LAB: Frog Dissection
Pictures: Modern Biology, Holt
Materials:
• optional gloves, preserved frog, dissection kit (tools), dissecting pan
Procedure: Part 1 – External Observations
1. Place a frog in your dissection pan. To determine the frog’s gender, 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.
2. Use the diagram below to locate and identify the external features of the frog’s head:
3. Cut the hinges of the mouth and open it wide. Use the diagram below to locate and identify the structures inside the frog’s mouth. Use a probe to help find each part:
vomerine teeth, maxillary teeth, internal nares, tongue, openings to the Eustachian tubes, esophagus, pharynx, and the slit-like glottis.
Both types of teeth are used for holding food in the mouth, frogs swallow their food whole and do not chew. In the pharynx, there are several openings: one into the esophagus ( the tube into which food is swallowed); one into the glottis (through which air enters the larynx, or voice box); and two into the Eustachian tubes (which connect the pharynx to the ear, and are used to equalize pressure in the inner ear while the frog is swimming).
4. Turn the frog on its back and pin down the legs.
5. Look for the opening to the frog’s cloaca, located between the hind legs. Use forceps to lift the skin and use scissors to cut along the center of the body from the cloaca to the lip. Turn back the skin, cut toward the side at each leg, and pin the skin flat. The diagram below shows how to make these cuts:
7. Once the body cavity is open, locate each of the organs below. Check the box (on your data sheet) to indicate that you found the organs. Use the diagrams below for reference if needed:
1. GENERAL
Fat Bodies –Feathery-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.
2. DIGESTIVE SYSTEM
Liver--The largest structure of the 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.
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 swallow their meals whole. Follow the stomach to where it turns into the small intestine. The pyloric sphincter valve regulates the exit of digested food from the stomach to the small intestine.
Pancreas –Look along the top edge of the stomach, there will be a thin pinkish/ red curved tube attached. This is the pancreas, which makes Insulin to aid in digestion. (hint: it kind of looks like a skinny tongue)
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.
Large Intestine--As you follow the small intestine down, it will widen into the large
intestine. The large intestine is also known as the cloaca in the frog. The cloaca is the last stop before wastes, sperm, or urine exit the frog's body. (The word "cloaca" means sewer)
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 frog’s mouth and find the esophagus, poke your probe into it and see where it leads.
3. CIRCULATORY & RESPIRATORY SYSTEMS
Heart - at the top of the liver, the heart is a triangular structure. The Amphibian heart has
3 chambers: The left and right atria (receiving chambers) can be found at the top of the
heart. Blood is brought to these chambers by veins. A single ventricle (pumping
chamber) can be located at the bottom of the heart. The oxygenated blood is pumped into arteries, which are blood vessels that carry blood away from the heart and to the rest of the body.
Lungs - Locate the lungs by looking underneath and behind the heart and liver. They are two spongy organs (Hint: they look like deflated balloons). Each lung is connected to the atria of the heart to deliver oxygenated blood
4. URINARY & REPRODUCTIVE SYSTEMS: Use a probe and scissors to lift
and remove the intestines and liver to expose the organs underneath.
The frog’s reproductive and excretory systems are combined into one system called the Urogenital system. The Excretory / Urinary system consists of the frog’s kidneys, ureters, bladder, and cloaca. The Reproductive organs include the testesandsperm ductsin themale;and theovaries, oviducts, and uteriin the female.
Use the above diagrams to identify the parts of the urinary and reproductive systems. Remove the peritoneal membrane, which is connective tissue that lies on
top of the red kidneys. Find the ureters; the urinary bladder; the testesand
sperm ductsin themale;and theovaries, oviducts, and uteriin the female. 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. Often the top of the kidneys have yellowish stringy fat bodies attached.
Ureters – skinny tubes that extend downward from each kidney. These carry liquid
waste to the bladder.
Bladder - An empty sac located at the lowest part of the body cavity. The bladder stores urine.
Cloaca - mentioned again as part of the urogenital system - urine, gametes (sperm or eggs) exit from the body here.
Depending on the gender of your frog, locate:
Testes - in male frogs, these organs are located at the top of the kidneys, they are pale colored and roundish.
Oviducts - females do not have testes, though you may see a curly-q type 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.
After all organs / structures have been found, you need to clean up.
Remove any pins and return to your tool box.
Dispose of all solid frog parts into the trash can.
Rinse and thoroughly DRY your pan.
Wipe / rinse / dry any tools if necessary.
Return all tools to your tool box.
Clean up your work area and wash your hands
