Frog Dissection


Most vertebrates are adapted for a life in one habitat, either on land or in water. The members of the class Amphibia live in both aquatic and terrestrial habitats at some point in their lives. The frog is a typical amphibian. The frog begins its life in the water, developing from an egg to a fishlike tadpole with gills and a tail. Eventually, the tadpole loses its tail and develops into an adult frog with lungs to breathe air and two pairs of legs for locomotion. Like all amphibians, frogs have an endoskeleton, are ectothermic, and have smooth, moist skins that take in oxygen..



        Explain the advantages of the frog's coloration.

        Identify and explain the functions of the structures in the mouth of a frog.

        Identify and label the internal organs of a frog.

        Describe the functions of the internal organs of a frog

In this investigation, you will study the body structure and internal organs of a frog to discover how amphibians are adapted for their dual lives



Text Box:  PRELAB


1. Answer Prelab questions 1 through 6 on the Lab Report.


View figure 33A-1 in lab instructions. Describe the incision you will make in the body wall of the frog.


(Read step #6 of Investigation Procedure) why will you have to repeat this incision?


(Read step #8 of Investigation Procedure) if you have a female frog, what will you see when you first open the frog? Why must these structures be removed before proceeding with the lab?


(Read step #9 of Investigation Procedure) what color will the liver appear __________ Gall bladder ___________?


Which organ do you think will be longer, the small or large intestine? Explain why.(this is your own opinion, you will find out the correct answer later)



(Read step #32 of Investigation Procedure) where will you find the kidneys?


2. Put on your plastic gloves and safety glasses.


3. Place a preserved frog in your dissecting tray. Observe and compare the coloration of the dorsal and ventral surfaces. Answer Prelab question 7 on the Lab Report.


Describe the difference in coloration of the dorsal and ventral surfaces of the frog. What do you think the advantage of these different skin colors might be?


4. Turn the frog so that its ventral surface (abdomen) is facing up. With scissors, make small cuts in the corners of the mouth where the upper and lower jaws meet.


5. Pry open the mouth as wide as possible. Locate the maxillary teeth by running your finger along the inner edge of the upper jaw. Find the two vomerine teeth in the roof of the mouth. Answer Prelab question 8 on the Lab Report.


What is the function of the maxillary and vomerine teeth?


6. Insert the tip of your probe into one of the external nostril openings. Follow the passageway to find the internal nostril opening.


7. In the back and near the sides of the upper jaw, find the openings of the eustachian tubes. Carefully insert the tip of your probe into one eustachian tube and follow it to see where it leads. Answer Prelab question 9 on the Lab Report.


To where does the Eustachian tube lead? What is its purpose?


8. At the back of the mouth, find the large, horizontal opening called the gullet. Use your probe to find the glottis, a vertical slit like opening just beneath the gullet. Answer Prelab question 10 on the Lab Report.


What is the gullet?


9. Examine the frog's tongue and note how it is attached. In a live frog, the tongue is covered with a sticky liquid. Answer Prelab question 11 on the Lab Report.


Describe the attachment of the tongue. How do you think the frog uses it to catch food?


10. Label the structures of the mouth on the diagram in item 12 of the Prelab section on the Lab Report.


Label the frog's mouth


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11. If you are leaving the laboratory, clean up your materials. Place your frog in a plastic bag. On a piece of masking tape, write your name, class, and date. Put the piece of tape on the bag and fasten it with a twist tie. Before you leave the laboratory, wash your hands thoroughly with soap and water; use a fingernail brush to clean under your fingernails.




1. Put on your plastic gloves and safety glasses.


2. Lay the frog in your dissecting tray with its ventral surface up and the head pointed away from you. Study Figure 33A-1 which shows the incisions you will make in the frog.


3. With forceps, lift the skin on the ventral surface of the frog near the hind legs. Insert the tip of your scissors through the skin where the hind legs meet.

4. Cut through the skin up the midline of the frog to the tip of the jaw.


5. Cut the skin laterally, just below the front legs and just above the hind legs as shown in Figure 33A-1. With your fingers, gently separate the skin from the underlying muscle tissue. Pull the flaps of skin as far as possible to the sides of the frog.


6. Using forceps lift the muscle layer on the ventral surface of the frog near the hind legs. With scissors, cut through the muscle layer, following the same incision pattern you used for the skin. When you reach the point between the front legs, you will be cutting through a bone that protects the heart. Be sure to keep the tips of your scissors tilted upward to prevent damaging the organs below.


7. When you have finished making your incisions, carefully pull the flaps of muscle to the sides. With pins, fasten the skin and muscle layers to the dissecting tray.


8. Observe the organs as they are positioned in the body cavity. If you have a female frog, the body cavity may be filled with tiny black and white eggs. With forceps, remove the eggs and place them in a corner of your dissecting tray.


9. At the anterior end of the frog, find the large, greenish-brown liver. Lift the lobes of the liver to locate the green, pea-shaped gall bladder. With scissors, carefully remove these organs. Answer Investigation questions 1and 2 on the Lab Report.


How does the liver aid in digestion?


What does the gall bladder do?


10. Locate the heart, a triangular-shaped organ lying between the forelimbs at the anterior end of the frog.


11. Surrounding the heart, you will notice a thin membrane called the pericardium. With a scalpel, cut the pericardium away from the heart. Do not cut through the blood vessels joining the heart. CAUTION: The scalpel is very sharp. Use it carefully.


12. Study the heart. Find the atria, the dark-brown structures, which make up the upper portion of the heart. Beneath the atria, observe the light-brown, cone-shaped ventricle. Answer Investigation questions 3 and 4 on the Lab Report.


Name the chambers of the frog's heart.


Describe how the blood flows through the heart of a frog.


13. With your probe, gently press on one of the atria and then on the ventricle. Note the muscularity of their walls.


14. On the ventral surface of the heart near the top scrape away soft tissue until you locate a Y-shaped artery that connects with the ventricle. This artery called the conus arteriosus, branches and forms two arches above the heart, which reunite below the heart to form the dorsal aorta.


15. Try to locate the major blood vessels in the frog. You may have to move the lungs to find some of them. Label the ventral view of the heart on the diagram in Investigation item 5 of the Lab Report. With scissors, cut through the blood vessel that joins the heart and remove the heart from the frog. Lay the heart in the dissecting tray so that the dorsal surface of the heart faces up. Observe the triangularly shaped sac called the sinus venosus.


16. Label the dorsal view of the heart in the diagram on the Lab Report.


Ventral and Dorsal Views of a Frog's Heart


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17. With your scalpel, make a longitudinal cut through the heart as shown in Figure 33A-3. Separate the halves of the heart.

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18. With your probe, carefully remove the dried blood filling the atria until you can see the atrial walls.










19. Compare the atria and the ventricle. Notice the thickness of the walls and the capacity of each chamber. Answer Investigation question 6 on the Lab Report.


Compare the thickness of the atria and ventricle wall and explain the difference


20. Locate the lungs, two small, black, saclike structures on either side of the frog's heart.


21. Insert the tip of an empty medicine dropper into the glottis in the frog's mouth. Squeeze the bulb and observe what happens to the lungs. Label the lungs on the diagram of the internal organs in Investigation item 7 of the Lab Report. Answer Investigation questions 8 and 9.


Label the internal organs of a frog


Text Box:  Compared to the size of a frog's body, its lungs are quite small. Does the size of a frog's lungs affect its ability to take in oxygen? Explain your answer.


What is the glottis?












22. Locate the large, muscular stomach. Follow the stomach anteriorly to find the short esophagus. Answer Investigation question 10 on the Lab Report.


What happens to food in the stomach of the frog?


23. Follow the stomach posterior to find the narrow, tube like small intestine. Answer Investigation question 11 on the Lab Report.


In the frog, what is the job of the small intestine?


24. Find the light-brown pancreas, which is held in the mesentery between the stomach and the intestine. In the mesentery near the stomach, also find a brown, bean-shaped organ called the spleen. The spleen produces red blood cells and filters out old blood cells. Answer Investigation question 12 on the Lab Report.


Does food ever enter the pancreas? How is the pancreas involved in digestion?


25. Find the yellow, flame-shaped, fat bodies. Notice where these structures are attached. Answer Investigation question 13 on the Lab Report.


What is the purpose of the fat bodies? Why are these structures important to the frog?


26. Trace the small intestine to the point where it becomes a wider tube. This is the large intestine. Compare the lengths and diameters of the small and large intestines. Answer Investigation questions 14 and 15 on the Lab Report.


Why is the small intestine is called "small" when it is actually much longer than the large intestine?


Give two reasons that might explain why the small intestine is so long.


27. The large intestine leads to the cloaca, the slightly enlarged portion of the digestive tract, which is just anterior to the cloacal opening or anus.


28. Remove the digestive tract by cutting through the anterior end of the esophagus and the posterior end of the large intestine.


29. With your fingers, carefully tear away the mesentery to free the digestive organs. Stretch out the digestive tract. Notice that it is actually just one long tube.


30. With your fingers, press on the stomach and note how it feels. With scissors, make an incision along the outer curve of the stomach. Spread the stomach walls apart. Study the contents of the stomach. Observe the texture of the stomach lining. Answer Investigation questions 16 through 18 on the Lab Report.


Why is the stomach such a muscular organ?


Describe the types of food you were able to identify in the contents of the stomach.


Describe the appearance of the stomach lining. How is the lining adapted to its function?


31. Label the digestive system of the frog on the diagram of the internal organs on the Lab Report.


. Label the internal organs of a frog


Text Box:  32. Now that the digestive tract has been removed, you should be able to see the excretory and reproductive systems. Locate the long, brown kidneys on either side of the backbone. Answer Investigation question 19 on the Lab Report.


What role do the kidneys play in excretion?


33. Find the urinary bladder, which looks like a transparent sac next to the large intestine. Connecting the kidneys to the bladder, are tiny, threadlike tubes called ureters. Try to find these.









34. Label the kidneys, ureters, urinary bladder, and cloaca on the diagrams of the male and female excretory and reproductive systems in Investigation item 20 of the Lab Report. Answer Investigation question 21.


Male and Female Excretory and Reproductive Systems


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Through which organ is liquid waste eliminated from the frog?


35. Find the reproductive organs in the frog. If you have a female and have removed the eggs, you also have removed the ovaries. Find the white, coiled oviducts and follow them down to the posterior end of the frog. Notice which structure they empty into. If you have a male frog, find the testes, small oval structures on top of the kidneys. Answer Investigation questions 22 and 23 on the Lab Report.


Describe the pathway an egg takes as it exits the body of a female frog.


Describe the pathway that sperm travel from the testes out of the frog.


36. Make sure you have labeled the ovaries, oviducts, and testes on the diagram of the excretory and reproductive systems in the Lab Report.


37. When you have finished dissecting your frog, dispose of it in the manner indicated by your teacher. Carefully wash and dry your dissecting tools and tray.


38. Before you leave the laboratory, wash your hands thoroughly with soap and water.



1. Carefully remove all of the skin from one hind leg. Identify all muscles present, move the leg and observe how the muscles respond.

2. Remove all of the muscles from the hind leg. Identify the major bones, move the leg and observe how the bones respond.

3. Study the brain of a frog (with permission of instructor). With a scissors, remove the skin and underlying muscle tissue from the dorsal surface of the frog's head. Now you should be able to see the frog's skull. Carefully shave away the bony skull between the eyes, using your scalpel, until you can see the brain. With forceps, remove other pieces of the skull to give you a clearer view of the brain. Refer to the illustration in your text or on computer to help you identify the parts of the brain.




1. Compare and contrast fish and amphibian body structures. (Use textbook/internet)


2. If you dissected a tadpole, what differences would you find from what you saw in the adult frog?


3. Describe where and how a frog might live during the change from tadpole to adult. Explain your reasoning.


4. Below using your notes and textbook, list the 3 major orders of the amphibian phylum, an example for each class and typical traits. (Use textbook, internet and notes)




Typical Traits



















5. Using notes and textbook describe the function of the following organs/structures.

Small Intestine


Large Intestine




Villi of small intestine