% ethanol and set down for a few minutes to
allow the embryo to float to the top. Open up the blunt end of the shell and
carefully peel back the shell membranes. Observe the embryo under the
dissecting scope.

2. Measure the heart rate and observe any other movement. Grab
the outer ring of the blastodisc near embryo with fine forceps in your non-cutting
hand. Try not to pinch any blood vessels. Quickly cut around embryo.

3. Keeping hold of the embryo with the forceps, pick up a plastic spoon with your
cutting hand and slide it under the embryo. Continue to hold onto embyo, lift
with the spoon and transfer to the petri dish filled with Howard's and examine
with the dissecting microscope. have your lab partner stand by with scissors in
case the embryo wasn't completely detached. If the dish is cloudy with yolk,
transfer embryo to a fresh dish of Howard's.

Note: an alternate method of embryo isolation is to break the egg open into a finger bowl of Howard's. This is similar to cracking an egg for culinary purposes, but the yolks of warm eggs are more fragile than those of refrigerated ones. First, place the egg in a horizontal position to allow the embryo to float to one side. Next, keeping the same orientation, crack the egg gently against the side of a finger bowl filled with Howard's. Submerge the egg and gently pull the ends appart to crack the egg open.

Instructor's prep list

©Cebra-Thomas, 2000

Last Modified: 13 April 2000

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