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Dissection of the Frog
Introduction
Dissection is the scientific technique that allows you to separate one tissue from another. Dissection of
an organism is not simply a matter of cutting and slicing. The dissection is to separate the structures of
one body system from the structures of the other systems. In this way, you can see for yourself the
marvellous way that an organism is put together. Of course, you will not see this unless you dissect with
care and follow all instructions.
You are reminded that all life is valuable. Respect your animal and do not waste this opportunity. Take
your time, read carefully, observe closely and learn lots!
Evaluation is based on:
1. A full original, hand-drawn diagram (drawn to scale) of the digestive system and related structures of
the frog. Each person must hand in their own diagram. Include and label all of the following:
(25 marks)
The Digestive System:
a) tongue (diagram of mouth)
b) vomerine teeth (diagram of mouth)
c) gullet (opening to the esophagus, in mouth)
d) esophagus
e) stomach
f) pyloric sphincter
g) small intestine
h) large intestine
i) cloacal opening (anus)
j) glottis (opening to the trachea, in mouth)
Related Structures:
a) liver
b) gall bladder
c) pancreas
d) mesentery (circulatory system)
e) fat bodies
f) spleen (circulatory system)
g) lungs (respiratory system)
h) heart (circulatory system)
i) kidneys (excretory system)
j) urinary bladder (excretory system)
k) reproductive organ (testes or oviducts)
2. How well you perform the dissection:
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Upon completion of the dissection, your teacher will examine your specimen and determine how
carefully the dissection was performed, and how well the instructions were followed.
Students are expected to be respectful to the animals and behave maturely.
Students are expected to be respectful and considerate of other students’ feelings.
Marking scheme for dissections (10 marks):
1. Cuts are made neatly as instructed.
2. Skin is pinned back.
3. Small intestine is intact and uncut.
4. Cuts are made to the sides of the mouth.
5. Eyes and ears have not been damaged.
6. The gall bladder and liver are intact.
7. The pancreas is intact.
8. The brain cavity is undisturbed.
9. The reproductive organs are undisturbed.
10. Students have behaved in a mature and responsible fashion. (5 marks)
3. An in-class quiz on Thursday March 29 based on identification of the various structures, and stating
their function. (20 marks)
Final Evaluation
When you are finished, go back over your diagram and be sure that you have included all of the above
listed organs and associated tissues. Include a title for your diagram (look up the scientific name for the
Northern leopard frog). Your diagram will be marked out of 20. Then, when you are satisfied that you
can identify each structure and you know its function, ask your teacher to come over to check your frog
to see how carefully it was dissected and you will be asked to identify several organs for a mark out of
10.
Step I: External features. Pick up the frog and wash it off with tap water. Feel the skin.
1. Describe the texture (feel) of the skin. ________________________________________________
2. Is there any sign of bristles, hair, feathers or scales? __________
3. What unusual function does the frog’s skin play? ________________________________________
4. To determine the frog’s sex, look at the hand digits, or fingers, on its forelegs. A male frog usually has
thick pads on its "thumbs," which is one external difference between the sexes, as shown in the
diagram below. Male frogs are also usually smaller than female frogs. Observe several frogs to see
the difference between males and females. Sex = ______________________
Step II: Find the circular “patch” behind each eye.
1. What is the name of this patch? ___________________________________
2. What is the function of this patch? ___________________________________________
Step III: Using the scissors, cut the mouth at each corner. Open the mouth and examine it. Rub your
finger over the upper and lower jaw and the roof of the mouth.
1. Are teeth present in the upper jaw? _________
2. Are teeth present in the lower jaw? ________
3. Find the teeth in the roof of the mouth.
4. What is the function of these teeth?
5. Where is the tongue attached to the mouth?
6. Label the diagram of the frog’s mouth:
A.____________________________________
B.____________________________________
C.___________________________________
D.____________________________________
E. ____________________________________
AB.___________________________________
AC.___________________________________
Step IV: Dissection (a coloured, labelled picture may be helpful when identifying the organs)
1. Place your frog on its back in the dissection tray.
2. Use four pins to pin the frog’s hands and feet to the tray.
3. Study the diagram to the right showing the required cuts to
open the abdomen.
4. Start by pinching a small fold of skin on the abdomen, just
in front of the back legs. Make a small snip into the
pinched fold of skin.
5. Insert the end of the scissors gently into the first cut. Hold
the scissors horizontally so you do not cut down into the
delicate structures below.
6. Keeping the scissors horizontal, follow the diagram and cut
the skin up to the rib cage, and then out to both sides.
7. Peel back the layer of skin.
8. Once the skin is peeled back, follow steps 4 to 7 to cut
through the muscle layer. Peel back the muscle to expose
the tissues below. Pin the muscle flaps to dissecting tray.
9. Take a few minutes and just look around at the major organs. Identify as many as you can.
10. Stop as necessary to sketch your diagram of the digestive system of the frog, follow these step-bystep instructions to identify the various organs. A careful dissection will take two full periods.
*If your specimen is a female, the body may be filled with eggs and an enlarged ovary. You may need to remove
these eggs to view the organs.
11. Store or dispose of your materials according to the directions from your teacher.
12. Clean up your work area and wash your hands before leaving the lab.
Additional Dissection
Once you have completed the required parts of this dissection, and your specimen has been marked, you
may ask for your teacher’s permission to explore additional parts of the body, such as the muscular
system, the skeletal system and the nervous system (including the brain).
Locate each of the organs below. Check the box to indicate that you have found the organs.
Fat Bodies --Spaghetti shaped structures that have a bright orange or yellow color, if you have a
particularly fat frog, these fat bodies may need to be removed to see the other structures. Usually
they are located just on the inside of the abdominal wall.
Peritoneum A spider web like membrane that covers many of the organs, you may have to
carefully pick it off to get a clear view. Remove the peritoneum from around the heart.
Liver--The largest structure of the the body cavity. This brown colored organ is composed of
three parts, or lobes. The right lobe, the left anterior lobe, and the left posterior lobe. The
liver is not primarily an organ of digestion, it does secrete a digestive juice called bile. Bile is
needed for the proper digestion of fats.
Heart - at the top of the liver, the heart is a triangular structure. The left and right atrium can be
found at the top of the heart. A single ventricle located at the bottom of the heart. The large
vessel extending out from the heart is the conus arteriosis.
Lungs - Locate the lungs by looking underneath and behind the heart and liver. They are two
spongy organs.
Gall bladder--Lift the lobes of the liver, there will be a small green sac under the liver. This is the
gall bladder, which stores bile. (hint: it kind of looks like a booger)
Stomach--Curving from underneath the liver is the stomach. The stomach is the first major site
of chemical digestion. Frogs swallow their meals whole. Follow the stomach to where it turns
into the small intestine. The pyloric sphincter valve is a thickened muscular band at the bottom
of the stomach, where it attaches to the small intestine. It regulates the exit of digested food from
the stomach to the small intestine. Use the tip of the scissors to cut open the stomach. Examine
the stomach contents to see what the frog has been eating.
Small Intestine--Leading from the stomach. The first straight portion of the small intestine is
called the duodenum, the curled portion is the ileum. The ileum is held together by a membrane
called the mesentery. Note the blood vessels running through the mesentery, they will carry
absorbed nutrients away from the intestine. Absorption of digested nutrients occurs in the small
intestine. . In the first loop of the small intestine (just beside the stomach), there is a
creamy/transparent white mass. This is the pancreas. Handle it gently.
Large Intestine--As you follow the small intestine down, it will widen into the large intestine. The
large intestine is also known as the cloaca in the frog. The cloaca is the last stop before wastes,
sperm, or urine exit the frog's body. (The word "cloaca" means sewer)
Spleen--Return to the folds of the mesentery, this dark red,“bean-like” spherical object serves as
a holding area for blood.
Esophagus--Return to the stomach and follow it upward, where it gets smaller is the beginning
of the esophagus. The esophagus is the tube that leads from the frogs mouth to the stomach.
Open the frogs mouth and find the esophagus, poke your probe into it and see where it leads.
Kidneys - flattened bean shaped organs located at the lower back of the frog, near the spine.
They are often a dark color. The kidneys filter wastes from the blood.
Testes - in male frogs, these organs are located at the top of the kidneys, they are pale colored
and roundish.
Oviducts - females do not have testes, though you may see a tightly coiled structure around the
outside of the kidney, these are the oviducts. Oviducts are where eggs are produced. Males can
have structures that look similar, but serve no actual purpose. In males, they are called vestigial
oviducts.
Urinary Bladder – A greyish-white, empty sac located at the lowest part of the body cavity in
front of the large intestine. The bladder stores urine.