Frog Dissection
Background Information:
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.
Dissecting Materials:
Dissecting pan
Dissecting tools (scalpel, scissors, probes, forceps)
Frog dissection packet and Frog diagrams
Frog dissection answer sheet
The External Anatomy and Oral Cavity:
1. Rinse a preserved frog well with water. Place it ventral (belly) surface down in your dissecting pan.Measure and record the length of your frog (cm).
2. Note the arrangement of the spots and the coloration of the frog. The color of the frog is caused by scattered granules in the epidermis and chromatophore cells in the dermis. Chromatophore cells are cell that contain pigments .Describe the coloration of your 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?
3. The body is divided into the head and trunk. Label each of these sections on your external diagram.
4. Touch the frog! Pick it up, squeeze it, feel it. Does it feel like the frog has bones similar to the bones humans have? Does the frogs skeleton feel harder or softer than the bones in your arm? Does the frog’s skeleton feel more like your rib bones or the top of your ear? Describe the texture of your frog by answering the above questions.
5. Locate the thin membrane that covers the eye from below. This is the nictitating membrane, which protects the eye when the frog is underwater and keeps it moist when the frog is on land. Label the nictitating membrane on your external diagram.
6. Notice the large tympanic membranes behind the eyes. These membranes function as eardrums to receive sound waves. Label the tympanic membranes on your external diagram.
7. Examine the forelimbs and hindlimbs of your frog, noting the number of digits on each limb.If your frog is male, it will have roughened pads near the thumbs. These are used to hold the female during amplexus. Determine the sex of your frog.
8. Open the mouth by cutting the jaws of your frog with the scissors.
9. Locate the maxillary teeth around the edge of the upper jaw. Maxillary teeth are not used for chewing but are instead used for holding and grasping prey.Label these teeth on your oral cavity diagram.
10. Locate the slitlikeglottis at the back of the throat. This opening leads to the respiratory system. Above the glottis is the opening to the esophagus. What is the function of the glottis?Label the glottis on your oral cavity diagram.
11. Find the Eustachian tubes at the posterior corners of the upper jaw. Eustachian tubes helped to equalize pressure within the ear. Gently probe these structures with your forceps to find out where they lead (externally). Label the Eustachian tubes on your oral cavity diagram.
12. If your frog is male, locate the opening that leads to the vocal sacs at the widest corner of the lower jaw. These amplify the male’s mating call. If applicable, label the vocal sacs on your oral cavity diagram.
13. Notice the shape of the tongue and where it is attached in the mouth of the frog. The tongue can be flipped forward rapidly in order to catch prey. Label the tongue on your oral cavity diagram.
14. Locate the nostril openings (nares) in the roof of the mouth. Between the nostril openings are two vomerine teeth. Feel these teeth and the maxillary teeth with your fingers. What is the function of the vomerine teeth?Label the internal nares and the vomerine teeth on your oral cavity diagram.
The Internal Anatomy:
15. Place your frog dorsal surface down in the dissecting pan. Open the frog by making a mid-line cut down the ventral, abdominal surface of your frog. Then make two transverse incisions: one at the forelimbs and one at the hindlimbs. You should cut through the skin and body wall muscles, avoiding damage to the internal organs.
16. Pin the skin and muscles down to the dissecting tray. Note, if your frog is female and contains black eggs, they must be removed carefully before the internal organs can be observed.
17. Locate the esophagus as seen in the oral cavity. Pass a probe from the esophagus into the stomach. Note that the lower end of the stomach is constricted. This constriction is the pyloric valve. It regulates the amount of food that enters into the small intestine.Label the esophagus, stomach and pyloric valve on your internal diagram.
18. Cut open the stomach and observe its lining. If food is present in the stomach, try to intentify the contents. Usually insect body parts can be seen. What is the name of the folds found within the stomach? What is the purpose of such folds?
19. Follow the digestive tract beyond the pyloric valve to the coiled small intestine. Label the small intestine on your internal diagram. Cut open the lower part of the small intestine. Note the villi, the many small folds in the lining of the small intestine. What is the purpose of the folded villi?
20. 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 where it expels materials from the digestive, reproductive and excretory systems.Label these structures on your internal diagram.
21. Note the large brown liver.What are the functions of the liver? How many lobes make up the frog’s liver? Lift the lobes of the liver to locate the greenish gallbladder. The gallbladder stores bile that is secreted by the liver. Bile aids in the digestion of fats.Label the liver and gallbladder on your internal diagram.
22. Locate the pancreas. The pancreas is a soft, irregular, pinkish organ that produces digestive enzymes, found lying in a membrane between the stomach and small intestine. What is the function of the pancreas? Label the pancreas on your internal diagram.
23. 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 nare open, and the throat contracts, forcing the air out through the nares.Locate the pinkish-gray lungs. Label the lungs on your internal diagram. Sequence the passage of air into and out of a frog.
24. Notice the three-chambered heart between the lungs and posterior to the trachea. The pointed ventricle is lighter in color than the two thin-walled atria. Label the heart’s chambers on your internal diagram.
25. Using your forceps, gently lift the stomach and find the spleen, a round reddish organ. The spleen filters the blood, taking out improperly functioning red blood cells. Label the spleen on your internal diagram.
26. 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, which secretes hormones. The kidneys filter the blood and produce urine, which drains into the urinary bladder, a thin-walled bag that attaches to the cloaca.How do wastes get from the kidney to the external environment?Label the kidneys and urinary bladderon your internal diagram.
Reproductive System:
27. 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. Locate in a male frog the white testes that can be found close to the kidneys. Look at the reproductive organs of both sexes. Label the appropriate gonads on your internal diagram.
28. 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?
If you have extra time:
29. Turn your frog so that the dorsal side once again faces up. Insert the point of your scissors through the skin at the base of the head and remove the skin from the head area. Bend the frog to determine the approximate region of the “neck.” Insert your scissors and clip across the upper spinal cord in the region of the neck. Locate the white spinal cord enclosed within the vertebrae.
30. 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. Locate the forebrain, midbrain, and hindbrain regions.