Frog Dissection: Step-by-Step Labeled Guide

A frog dissection takes one to three class periods. You start with the outside of the frog and its mouth, then open the belly in two layers (skin, then muscle), identify the organs in place, and finally remove the digestive tract to see the kidneys, gonads and nerves. The brain and leg muscles are optional extras.

1. External anatomy and sexing the frog
2. The mouth: teeth, tongue and openings
3. Skin and body-wall muscles
4. The body cavity: liver, heart, lungs, digestive organs
5. Kidneys, bladder and reproductive organs
6. Nerves, leg muscles, brain and skeleton

For the full anatomy behind each step, with the skeleton, muscles and a frog vs human comparison, see our frog anatomy page.

You need a preserved frog, a dissecting tray, scissors, forceps, a probe, pins and protective equipment. Most school frog dissection kits contain everything except the specimen and the gloves. A double-injected frog (red latex in the arteries, blue in the veins) makes the blood vessels much easier to follow.

Materials

• Preserved frog: usually a northern leopard frog (Lithobates pipiens) or a bullfrog (Lithobates catesbeianus), ideally double-injected (arteries red, veins blue)
• Dissecting tray with wax or a cutting pad
• Fine scissors (one blunt-tipped blade is best), forceps and a blunt probe
• Scalpel or razor blade (only for the brain and skin of the head)
• 6–10 dissecting pins
• Hand lens or magnifying glass
• Nitrile gloves, safety goggles and a lab apron
• Paper towels, a wash bottle of water, and a labeled zip bag for storing the specimen between sessions
• Lab worksheet or the diagrams on this page

Safety

• Preserved frogs are stored in fluid that may contain formaldehyde, which irritates the eyes, nose and skin. Rinse the frog under running water, work in a well-ventilated room, and keep the fluid off your skin.
• Wear gloves and goggles the whole time. Never touch your face, eyes or mouth while working.
• Cut away from your body and your lab partner. Use scissors, not a scalpel, to open the body: lift the skin with forceps first so you cut only the layer you mean to.
• Hand sharps (scalpels, pins) to others handle-first and put them back on the tray, never loose on the bench.
• No food or drinks in the lab. Wash your hands with soap when you finish.
• Put all frog tissue in the waste container your teacher provides. Never put it in the sink or regular trash.

Open a frog in two layers. First cut only the skin, along the midline from the cloaca to the chin, with side cuts behind the forelimbs and in front of the hind limbs. Then cut the muscle wall separately, slightly to one side of the midline, and finally cut through the breastbone to reach the heart.

This is what a dissected frog looks like from the belly side: the three liver lobes with the gallbladder, the heart in front of them, the lungs at the sides, the stomach on the frog's left, the coiled small intestine in its mesentery, the yellow fat bodies, and the bladder and cloaca at the back.

Follow these steps in order. They are the same steps as in the 3D guide above, so you can open each one in the model to see what you should find.

1. 1. External anatomy

   1. Rinse the frog and lay it on its belly in the tray. Identify the head, trunk, forelimbs and hind limbs. Frogs have no neck and no tail.
   2. Find the large eyes and press gently on one: it sinks into the head. Lift the lower lid to see the thin, see-through nictitating membrane.
   3. Behind each eye, find the round tympanum (eardrum). Compare its size with the eye.
   4. Find the two nostrils (external nares) at the tip of the snout, the pale dorsolateral folds down the back, and the cloacal opening at the back end.
   5. Count the digits: four fingers on each hand, five toes on each foot. Spread the toes to see the webbing.
   6. Note the colour pattern: darker on the back with spots, pale on the belly.

   Identify: Skin, Eye, Nictitating membrane, Tympanum, External nares, Dorsolateral folds, Forelimb (arm), Hand, Thigh, Shank, Webbed foot, Cloacal opening.

2. 2. Male or female?

   1. Look at the inner finger (thumb) of each hand. A swollen, dark pad at its base, the nuptial pad, means the frog is male.
   2. Look behind the angle of the jaw for loose folds of skin: the vocal sacs of a male leopard frog.
   3. Females are usually larger and have no thumb pads. If you are not sure, you will find out when you see the gonads in step 9. Use the Female/Male switch to compare.

   Identify: Nuptial pad, Vocal sacs, Hand.

3. 3. Inside the mouth

   1. Open the mouth wide. If it is stiff, cut through the angle of the jaw on each side with scissors.
   2. Run a finger along the upper jaw to feel the tiny maxillary teeth. Feel the lower jaw: it has no teeth.
   3. On the roof of the mouth, find the two patches of vomerine teeth and, just beside them, the internal nares. Push a probe into a nostril and watch it come out of an internal naris.
   4. At the back corners of the roof, find the large openings of the Eustachian tubes, which lead to the eardrums.
   5. Lift the tongue with forceps: it is attached at the front and free at the back, with a forked tip.
   6. Behind the tongue, find the slit-like glottis on a small mound, and the wide opening of the gullet (esophagus) behind it. In a male, look for the vocal sac openings near the jaw angles.

   Identify: Buccal cavity, Maxillary teeth, Vomerine teeth, Internal nares, Eustachian tube openings, Tongue, Glottis, Vocal sac openings, Esophagus.

   

4. 4. Remove the skin of the belly

   1. Turn the frog on its back and pin it through the hands and feet, angling the pins away from the body.
   2. Lift the belly skin with forceps near the cloaca and make a small cut. Cut forward along the midline to the chin, keeping the scissor tips pointed up.
   3. Make side cuts behind the forelimbs and in front of the hind limbs, and pin the skin flaps back. Note how loosely the skin is attached: the space under it is a lymph sac.
   4. Look at the inside of the skin: it is full of blood vessels, because frogs breathe through their skin.
   5. Identify the muscles of the body wall: the rectus abdominis with the white linea alba down the middle, the external obliques on the sides, the pectoralis on the chest, and the mylohyoid under the throat.

   Caution: The abdominal vein runs along the inside of the linea alba. Cut the muscle wall slightly to one side of the midline so you do not cut it.

   Identify: Rectus abdominis, External oblique, Pectoralis, Mylohyoid, Abdominal vein.

5. 5. Open the body cavity

   1. Lift the muscle wall with forceps and cut through it a little to one side of the midline, from the pelvis forward to the chest.
   2. Cut carefully through the sternum and pectoral girdle to expose the heart. Make side cuts and pin the flaps back.
   3. If your frog is a female full of eggs, carefully lift out one ovary to make room.
   4. Without moving anything, identify the organs you can see: the three lobes of the liver, the heart in front of them, the lungs on each side, the yellow fat bodies, and the coiled intestines.

   Caution: Lift the body wall away from the organs before every cut. The tips of the scissors should always be visible.

   Identify: Liver, Ventricle, Lungs, Fat bodies, Stomach, Ileum, Abdominal vein, Ovaries.

6. 6. The heart and great vessels

   1. Find the heart inside its thin sac (pericardium) at the front of the body cavity. Snip the sac open.
   2. Identify the single cone-shaped ventricle and the two atria in front of it. The right atrium is the larger.
   3. Find the conus arteriosus leaving the front of the ventricle and branching into left and right trunks.
   4. Follow each trunk as it divides into three arches: the carotid (to the head), the systemic (around the gullet to the dorsal aorta) and the pulmocutaneous (to the lungs and skin).
   5. Lift the heart forward with a probe to see the thin, dark sinus venosus on its dorsal side, where the venae cavae enter.
   6. Optional heart dissection: remove the heart, cut it lengthwise and look for the spongy inner wall of the ventricle and the spiral valve in the conus.

   Identify: Ventricle, Right atrium, Left atrium, Conus arteriosus, Sinus venosus, Carotid arches, Systemic arches, Pulmocutaneous arches, Anterior venae cavae, Posterior vena cava, Pulmonary veins, Dorsal aorta.

7. 7. The digestive system

   1. Lift the liver lobes forward. Count the lobes and find the small green gallbladder between them.
   2. Find the gullet (esophagus) entering the stomach on the frog's left. Follow the J-shaped stomach back to the firm pyloric sphincter.
   3. Follow the duodenum forward from the pylorus. In the loop between the stomach and duodenum, find the thin, pale pancreas.
   4. Spread the coils of the ileum (small intestine) and see the thin mesentery holding them. Find the small dark-red spleen in the mesentery.
   5. Follow the wide large intestine back to the cloaca.
   6. Optional: cut open the stomach and look at its folded lining (rugae) and any food inside.

   Identify: Liver, Gallbladder, Esophagus, Stomach, Pyloric sphincter, Duodenum, Pancreas, Ileum, Mesentery, Spleen, Large intestine, Cloaca.

8. 8. The lungs and breathing

   1. Push the liver and heart aside to find the two lungs, dorsal and lateral in the front of the body cavity.
   2. Note how thin and spongy they are. Insert a straw or pipette through the glottis and blow gently to inflate them.
   3. Find the short larynx between the glottis and the lungs. Frogs have almost no trachea.
   4. Remember that the frog also breathes through its skin and the lining of its mouth.

   Identify: Lungs, Larynx, Glottis, Pulmocutaneous arches.

9. 9. Kidneys, bladder and fat bodies

   1. Cut the esophagus and the large intestine near the cloaca, and lift the digestive tract out of the body cavity (or fold it to one side).
   2. Find the two long, dark-red kidneys against the back wall, and the thin yellow-orange adrenal glands on their surface.
   3. Find the ureters (Wolffian ducts) running from the kidneys to the cloaca, and the thin, two-lobed urinary bladder at the back of the cavity.
   4. Find the fat bodies attached in front of the gonads.

   Identify: Kidneys, Adrenal glands, Ureters (Wolffian ducts), Urinary bladder, Fat bodies, Cloaca, Dorsal aorta, Posterior vena cava.

10. 10. Female reproductive system

    1. In a female, the two ovaries are lobed sacs of black-and-white eggs, often filling much of the body cavity.
    2. Find the long, white, coiled oviducts along the sides of the cavity. Trace one forward to its funnel-shaped opening (ostium) beside the base of the lung.
    3. Trace each oviduct back to its thin, expanded end, the ovisac, which opens into the cloaca.
    4. Trade frogs with a group that has a male so you see both sexes.

    Caution: Switch the model to Female to see these organs.

    Identify: Ovaries, Oviducts, Ovisacs, Fat bodies, Kidneys.

11. 11. Male reproductive system

    1. In a male, find the two small, yellowish, bean-shaped testes attached to the front of the kidneys.
    2. Sperm pass through tiny ducts into the kidney and then down the Wolffian duct, which carries both urine and sperm to the cloaca.
    3. Compare: males have no oviducts and much smaller gonads than a female full of eggs.

    Caution: Switch the model to Male to see these organs.

    Identify: Testes, Ureters (Wolffian ducts), Kidneys, Fat bodies.

12. 12. Spinal nerves and the sciatic plexus

    1. With the organs removed, look at the roof of the body cavity on each side of the backbone for white spinal nerves.
    2. Find the brachial nerve running out to each forelimb.
    3. At the back, find spinal nerves 7, 8 and 9 joining to form the sciatic plexus beside the urostyle.
    4. Follow the plexus into the thigh, where it continues as the sciatic nerve.

    Identify: Spinal nerves, Brachial nerve, Sciatic plexus, Sciatic nerve, Spinal cord, Urostyle.

13. 13. Muscles of the leg

    1. Remove the skin from one hind leg like pulling off a stocking.
    2. On the underside of the thigh, find the thin sartorius crossing diagonally, the adductor magnus beside it, and the gracilis muscles along the back edge.
    3. Turn the frog over. On the dorsal thigh, find the large triceps femoris at the front, the slender iliofibularis in the middle and the semimembranosus behind it.
    4. Separate the iliofibularis and semimembranosus with a probe to find the yellowish-white sciatic nerve between them.
    5. On the shank, find the big gastrocnemius (calf) and follow it to the Achilles tendon at the heel. Find the tibialis anticus and peroneus on the front and side of the shank.

    Identify: Triceps femoris, Sartorius, Adductor magnus, Gracilis major and minor, Iliofibularis, Semimembranosus, Sciatic nerve, Gastrocnemius, Achilles tendon, Tibialis anticus longus, Peroneus.

14. 14. The brain

    1. Turn the frog on its belly. Cut away the skin on top of the head between the eyes.
    2. With fine scissors or a scalpel, shave thin layers of bone from the top of the skull, starting between the tympana and working forward, until the brain shows.
    3. Identify, from front to back: the olfactory lobes, the two cerebral hemispheres, the diencephalon, the two rounded optic lobes, the narrow cerebellum and the medulla oblongata.
    4. If you can, lift the brain to see the optic nerves crossing underneath.

    Caution: The skull is thin and the brain is soft. Remove bone in tiny pieces; never push the scissors deep.

    Identify: Olfactory lobes, Cerebral hemispheres, Diencephalon, Optic lobes, Cerebellum, Medulla oblongata, Optic nerves, Spinal cord, Cranium (braincase).

15. 15. The skeleton (demonstration)

    1. Use a prepared frog skeleton or the 3D model. Count the vertebrae: eight presacral, one sacral, then the long urostyle. Note that there are no ribs.
    2. Find the skull bones: frontoparietals, nasals, upper jaw, squamosal, pterygoid and parasphenoid.
    3. Find the pectoral girdle and sternum, the humerus and the single radio-ulna.
    4. Find the long ilia, the femur, the single tibiofibula and the two long ankle bones (astragalus and calcaneum).

    Identify: Cranium (braincase), Frontoparietals, Upper jaw, Lower jaw, Presacral vertebrae, Sacral vertebra, Urostyle, Suprascapula, Pectoral girdle, Sternum, Humerus, Radio-ulna, Ilium, Ischium and pubis, Femur, Tibiofibula, Astragalus and calcaneum, Foot bones.

To dissect the frog heart, first identify it in place: the cone-shaped ventricle points backward, the two darker atria sit in front of it, and the conus arteriosus leaves the front of the ventricle. Then cut the large vessels and lift the heart out to look at it from both sides.

• Ventral side: right and left atria, ventricle, conus arteriosus and the truncus dividing into the carotid, systemic and pulmocutaneous arches.
• Dorsal side: the thin, dark sinus venosus, where the two anterior venae cavae and the posterior vena cava join before entering the right atrium.
• Inside: cut the ventricle lengthwise. Its wall is thick and spongy, with no dividing wall (septum). In a large frog, open the conus to find the twisted spiral valve.

Count the chambers as you go: two atria plus one ventricle make three. The sinus venosus and conus are extra parts but are not counted as chambers.

The frog brain is small and soft, so remove the skull roof in thin pieces from the top of the head. Start between the tympana, work forward to the eyes, and stop when you see the white and yellowish brain.

• Olfactory lobes, at the front, joined in the middle: smell.
• Cerebral hemispheres, two long smooth lobes: behavior and smell.
• Diencephalon, small, with the pineal body on top.
• Optic lobes, two large round lobes: vision.
• Cerebellum, a thin ridge across the brain: balance.
• Medulla oblongata, tapering into the spinal cord: breathing and heart rate.

Only the reproductive organs differ, and in a female full of eggs they change how the whole dissection looks. Check the thumbs first: dark nuptial pads mean a male.

• Female: two lobed ovaries packed with black-and-white eggs, often filling the body cavity; long, white, coiled oviducts along the sides, opening at a funnel (ostium) near the base of each lung and ending in thin ovisacs at the cloaca.
• Male: two small yellowish testes on the front of the kidneys; the Wolffian ducts carry both urine and sperm; vocal sac openings in the floor of the mouth; nuptial pads on the thumbs.

Both sexes have fat bodies, kidneys, adrenal glands, a urinary bladder and a cloaca. Trade frogs with another group so that everyone sees both sexes, or use the Female/Male switch on the 3D model.

Frogs are dissected because they are vertebrates with the same organ systems as humans, packed into a small body that is easy to open and see. In one specimen a student can find a brain and spinal cord, a heart, lungs, a liver with a gallbladder, a stomach and intestines, a pancreas, kidneys and gonads, and compare each with the human version. The frog's differences, such as a three-chambered heart, a cloaca and skin breathing, also teach how animals adapt to living in water and on land.

Frogs have been standard teaching animals for well over a century. The classic British lab guide by Milnes Marshall, first published in the 1880s and still in print in 1916, walks students through the frog's skin, mouth, body cavity, skeleton, muscles and nerves in much the same order that school worksheets use today. (Marshall & Gamble, The Frog: An Introduction to Anatomy, Histology and Embryology, 11th ed. (Internet Archive).)

Frogs are also central to the history of physiology. The frog sciatic nerve and calf muscle are so large and robust that they are still used to teach how nerves carry signals; MIT's quantitative physiology course, for example, begins with a lab in which students dissect out a living sciatic nerve. (MIT OpenCourseWare: frog dissection manual (sciatic nerve preparation).)

Many do, but fewer than before, and most schools now offer an alternative. Frog dissection is still common in US middle school (often 7th grade life science) and high school biology, but many states and school districts have dissection-choice policies that let students who object complete an alternative assignment, such as a virtual dissection, a model or a detailed diagram study.

Alternatives can teach anatomy about as well as dissection. A systematic review of 50 studies from 1968 to 2020 found that humane teaching methods, including models, videos, computer simulations and virtual reality, produced learning outcomes that were superior to those of harmful animal use in 30% of the studies, equivalent in 60% and inferior in 10%. Frogs were the second most common animal in the studies, after dogs. (Zemanova & Knight (2021): educational efficacy of humane teaching methods, Animals.)

That is why this page is built so it can be used either way: beside a real specimen, or on its own as a virtual frog dissection. The 15 steps, the labeled diagrams and the lab questions are the same for both.

Preserved frogs sold for dissection come from biological supply companies. They are usually northern leopard frogs or American bullfrogs, sometimes grass frogs of other species, and they are sold injected with colored latex (single- or double-injected) and stored in a preservative. Suppliers do not always say where or how their frogs were obtained, so specimen size, sex and condition vary from batch to batch.

Sourcing has an ecological side. Living animals bought from supply houses are usually not native to the place where they are studied, and teachers who release them can start invasions. Genetic studies linked red-legged frogs that damaged native frog populations in Alaska to frogs from Washington's Columbia Basin, and an Alaskan teacher had bought red-legged frogs from a Pacific Northwest supply house and released them in the 1970s. (Audubon (2013): teachers inadvertently spread invasive species.)

Bullfrogs themselves are an invasive species in many regions where they have been introduced. If your class keeps live frogs or tadpoles, never release them outdoors; ask your teacher how they will be cared for or disposed of. (Animal Diversity Web: Lithobates catesbeianus (American bullfrog).)

Older specimens were fixed in formalin, a solution of formaldehyde gas in water, typically about 37% formaldehyde by weight with some methanol. Formaldehyde irritates the eyes, nose and throat, can cause skin allergies, and is classed as a cancer hazard, which is why workplaces have strict exposure limits for it. (OSHA fact sheet: formaldehyde.)

Most school specimens today are fixed and then transferred to a holding solution with little or no free formaldehyde, but you should still treat every specimen as if it could irritate your skin and eyes: wear gloves and goggles, rinse the frog in water before you start, work in a ventilated room and wash your hands afterward. If the smell is strong or your eyes sting, tell your teacher.

The sciatic nerve is formed from spinal nerves 7, 8 and 9 on each side of the urostyle and runs into the thigh. In the classic guide you find it on the dorsal side of the thigh after separating the muscles there; the gastrocnemius forms the calf and ends in the Achilles tendon, which passes under the ankle into the sole. (Marshall & Gamble, The Frog: An Introduction to Anatomy, Histology and Embryology, 11th ed. (Internet Archive).)

If you dissect the nerve, handle it gently. Physiology manuals that use living preparations stress cutting parallel to the nerve, keeping it moist and never stretching it, and the same care keeps a preserved nerve intact for your teacher to check. (MIT OpenCourseWare: frog dissection manual (sciatic nerve preparation).)

These are the terms most middle school frog dissection worksheets use. Learn them before the lab and the steps above will make sense.

Anterior / posterior

Toward the head / toward the back end.

Dorsal / ventral

The back side / the belly side.

Amphibian

A vertebrate that usually starts life in water (as a tadpole) and lives on land as an adult.

Cloaca

The single chamber and opening for feces, urine and eggs or sperm.

Tympanum

The frog's eardrum, behind the eye.

Nictitating membrane

The see-through eyelid that covers the eye.

Vomerine teeth

Two small patches of teeth on the roof of the mouth.

Glottis

The slit that opens into the lungs.

Esophagus

The tube from the mouth to the stomach.

Mesentery

The thin sheet that holds the intestines in place.

Fat bodies

Yellow, finger-shaped fat stores used for energy.

Atrium / ventricle

Upper, receiving chambers of the heart / the lower, pumping chamber.

Oviduct

The tube that carries eggs from the body cavity to the cloaca.

Urostyle

The rod-shaped bone at the end of a frog's backbone.

Answer these during or after the dissection. Click a question to reveal the answer. Teachers can use them as a frog dissection worksheet; students who complete the virtual dissection can answer them too.

1. How did you decide whether your frog was male or female?

   From the outside: males have a swollen, dark nuptial pad on each thumb and vocal sacs behind the jaw. Inside: males have small yellow testes on the kidneys; females have ovaries full of black-and-white eggs and long, coiled oviducts.

2. How many fingers and toes does a frog have, and which feet are webbed?

   Four fingers on each hand and five toes on each foot. Only the hind feet are webbed.

3. What is the tympanum, and what connects it to the mouth?

   The tympanum is the eardrum, the round membrane behind the eye. The Eustachian tube connects the middle ear behind it to the back corner of the roof of the mouth.

4. Where are the frog's teeth, and what are they used for?

   Maxillary teeth line the upper jaw and two patches of vomerine teeth sit on the roof of the mouth. The lower jaw has none. The teeth grip prey; frogs do not chew, they swallow food whole.

5. Why is the tongue attached at the front of the mouth?

   So it can be flipped forward out of the mouth, catch prey on its sticky surface, and flip back in. A tongue attached at the back could not reach as far.

6. Name the openings you found in the mouth and what each leads to.

   Internal nares (to the nostrils), Eustachian tubes (to the middle ears), glottis (to the larynx and lungs), esophagus (to the stomach) and, in males, vocal sac openings (to the vocal sacs).

7. Why should you cut the body wall to one side of the midline?

   To avoid cutting the abdominal vein, which runs along the inside of the belly wall on the midline.

8. Which organ is the largest in the body cavity, and how many lobes does it have?

   The liver. It has three lobes in lab manuals: right, left anterior and left posterior.

9. Trace the path of food from the mouth to the cloaca.

   Mouth → esophagus → stomach → pyloric sphincter → duodenum → ileum (small intestine) → large intestine → cloaca → cloacal opening.

10. What does the gallbladder store, and where does it empty?

    Bile made by the liver. It empties through the bile duct, which passes through the pancreas into the duodenum.

11. How many chambers does the frog heart have? Name them.

    Three: the right atrium, the left atrium and one ventricle. The sinus venosus (on the dorsal side) and the conus arteriosus are extra chambers that are not counted.

12. Which atrium receives oxygen-rich blood from the lungs?

    The left atrium, through the pulmonary veins. The right atrium receives blood from the body through the sinus venosus.

13. Name the three pairs of arterial arches and where each takes blood.

    Carotid arches to the head; systemic arches to the body (they join to form the dorsal aorta); pulmocutaneous arches to the lungs and skin.

14. Frogs have no ribs and no diaphragm. How do they fill their lungs?

    By buccal pumping: they lower the floor of the mouth to draw air in through the nostrils, close the nostrils, and raise the floor to push the air through the glottis into the lungs.

15. What are the fat bodies for?

    They store energy (fat) for hibernation and for producing eggs or sperm in the breeding season.

16. What is unusual about the male frog's Wolffian duct?

    It carries both urine from the kidney and sperm from the testis to the cloaca.

17. What three systems empty into the cloaca?

    The digestive, urinary and reproductive systems.

18. Which spinal nerves form the sciatic plexus, and where does the sciatic nerve run?

    Spinal nerves 7, 8 and 9. The sciatic nerve runs down the dorsal side of the thigh between the iliofibularis and semimembranosus muscles.

19. Which brain lobes are largest in the frog, and what does that tell you?

    The optic lobes and cerebral hemispheres. Large optic lobes show that vision is very important to a frog, which hunts moving prey by sight.

20. Name two bones that are fused in the frog but separate in humans.

    The radius and ulna (fused into the radio-ulna) and the tibia and fibula (fused into the tibiofibula). The frontal and parietal bones are also fused into the frontoparietal.

The 3D viewer at the top of this page is a free online frog dissection simulation. Each of the 15 steps sets the skin, hides the layers you would have removed, opens the mouth when needed, and lists the structures to find; click any of them for its location and function. Students who opt out of a real dissection can work through the same steps, label the same diagrams and answer the same worksheet. It also works as a preview before a real lab, so that class time is spent finding organs rather than looking for them.