Name: _________________________________
Date: _________________
Period: _____
Frog Dissection
Introduction: Amphibians are more complex than fishes. The amphibians’
lungs and four legs helped to make life on land possible. By examining the
anatomy of amphibians, one can see many structures that are basic to both
amphibians and all the more advanced vertebrates.
Purpose:
In this lab, you will dissect a frog in order to observe the external and internal structures of frog
anatomy.
Objectives:
* Describe the appearance of the frog’s external features
* Describe the appearance of various organs found in the frog.
* Name the organs that make up various systems of the frog.
Materials:
• safety goggles and gloves
• forceps
• preserved frog
• dissecting pins (6–10)
• dissecting tray and paper towels
• dissecting probe
• scissors
• ruler
Procedure: *PLACE A CHECK ABOVE THE STRUCTURES THAT YOU FIND AND CIRCLE
THE ONES THAT YOU DON’T FIND.
Part A: External Anatomy
1. Put on safety goggles, gloves, and go to your lab station.
2. Place a frog on a dissection tray. 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.
3. Note the arrangement of the spots and the coloration of the frog. The color of the frog is caused
by chromatophore cells in the epidermis. Chromatophore cells are cells that contain pigments.
4. Remove a 1 cm x 1 cm section of the dorsal (top) skin containing one of the frog’s spots. Make
a wet mount of this piece of skin.
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5. Place the wet mount on the compound microscope under low power and then medium.
Chromatophores are usually star-shaped. Dispersal of the pigment into the rays makes the skin
darker. When the pigment is concentrated in the center of the chromatophores, the skin is lighter.
6. In the space below, make a drawing of the chromatophores. Return the microscope.
7. Stretch the rear legs back, making them as straight as possible. This simulates its position at
the end of a swimming stroke.
8. Examine the frog’s rear leg below the “knee.” What is the ratio of the length of the leg to the
length of the foot? ______________ Compare these figures with the ratio of the length of your own
leg to the length of your foot. _________________
9. Measure the frog’s thigh. Compare its length with the length of the leg and foot. ____________
_______________________________________________________________________________
How are the rear appendages adapted to the frog’s way of life (list two)?
_______________________________________________________________________________
_______________________________________________________________________________
10. Refer to Figure 1 on page 6 of the lab. Locate the thin membrane that covers the eye. This is
the nictitating membrane, which protects the eye when the frog is under water and keeps it moist
when the frog is on land.
11. Notice the large tympanic membranes behind the eyes. These membranes function as
eardrums to receive sound waves. The external nares are the frog’s nostrils
How is the structure of the eyes, tympanic membrane and nares adapted to the frog’s way of life?
_______________________________________________________________________________
_______________________________________________________________________________
Compare the position of the frog’s eyes with the position of human eyes. How is the positioning of
the frog’s eyes an adaptive advantage for the frog? _____________________________________
_______________________________________________________________________________
_______________________________________________________________________________
Compare the colors of the dorsal and ventral surfaces of the frog. Of what adaptive value to the
frog is each of these colorations? ____________________________________________________
_______________________________________________________________________________
_______________________________________________________________________________
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Part B: Oral Cavity
Use Figure 2 on page 6 for this part of the lab.
1. Open the mouth by cutting the jaws with the scissors. Feel around the edge of the jaw and you
will find the maxillary teeth. They hold food but are not used for chewing.
2. Locate the nostril openings (internal nares) in the roof of the mouth. Between the nostril
openings are two vomerine teeth. Feel these teeth with your fingers. These teeth are also used to
holding prey.
3. The eustachian tubes are on either side near the back of the mouth. Insert a probe into one of
the eustacian tubes. Where does the probe come out? _____________________________ The
eustachian tubes equalize pressure within the ear.
4. In a male (whether you have one or not) locate the opening that leads to the vocal sacs.
They amplify the male’s mating call.
5. Locate the slit-like glottis at the back of the throat. This opening leads to the respiratory system.
Above the glottis is the opening to the esophagus called the gullet.
6. Find the tongue. Describe where the tongue is attached to the mouth. __________________
How does our tongue attachment compare? _________________________________________
Part C: Digestive System
Refer to Figures 3 and 4 for this part of the lab.
1. Place your frog in the dissection tray ventral side up. Secure the feet and hands down, angling
the pins so they don’t interfere with the dissection. Refer to Figure 3 for the cuts that should be
made in the frog. Note: This can be done with the scissors, a scalpel is not necessary.
2. If your frog is a female, the most obvious structures will be the black eggs and the white ovaries.
The eggs and ovaries must be carefully removed before the internal organs can be observed. (If
you have a male, find a group with a female and have them show you the eggs and ovaries!)
3. The next large structure is the liver. It is located between the forelimbs and covers most of the
organs. How many lobes does the liver contain? _________________
4. Lift the liver and located behind the middle lobe is the bag-like, green colored gallbladder. The
gallbladder stores bile that is secreted by the liver. Bile aids in the digestion of fats. Try to locate
the bile duct leading from the liver to the gallbladder.
5. With scissors, carefully remove the liver and gallbladder from the body. The remaining organs of
the digestive system are easier to see with the liver removed.
6. Study the exposed organs, note that most of the organs are held in place by thin, transparent
tissues called mesenteries.
7. Locate the esophagus as shown in Figure 4. follow the esophagus to the stomach. Note that
the lower end of the stomach is constricted. This constriction is the pyloric sphincter. It regulates
the amount of food that enters the small intestine.
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8. Cut open the stomach and observe its lining. If food is present in the stomach, try to identify it.
Usually insect body parts will be present.
9. Follow the digestive tract beyond the pyloric sphincter to the coiled small intestine. The first
portion that usually runs parallel to the stomach is the duodenum. The duodenum is where much
of the digestion of food takes place.
10. Follow the digestive tract below the small intestine where it widens into the large intestine, or
colon. The colon ends in the rectum, which in turn opens into the cloaca. The cloaca opens to
the outside of the frog. The digestive, reproductive, and excretory systems all open into the cloaca.
11. Locate the pancreas, a soft, irregular, pinkish organ that produces digestive enzymes, found
lying in a membrane between the stomach and duodenum.
Part D. Respiratory and Circulatory Systems
Refer to figure 4 for this part of the Procedure.
Air is drawn into the mouth by expansion of the throat. The external nares close, then the throat
muscles contract and air is forced into the lungs through the glottis. Air is expelled as the nares
remain closed, the throat expands, and air enters the mouth again from the lungs. The glottis
closes, the nares open, ant the throat contracts, forcing the air out through the nares.
1. Using a pipette, probe the glottis to see where it leads. Locate the trachea, the passageway
between the glottis and the lungs.
2. Locate the pinkish-gray lungs. Squeeze the end of the pipette to expand the lungs.
Sequence the passage of air into and out of the frog. _________________________________
____________________________________________________________________________
____________________________________________________________________________
3. Notice the three-chambered heart between the lungs and posterior to the tracea. The heart is
encased in a membranous sac called the pericardium. Carefully open it up if not already
open.The pointed ventricle is lighter in color than the two thin-walled atria. The large artery on
the ventral surface of the heart is the coronary artery.
4. Lift the stomach and find the spleen, a round red organ. The spleen filters the blood, taking out
improperly functioning red blood cells.
Part E. Excretory and Reproductive Systems
Refer to Figure 5 for this part of the Procedure.
1. Examine the kidneys that lie against the dorsal body wall in the posterior region of the body
cavity. Each kidney has a yellow stripe, known as the adrenal body, that secretes hormones.
The kidneys filter the blood and produce urine, which drains into the urinary bladder, a thinwalled bag that attaches to the cloaca.
2. A female frog has two lobed, grayish ovaries that lie close to the kidneys. In a mature female,
the two ovaries might be filled with black and white eggs.
3. Locate in a male frog the white testes that can be found close to the kidneys. Look at the
reproductive of both sexes.
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4. Examine the yellow, fingerlike fat bodies attached near the kidneys. Compare their size with
those in a frog of the opposite sex. The fat bodies provide nourishment for the gametes.
Are the fat bodies larger in male or female frogs? ____________________________________
Why is this so? _______________________________________________________________
____________________________________________________________________________
Part F. The Brain
Refer to Figures 6 and 7 for this part of the Procedure.
1. Turn over your frog so that the dorsal side once again faces up.
2. Insert the point of your scissors through the skin at the base of the head and remove the skin
from the head area.
3. Bend the frog to determine the approximate region of the “neck.”
4. Inert your scissors and clip across the upper spinal cord in the region of the neck.
5. Locate the white spinal cord enclosed within the vertebrae.
6. Use your forceps to remove the bone above the spinal cord, working forward until you have
reached the nostril area. You will be exposing the brain, as shown in figure 6.
7. Locate the spinal cord and the olfactory lobes, cerebrum, optic lobe, cerebellum, and
medulla oblongata of the brain, using Figure 6 as a guide.
The largest parts of the frog’s brain are the olfactory lobes and optic lobes, the centers of smell
and vision, respectively. How is this adaptation an advantage for the frog’s lifestyle? ________
____________________________________________________________________________
____________________________________________________________________________
Part G: Muscles
Introduction. An organism’s muscles serve two main functions: (1) Muscles are attached to bones
by tendons which pull on the bones during contraction and thus cause the bone to move (articulate),
and (2) the muscles also give protection to the organs. Ligaments are not muscle tissue, but give
support and stability to joints.
Muscles work in antagonistic pairs. This means that when one muscle contracts, the opposite
muscle of the pair relaxes to allow the resulting articulation (relative movement of limbs). Without
this process, movement would be impossible.
1. Carefully remove the skin from one leg of your frog without damaging the muscles beneath. Use
scissors for the task. Make sure you remove the skin all the way down to the frog’s foot.
2. Look at the frog’s leg from the ventral (the underside of the frog). Locate the muscles listed it the
pictures in Figure 8. It may be necessary to separate muscles from one another, as they are held
together and covered by membrane. Use pins as in the picture below to mark the muscles you have
found. (Do not poke the muscles with the pins!)
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3. After locating the ventral muscles, flip your frog over and locate the dorsal muscles
4. The frog’s leg is made up of four major bones, listed here proximal (nearer the torso) to distal
(further from the torso): femur, tibiofibula, calcaneus, and astragalis. The calcaneus and astragalis
run side by side in the 3rd segment of the frog’s leg.
5. Cut neatly through the gastrocnemius and peroneus to expose the tibiofibula bone.
6. Cut neatly through all the ventral thigh muscles to expose the femur. You should also notice the
femoral artery (pink) and the femoral vein (blue). Which of these two supplies the leg with blood?
Which one drains the leg of blood? ___________________________________________________
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Figure 8. Ventral Muscles of Frog Thigh
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Figure 4
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Figure 5
Figure 6
Figure 7
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