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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!
Final 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 a title for your diagram (look up the
scientific name for the Northern leopard frog). Use the marking scheme provided to double check your
drawing. Include and label all of the following: (25 marks)
The Digestive System:
a) esophagus
b) stomach
c) pyloric sphincter
d) small intestine
e) large intestine
f) cloaca
g) pancreas
h) gall bladder
i) liver
Related Structures:
a) mesentery (circulatory system)
b) fat bodies
c) spleen (circulatory system)
d) lungs (respiratory system)
e) heart (circulatory system)
f) kidneys (excretory system)
g) urinary bladder (excretory system)
h) reproductive organ (testes or oviducts)
2. How well you perform the dissection (10 marks):

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.
Items to be considered:
1. Cuts are made neatly as instructed.
2. Skin is pinned back as shown.
3. All organs are intact and uncut.
4. Cuts are made to the sides of the mouth.
5. Eyes, ears, brain cavity and cloaca have not been damaged.
6. Students have behaved in a mature and responsible fashion.
7. Lab station and equipment has been thoroughly cleaned at the end of each class.
3. An in-class quiz during the dissection that asks students to identify various organs and a powerpoint
quiz on Thursday Dec 11 based on identification of the various structures, and stating their function. (25
marks)
4. Supplemental Questions about the frog are answered (next page). (10 marks)
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. ______________________________________________ (0.5)
2. Is there any sign of bristles, hair, feathers or scales? _______________
(0.5)
3. What unusual function does the frog’s skin play? ________________________________________ (1)
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. What do you think the sex of your frog is? ____________
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? ___________________________________________
(1)
(1)
Step III: Using the scissors, cut the mouth at each corner. Open the mouth and examine it. Rub your
finger or run a probe over the upper and lower jaw and the roof of the mouth.
1. Are teeth present in the upper jaw? _________
(0.5)
2. Are teeth present in the lower jaw? ________
(0.5)
3. Find the teeth in the roof of the mouth.
4. What is the function of these teeth?
(1)
5. Where is the tongue attached to the mouth?(0.5)
6. Label the diagram of the frog’s mouth:
A.____________________________________
B.____________________________________
C.___________________________________
D.____________________________________
E. ____________________________________
AB.___________________________________
AC.___________________________________
(3.5)
Step IV: Dissection (the powerpoint presentation 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. Call your teacher over if this is the case.
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.
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.
Mesentery 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 mesentery from around the heart.
Liver--The largest structure of 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 do not chew their prey and 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.
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 comes to an end at the cloaca in the frog. The cloaca is the last stop before
wastes, sperm, or urine exit the frog's body. The exit of the cloaca can be seen on the back of the
frog (The word "cloaca" means sewer)
Spleen--Return to the folds of the mesentery around the small intestine, 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 gullet, 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 round. 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 and also connects to the cloaca.
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). Use
the tip of the scissors to cut open the stomach. Examine the stomach contents to see what the
frog has been eating.