2nd Quarter Science Project
PROBLEM: What are the external an internal features of the frog?
1. To be acquainted with the external anatomy of the frog.
2. To be acquainted with the internal anatomy of the frog.
3. To locate the structures, organs, and systems of the frog.
4. To assess the function of structures from observing the actual anatomy of the organism.
5. To learn and practice dissection technique.
Materials and Equipment
– Dissection tray
Frogs are part of phylum Chordata and are in the class Amphibia. Although the salamander might be more “typical” amphibian, the frog is fun to dissect and a good learning experience.
Introduction to Dissection
1. To successfully follow dissection, it is essential to be familiar with the following terms:
Dorsal – the back or upper surface of an organism
Ventral – the stomach or lower surface of an organism
Anterior – head end of an organism
Posterior – tail end of an organism
2. Dissecting involves the use of sharp cutting instruments like the scalpel and scissors. Use care!
3. Important: Whenever using scissors to cut into a specimen, make sure to keep the tip of the scissors pointed up so as not to dig down into the specimen, damaging the organs to be viewed.
4. Gloves: It is advisable to buy gloves and use them for dissecting.
5. Making and labeling drawings for dissection labs:
a) Make drawings as accurate as possible to what you actually see.
b) When adding labels, the label line should be straight and should not cross each other. The line should not have arrows on them and should go directly to the object they indicate and touch it or be drawn into it. Although the label lines may be horizontal, diagonal, or vertical, the label writing must always be horizontal. Refer to the diagram above.
Part A: External Features
1. Place a frog, dorsal side up, in a dissecting pan. You will be finding and identifying distinctive structures. Refer to the diagram of the external structures as needed.
2. Locate the following structures on your own specimen and label them on the picture in question 9 of Part 1 of the questions section: eyes, nostrils, tympanic membrane, nictitating membrane, thumb, foreleg, and webbed hindleg.
3. Carefully examine the legs of the frog: Record the answers to the questions in Part 1 of the questions section.
a. Measure the length of the foreleg and hindleg.
b. Measure the length of the whole frog from nose to legs stretched out behind.
c. Count how many digits there are on the foreleg and hindleg.
d. Check to see if the forelegs are webbed. Check to see if the hindlegs are webbed.
e. Locate the thumbs on each foreleg. In males, the thumb is thickened and large.
4. Focus on the head region. Look carefully at the bulging eyes. Notice how they are situated, to enable the frog to see to the front and to the sides. Also, find the nictitating membrane – a transparent eyelid that moves from the bottom of the eye to the top. What is the purpose of this eyelid? Record your answers in Part 1.
5. The tympanic membrane is a circular membrane located below the eye. What is the purpose of this membrane?
6. Examine the mouth of the frog. To open the mouth wide, use the scissors to cut the hinges joints at both corners of the mouth. Spread the mouth open. Refer to the diagram of the mouth to find the following structures:
a. Find the tongue. Locate where it is attached to the floor of the mouth.
b. Find the glottis, gullet, esophagus, Eustachian tubes (on the sides of the upper jaw), vocal sacs (on the sides of the lower jaw), nostrils (externally and internally), and teeth.
c. The gullet is the opening into the esophagus.
d. Look to see if there are vocal sacs. If not, perhaps your specimen is a female. Only males have these openings which are used for croaking.
e. Locate the external and internal nostrils. Use a probe to stick through the nostrils from the outside in.
f. Find two sets of teeth. Rub your finger along the top jaw to feel the maxillary teeth. Find the vomerine teeth located on the roof of the mouth.
Part B: Internal Features
Note: Use goggles.
7. Place the frog ventral side up on the dissecting pan/tray.
8. Refer on the dotted lines on the diagram at the bottom. Using your scalpel, make a small opening through the skin slightly anterior of the anus. Insert the scissors and cut anteriorly to the tip of the lower jaw. (Make sure you are only cutting the skin.) Make additional cuts across the bottom of the forelegs and the top of the hindlegs extending the cuts to the mid-body. Cut the two flaps of skin off, exposing the muscle layer. Cut away the skin between the forelegs and the lower jaw also. Examine the skin. Look at the underside of the skin. Answer question 9 of Part 2 of the questions section.
9. To expose the cavity, it is necessary to cut away the muscle layer. To do this, repeat the procedure for cutting away the skin. Make an incision just anterior of the anus and follow the same cutting pattern. You will find that it will be more difficult to cut along the middle up to the lower jaw because when you reach the fore legs, you must cut through the sternum (breastbone). Continue cutting, using the pattern for the skin, until you have cut away the muscle tissue, exposing the organs.
Do not cut too deeply. It is essential to keep your scissor tips pointing upward while cutting to avoid damage to the internal organs and insuring that you are only cutting the muscle layer.
10. If your specimen is a female, when the body cavity is exposed, you may see a mass of black and white eggs. You will need to remove these carefully in order to locate the other organs. To remove, lift it up with your fingers and find the place where they are attached. Work them by pinching them off from that attachment and pulling them out. (Also note, you may still have a female specimen even though there are no black and white eggs present.)
11. Once the interior structures are exposed clearly, start to locate the structures of the different systems of the frog. Label the diagram in Part 2 of the questions section. Refer to the diagram of the internal structures of the frog if necessary.
12. When the frog ingests its food, it passes along the esophagus to the stomach. From the stomach, the food passes through the small intestines, through short large intestines where indigestible food passes into the cloaca and then is eliminated from the body through the anus. The cloaca is a versatile organ, being the passageway for wastes, bot solid and liquid, as well as the reproductive gametes, the sperm or eggs.
13. The most prominent organ you will see is a large, reddish multi-lobed organ, the liver. Gently lift up the lobes of the liver and find a small greenish sac, the gall bladder. Remove the live by feeling under it to find its attachment and gently pinch it off there and pull it out. Count how many lobes it has, record your answer in Part 2 of the questions section.
4. The stomach, a beige organ should be visible now. Follow it anteriorly to find the esophagus and posteriorly to find the small and large intestine and the cloaca.
15. Locate the pancreas—a dark, grainy flat organ that lies between the stomach and the small intestines.
16. The spleen is located along the intestines. It is a small, dark, round organ.
17. Carefully: Remove the digestive system by making a cut at the esophagus and then pulling up carefully on the stomach and along the intestines to the urinary bladder (looks like a clear, deflated balloon). Cut just anterior of the urinary bladder. If there are mesenteries (a clear, stringy-like membrane that holds body structures in place), tease them carefully away from the organs with a probe. Gently pull the organs out in one piece. *If you do not do this carefully, you could damage structures of the excretory and reproductive systems.
18. Cut open the stomach to see if there is any recognizable food left there.
19. Label the following structures on the diagram in Part B: liver, gall bladder, stomach, esophagus, small intestines, large intestines, cloaca, pancreas, spleen, and anus.
20. The frog’s three-chambered heart is the central organ of the circulatory system. Its two atria and one ventricle pump blood through the system of veins and arteries, much like a mammalian heart. The atria are soft in texture and the ventricles are muscular.
21. Locate the heart enclosed in its special sac, called the pericardium. With a probe, tease away the pericardial sac from the heart.
22. The frog receives oxygen in three ways, through its skin, through the lining in its mouth and through the lungs. When it does not need much oxygen, breathing through its skin is sufficient; if more oxygen is needed, it can supplement its oxygen supply through its mouth lining, and for maximum need, the frog’s lungs are added.
23. The lungs, two filmy or spongy organs, lie dorsal to the heart. They are connected to the trachea which opens into the mouth cavity. Find the lungs. The trachea can be found by inserting a probe down the glottis. Label the following structure on the picture in Part 2: lungs and trachea.
24. The wastes and excess water are filtered by the kidney and then travel through the ureters to the cloaca and finally to the urinary bladder where it is stored until eliminated.
25. The kidneys are located under the reproductive structures, and are attached to the dorsal wall by the mesentery. Carefully remove the mesentery from one of the kidneys.
Trace the excretory system by following the ureters, found at the posterior end of the kidneys, to the cloaca and the urinary bladder. The urinary bladder looks like a deflated transparent sac usually pressed against the body wall.
26. Label the kidney, ureters, cloaca, and urinary bladder on the picture in Part 2.
27. In the female frog, the ovaries sit above the kidneys as a large, lobed structure. When the ovary fills with eggs, it bursts, spilling the eggs into the body cavity. The eggs travel down the oviducts to the uterus where they are stored until expelled through the cloaca. The male frog has two oval testes. The sperm they produce travel through the kidneys to the cloaca.
28. For a male: Locate the mass of yellow feathery fat bodies. Attached to their posterior end are the small yellowish oval testes. Lift one of the testes to see if you can
locate the thin coiled tubules that connect it to the kidneys.
For a female: If there were not a mass of black and white eggs, the ovaries look like
lumpy sacs located between the yellow fat bodies and the kidneys. The oviducts are
thin and coiled leading to the uterus. If there were a mass of eggs when you first
exposed the body cavity, examine the area around the yellow fat bodies for what might
be left of the coiled oviducts or ovaries.
29. Carefully remove the reproductive structures.
30. Label the testes or ovary (eggs), oviducts on the picture in Part 2.
31. The frog’s nervous system is made up of the central nervous system consisting of the brain and spinal cord, along with the peripheral nervous system which are all the nerves that transmit impulses to the sense organs and the muscles. The brain has five lobes: the cerebrum, the optic lobes, the cerebellum, olfactory lobes, and the medulla oblongata.
26. The brain is well protected so can be a
challenge to expose. First remove the skin
from the dorsal side of the head. Crack the
skull (without smashing the head) and chip
away the skull to reveal the brain.
Questions for Dissection: Frog
Part 1: External Features
1. Answer the following questions:
a. What is the length of the foreleg?__________ Hindleg?__________
b. How do they compare and why?
c. What is the length of the frog’s body? _______________________
d. What is the ratio of the frog’s hind legs to its body length? _______
e. How many digits are on the foreleg? _______ Hindleg?_________
f. Are the forelegs webbed?________ Are the hindlegs webbed?______
2. What is the purpose of the nictitating membrane?
3. What is the function of the tympanic membrane?
4. Why is the tongue attached where it is?
5. Why does the gullet, the opening into the esophagus, have to be so big?
6. Do female frogs croak? Why or why not?
7. What do the maxillary teeth feel like?
What do the volmerine teeth feel like?
8. Fill in the pictures of the external structures with the appropriate labels: eyes, nostrils, tympanic membrane, thumb, nictitating membrane, foreleg, hind leg, and webbed hind foot.
Part 2: Internal Features
1. Is the skin thick or thin? Are there a lot of blood vessels under the skin? Why is this important?
2. How many lobes does the liver have?
3. What part does the pancreas and spleen play in digestion?
4. Draw the brain in the space below and label the olfactory nerve, olfactory lobe, cerebrum, diencephlon, optic lobe, cerebellum, fourth ventricle, medulla oblongata, and spinal cord.
5. Fill in these pictures of the internal structures with the appropriate labels as you can: liver, gall bladder, stomach, esophagus, small intestines, large intestines, cloaca, pancreas, spleen, heart, lungs, kidney, urinary bladder, ovary, and oviduct.