Frog Lab
Purpose:
Locate and identify the various structures of the frog’s major systems and relate the internal
structures of the frog to their respective functions.
Background: None
Variables:
Different frogs
Materials:
Preserved frogs, scalpels (WARNING: scalpels are sharp and dangerous, handle with care),
scissors, forceps, dissecting trays, probes, plastic bags and dissecting pins.
Procedures:
Part 1 “External Structures, the Mouth and the Brain”
External Structures & the Mouth
1. Obtain a preserved frog and a plastic bag from your instructor. Put your group’s surgeon’s name, and the
period # on the plastic bag. Place your frog on its ventral side (stomach) down on the dissecting tray.
2. Identify the dorsal, ventral, anterior and posterior regions of the frog and answer question 1 from data and
results section.
3. Locate the forelegs and the hind legs. Examine the hands of the frog. If the hands have enlarged thumbs,
the frog is a male. Answer questions 2-4.
4. Locate the two large, protruding eyes. Lift the outer, upper eyelid using a probe. Beneath the upper lid is an
inner lid, attached to lower eye called the nictitating membrane. Answer question 5.
5. Posterior to each eye is a circular region of tightly stretched skin. This region is the tympanic membrane, or
eardrum. Locate the tympanic membrane and answer question 6 and 7.
6. Just behind the tip of the frog's head are two small openings called external nares or nostrils. Answer
question 8 and 9.
7. Besides answering questions in the data and result section you will also be required to correctly label several
figures. In the data collection section of write-up label structures A-I from figure 1 below.
a. Figure 1: Anterior, posterior, dorsal, ventral, external nares, eye, nictitating membranes, tympanic
membrane, forelimb, hand, hind limb, webbed foot, and mouth.
Figure 1
8. Pry open the mouth of your frog with your fingers and insert the scissors into the corner of the mouth.
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9. Cut through the jawbone at the corner of the mouth and cut back about 1/2"-3/4", which will loosen the
jaw.
10. Repeat the previous 2 steps for the other side of the mouth.
a
The tongue is the most noticeable structure in the mouth. Observe where the tongue is attached and
note the two projections at the free end. Answer question 10.
11. At the back of the mouth, locate the large horizontal opening, the gullet or esophageal opening. In front of
the gullet is a small fold of skin with a vertical slit in it, the glottis. Answer question 11 and 12.
12. Look for two openings on the back, sides of the floor of the mouth. These are the openings to the vocal
sacs. They are present in male frogs but not in female frogs, also your cut may go through this hole.
13. Examine the roof of the mouth. Near the front center of the roof of the mouth are two small bumps. These
bumps are the vomerine teeth. On either side of the vomerine teeth are the openings of the internal nares
or nostrils. Run your fingers along the outer edge of the top jaw. These small teeth are the maxillary teeth.
Answer question 13.
14. The openings of the Eustachian tubes are on either side near the back of the mouth. Insert a probe into an
opening of one Eustachian tube. Gently determine where the probe stops. Answer question 14.
15. Label structures A-H on fig. 2 in your data collection section.
a
Figure 2: Internal nares, vomerine teeth, maxillary teeth, gullet opening, glottis, eustachian tubes
openings, vocal sac openings (males only) and tongue.
Data Analysis:
Part 1
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
Which side of the frog is darker? Why? *Hint frogs are ectothermic.
List two visible differences between the forelimbs and hind limbs. (2 pts)
Why are the hind and forelimbs different?
What is the sex of your frog?
Where is the nictitating membrane attached? What is its function? (2 pts)
What is the function of the tympanic membrane?
Why do frogs croak?
What parts of the body remain above the surface when the frog floats?
How does their position benefit the frog?
Why is the tongue attached at the front of the mouth?
Where does the gullet lead to?
Where does the glottis lead to?
What are two differences between the vomerine and maxillary teeth?
Where do the eustachian tubes lead?
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Clean-up
 Get a ziplock plastic bag from the teacher and use a sharpie pen to mark your name and period number on the bag.
 Place frog in marked plastic bag and place it in the bin marked with your period number.
 Rinse off, dry your tools and put them back in your tool kit, be careful with the scalpel.
 Use the plastic wiper blades to clean off table top, don't waste paper towels cleaning table tops.
 Throw away (in the garbage) any small pieces of the frog that you are not responsible for knowing.
 Rinse and dry off your tray and leave it at your station with your tool kit. Have instructor inspect your tool kit and
work station.
Part 2 “The Internal Structures”
1.
Turn your frog over, so that the ventral side is facing up and it is lying on its
dorsal side.
2.
With forceps, lift the loose skin of the abdomen. Carefully insert the tip of a
pair of scissors beneath the skin. CAUTION: to avoid cutting yourself, cut
in a direction away from your hands and body.
3.
Complete all the cut lines indicated on figure 3. First cut
A->B, then C->D, finally cuts E->F. Cut far down the sides between
points C -> E and D -> F.
4.
Remove the loose skin from the frog and throw it away in the garbage.
5.
Now repeat steps 1-3 cutting through the muscle layer. Keep scissors points
up and don't cut too deep to avoid damaging the organs below
the muscles!
Points will be deducted from lab groups if frog organs or tissues are
found in their sink.
6.
Remove the muscles and rib cage and throw it away in the
garbage
7.
Label figure 3: right atrium, ventricle, left atrium, urinary
bladder, fat body, liver, and small intestine.
8.
If the frog is a mature female, the most obvious organs will be
the ovaries. The ovaries are white sacs swollen with tiny blackand-white eggs. Carefully lift the ovaries from the body cavity,
cut the attachments with scissors, and remove the ovaries from
the frog (be careful not to rupture the ovaries with the scissors. If the
ovaries are ruptured, they can spill out a mess of eggs).
9.
Place the ovary and oviducts on the correct place on
Organ Display Sheet.
10. Locate the heart. The heart is encased in a membranous sac called the
pericardium. With the tip of the scissors, carefully cut open the pericardium.
FIGURE 3
FIGURE 3
11. Note the vessels attached to the heart. The large artery on the ventral surface of the heart is the coronary
artery. Two atria are on either side of the artery and they are a dark, redish color. The ventricle is the
triangular, muscular, whitish part at the tip of the heart. Gently probe the different lobes of the heart
and answer questions 1 & 2.
12. Place the pericardium and heart on the Organ Display Sheet. Make sure it is lined-up properly.
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13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
Label figure 4: liver, stomach, urinary bladder, pancreas, gallbladder, lung, fat body, large and small
intestine and spleen.
The large reddish-brown (color may
vary towards green in some frogs)
organ in the upper part of the
abdominal cavity is the liver. With you
fingers or a probe, lift and separate
the lobes of the liver. Behind the
middle lobe, look for a greenish, or
semi-transparent, pea-shaped sac. This
sac is the gallbladder. Answer
question 3 & 4
With scissors, carefully remove the
liver and gallbladder from the body.
The remaining organs of the digestive
system are easier to see with the liver
removed.
Place the liver and gall bladder on
the Organ Display Sheet.
Locate the stomach, which is a
comma-shaped structure in the middle
of the abdomen. At the end closest to
the head is the esophagus. The other
Figure 4
end of the stomach leads to a coiled,
small intestine.
Cut the stomach open the long way
and look in side.
Place the stomach on the Organ Display Sheet.
Pull the loops of the small intestine away from the body, but leaving it within the frog. You will notice
transparent tissue holding the coils of intestine together; this transparent tissue is the mesentery.
Inside the first loop of the small intestine is a thin, white organ called the pancreas. It is typically found
amidst connective tissue between the stomach and the small intestines. It releases enzymes to break
down food in the small intestine.
Place the small intestine and pancreas on the Organ Display Sheet.
Also in the mesentery around the intestines (lift the stomach to see it) is a small, brown, pea-shaped
organ called the spleen. This organ is part of the circulatory (cleans out worn out red blood cells) and
lymphatic systems (antibody production). Answer question 5.
Place the spleen on the Organ Display Sheet.
The small intestine ends in a large bag-shaped organ, the large intestine. The last organ of the digestive
system is the cloaca, a saclike organ at the end of the large intestine. Undigested food leaves through the
anus. Answer question 6-8.
Answer question 9 & 10.
27. Locate the two lungs. They are small, spongy brown sacs that lie behind and to the right and left of the
heart, right under the armpits.
28. Take a thin, small straw or plastic pipette and place it in the glottis. Inflate the lungs by blowing in straw
or squeezing bulb of pipette. Answer question 11.
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29. Place the lungs on the Organ Display Sheet.
30. The reproductive system and urinary system of the frog are closely connected and can be studied as the
combined urogenital system. At the back of the abdominal cavity are two long, reddish-brown organs,
the kidneys. The yellow, fingerlike lobes attached to the kidneys are fat bodies. A small, twisted tube
called the ureter leads from each kidney into the thin, membranous, saclike urinary bladder. The bladder
is connected to the cloaca.
31. Place the kidneys and urinary bladder on the Organ Display Sheet.
32. Locate the reproductive organs of the frog. If your frog is a male, it possesses testes, tiny white or yellow
oval organ found on the ventral surface of the kidneys.
33. If your frog is female, it possessed egg-filled ovaries (you removed earlier-duh). If your frog is an
immature female, the pale oval ovaries are located ventral to the kidneys. Leading from each ovary is a
long coiled tube called the oviduct. The oviducts extend along the sides of the body cavity. The oviduct
eventually joins the cloaca. Answer question 12.
34. Place the testes, or ovaries and eggs, on the Organ Display Sheet.
35. Label Figure 5 in your data section (note that there will be some similarities between the two sexes)
*You must be familiar with both sexes, therefore you may have to find a group with an opposite sex
frog:
Male (A-F): testes, fat body, kidneys, cloaca, ureter and urinary bladder.
Female (A-H): oviducts, ovary with eggs, fat body, kidney, cloaca, ureter, and urinary bladder.
36. If you finish early review all
parts of the frog. This
means you should know all
the parts you had to label as
well as the function of
them (when applicable).
Figure 5
THE BRAIN
Use your scalpel to cut along the dorsal mid-line, from the tip of the nose to the back of the skull, approximately in line with the top of the
arms or shoulders. Don’t press too hard because the skull is soft, you only have one frog and there is a dissection grade separate from the
lab write up.
1.
In order to expose the brain you must shave the skull down (this is like whittling a piece of wood). Cut back
folds of skin on the skull moving towards the tympanic membranes (perpendicular to line “A” in figure
1).*note: you will find it easier to remove the excess skin and muscles from the top of the skull and throw it
away.
2.
The exposed skull will appear to be off white in color. You will now begin to carefully shave away layers of
skull tissue and surrounding muscle, by facing your frog away from you and slide your scalpel away from
you towards the tip of the frog's nose, until the brain is exposed.
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3.
You should stop periodically to see if you are still scraping skull tissue or if you are scraping the softer brain
matter which is very similar in color, but different in texture. If you don’t know what the brain looks like
then see text or attached diagrams. This shaving process is much like whittling a stick or peeling a potato.
4.
Be careful because the olfactory lobe is close to the surface, while the spinal cord
and medulla oblongata are deeper down. You must remove a small piece of
brownish membrane to expose the triangular medulla and you must dig through
hard vertebrae to expose the nerve cord.
5.
There will be a thin, transparent layer of tissue just before the brain is completely
exposed and the shape of the brain is the same as figure 6. Label figure 6: nerve
cord, olfactory bulb, optic bulb, cerebellum, medulla, and cerebrum (cerebral
cortex).
6.
Answer questions 13-16.
7.
Remove the brain and place it on the Organ Display Sheet.
Figure 6
Clean-up
• Put your frog in the marked storage bag and return your frog to your appropriate period bin.
• After your instructor has checked your Organ Display Sheet, place your sheet and all of the
organs into the trash.
• Rinse off and dry your tools, be careful with the scalpel.
• Use the plastic wiper blades to clean off tabletop.
• Rinse and dry off your tray.
• Have instructor inspect your tool kit and workstation.
Data Analysis:
Part 2
1. In appearance, what is the difference between the atria and ventricle.
2. Why do the ventricles need to be more muscular than the atria?
3. How many lobes does the liver contain?
4. Where is the gall bladder located? How might its location reflect its function?
5. What is the function of the spleen (Hint: it performs the same function in humans)?
6. What holds all of the digestive organs together?
7. How does the location of the pancreas fit with its function?
8. Why do we call the large intestine large and the small intestines small? What are their functions?
9. What is the function of the stomach?
10. Describe the inside walls of the stomach.
11. What is an obvious physical property or characteristic of lung tissue?
12. What body system(s) feed into the cloaca?
13. What part of the brain is used to for the sense of smell?
14. Which part of the brain is the most advanced part and is the control center?
15. Looking at the overall size of the different parts of the brain, what senses would be the most advanced in
frogs?
16. In which kingdom does the frog belong? What phylum? What class?
Conclusion:
List at least five similarities between the structures of the frog and humans.
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Name ____________________
Student Answer Sheet
The purpose of this lab was
List the safety warnings
Figure 1
A. __________________________
B. __________________________
C. __________________________
D. __________________________
E. __________________________
F. __________________________
G. __________________________
H. __________________________
I. __________________________
Figure 2
A. __________________________
B. __________________________
C. __________________________
D. __________________________
E. __________________________
F. __________________________
G. __________________________
H. __________________________
Part 1 Questions
1. _________________________________________________________________________________
2. _________________________________________________________________________________
3. _________________________________________________________________________________
4. _________________________________________________________________________________
5. _________________________________________________________________________________
6. _________________________________________________________________________________
7. _________________________________________________________________________________
8. _________________________________________________________________________________
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9. _________________________________________________________________________________
10.
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11.
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12.
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13.
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14.
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Figure 3
A. __________________________
B. __________________________
C. __________________________
D. __________________________
E. __________________________
F. __________________________
G. __________________________
Figure 4
A. __________________________
B. __________________________
C. __________________________
D. __________________________
E. __________________________
F. __________________________
G. __________________________
H. __________________________
I. __________________________
J. __________________________
Figure 5
A. __________________________
B. __________________________
C. __________________________
D. __________________________
E. __________________________
F. __________________________
G. __________________________
H. __________________________
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Figure 6
A. __________________________
B. __________________________
C. __________________________
D. __________________________
E. __________________________
F. __________________________
Part 2 Questions
1. ____________________________________________________________________________________
2. ____________________________________________________________________________________
3. ____________________________________________________________________________________
4. ____________________________________________________________________________________
5. ____________________________________________________________________________________
6. ____________________________________________________________________________________
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16. ____________________________________________________________________________________
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Organ Display Sheet
Ovaries and eggs
Oviducts
Right Atrium
Ventricle
Liver
Left Atrium
Heart
Pericardium
Gall Bladder
Stomach
Small Intestine and Pancreas
Urinary Bladder
Spleen
Lungs
Testes
Kidneys
Brain
(Label parts for extra credit)
Page 10
Large Intestine
Fat Bodies
Page 11