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Biology 2005
DO NOT WRITE ON THIS DISSECTION GUIDE!DO NOT WRITE ON THIS DISSECTION GUIDE!
Bullfrog Dissection Guide
Objective:
Vertebrates are the most complex organisms as far as systems and structures. The frog, although an
amphibian, is a good representative of the vertebrates and may be the only vertebrate that many people
examine closely both externally and internally. This dissection and the information gained will add to
the student knowledge of comparative anatomy when combined with the information from the
earthworm and crayfish.
Materials List:
Dissection Tray
Blunt ended dissection probe (2)
Scissors
Hand lens, or Stereomicroscope
Bone cutters (last day only)
Paper towels
Dissection ruler
Optional:
A pair of gloves.
Dissection pins (optional)
colored pencils - student
pencil - student
paper for lab write up- student
2-gallon "zip lock" bag -student, or provided
permanent marker - student
Procedures:
Answers to any questions asked in the Procedures section will be
written in your own lab write up. Make sure to give the procedure
number with the answer. This should be neat and well written. The
answers should be full sentences.
At some point in the dissection, you will have to bag your specimen for storage until the
following day, and for the next. To do this, wrap your specimen in a paper towel that has
been slightly wetted. Place this into the bag and seal it. Make sure your group name, or
identification is on the bag in permanent marker.
1. You will be working with 4 or more partners on this investigation. One of the partners will be the
reader of the procedures, and the others will be the dissectors. You may want to change on a regular
basis so that all members have the ability to read, and/or dissect. Note that all members of the
group will be responsible for the structures and functions of all internal and external parts of the
frog, so do not move to another step until all members of the group get the information.
2. One of the members of the group will go and get the larger lab tray with the materials and return to
the desk. Take out the materials.
3. Fold the paper towel so that it will cover the black wax on the bottom of the lab tray.
4. Go to the teacher's desk and get the specimen with the tray and another paper towel. Take it to the
sink and rinse it off to remove any extra dissection fluid.
5. Return to your desk. With your partners, roll the frog over and note any coloration changes on your
lab write up. Why would the frog need to have coloration like this? Answer this in your lab write
up. You will note that the frog has a cut on the medial line on the ventral side of the torso, and cuts
on one or both legs. These cuts are intentional. They have been injected with red and blue dye so
that the arteries and veins can be visible.
EXTERNAL:
6. Place your specimen dorsal side up.
7. At the anterior of the body, you will find the head area. Prominent on the head are the eyes. They
have the ability to move into or out of the head to allow the frog to protect them from predators, or
see prey. Push on them and see what they do. Refer to diagram 1.
8. The eye is covered by the eyelid on top and an unusual feature on the bottom. The nictitating
membrane is a clear cover that the frog uses underwater as you would use swim goggles. It may
look opaque in the preserved specimen. Using the probe, pull it up away from the eye gently so
that all in the group can identify it. Refer to diagram 1.
9. Posterior and lateral to the eye is a dark round patch of skin. Touch it. It gives to pressure. It is the
tympanic membrane. It is an external eardrum for the frog.
10. At the most anterior point of your frog, you will see the opening of the mouth. Just dorsal to the
mouth you will see the external nares. Note: They are not nostrils. Nostrils are fleshy coverings
of the nasal opening into the skull. Since there are no fleshy coverings, there are no nostrils. Refer
to diagram 1
11. Lay your frog on its dorsal side.
12. Externally, there is only one way to determine the sex of the frog. Look closely at the thumbs of
your frog. Males have a notably larger thumb than the other digits on the forelimb. Females have a
thumb that is more or less uniform with the size of the other digits. Do you have a male, or a
female frog? Look at other frogs in the room to compare sizes of thumbs to be sure of your answer.
Refer to diagrams 2 and 3 for a comparison to your frog. Males use these strong digits to hold a
female that is ready to lay eggs so that other males do not take her away. Strongest male fertilize
the eggs.
MOUTH:
13. The mouth of the frog should be difficult to open. The
muscles of the jaw hold it very tight in a preserved
specimen. You will have to break the jaw and sever
the muscles to be able to see the interior of the mouth.
14. Using a probe or your finger, open the mouth enough
to get the scissors inside. Move the scissors to the
corner of the mouth. Moving back along a lateral line,
begin to cut the tissue of the jaw.
15. You will soon reach the bone of the lower jaw. With
a cut that is parallel to the line of the upper jaw, cut
through the bone. You should feel a loosening and
hear a snapping sound when you have successfully
broken the bone. Repeat steps 14 and 15 on the other
corner of the mouth. See the lines at the corners of the
mouth on the figure to the right.
16. The mouth will be easier to open although it will not lay open on its own. You will need to hold it
open to view the inside structures.
17. Look into the mouth. The most prominent thing in the mouth is the tongue. It is on the lower law
at the most anterior point. Notice that it attaches at the front of the mouth. It does not come out of
the mouth much more than its length (end of the myth). Refer to diagram 4.
18. If you have a male, on either side of the tongue and more posterior are the vocal sac openings.
They may have been damages when you were cutting the jaw, but they are there. It may look like a
fold in the skin. Refer to diagram 4. Female frogs do not have these structures, and make no
sound. Look at another group's frog to identify the vocal sac openings.
19. At the back of the throat, there are two lines. One goes laterally, and the other sagittally (up and
down). The lateral opening leads to the stomach and is called the gullet. The sagittal opening is
ventral to the gullet and leads to the lungs. This is the glottis. Refer to diagram 4. Humans have
only one opening going to the stomach and the lungs. What would be an advantage of having two
different openings going to these two places?
20. On the roof of the mouth and to the sides, you will find openings that seem to lead up toward the
tympanic membrane. These openings are the eustachian tubes. Using your probe, explore the
interior of the tube. When you get deep enough, your probe will be touching the inside lining of
the tympanic membrane. These tubes equalize the pressure inside the ear while the frog is diving
into water. Refer to diagram 4.
21. Along a medial line from the gullet towards the upper lip, there are two barbs. These barbs,
pointing backwards, are called the vomerine teeth. They hold prey. Refer to diagram 4.
22. Just anterior and lateral to the vomerine teeth you will find two small openings. These are the
internal nares. Using a probe, push into, and to the side of one of the internal nares. The probe
should lead out of the external nare. Why would this be an advantage to the frog?
23. Using your finger rub back and forth inside the upper lip of the frog. The sandpaper structures that
you feel are the maxillary teeth. They help the vomerine teeth to hold prey. Refer to diagram 4
INTERNAL:
24. Lay your frog in the tray ventral side up.
25. You should have an incision into the ventral side of your frog. Notice that the incision goes
through the skin and the underlying muscle. Using this opening cut the skin (not both skin and
muscle) in a posterior direction as close to the medial line as you can until you have come to the
beginning of the legs. Refer to diagram 5.
26. Using the same opening cut in an anterior direction as close to the medial line as you can. Stop
when you are between the corners of the mouth. Refer to diagram 5
27. Refer to diagram 5 and make lateral incisions in the skin only at the posterior edge of the body
cavity along the line of the body cavity and the hip. Cut until you are about 2 cm. from the middorsal line on the back. Repeat on the other side.
28. Cut the skin on the chest of the frog laterally along a line that would go across to the posterior edge
of the forelimbs. Continue under and around the forelimb until you are 2 cm. from the mid-dorsal
line. Repeat on the other side. Refer to diagram 5
29. Notice that the skin is held loosely on the body of the frog. At a point at about the place where the
light skin of the ventral becomes the dark skin of the dorsal, the skin is held onto the underlying
muscle. You must detach this connection. After you do this, the skin will (with some help) lay
down on the tray.
30. Using the same incision points as with the skin, cut through the underlying muscle. Be careful to
pull up with the scissors so you won't damage the organs of the body.
31. You will have to cut through the sternum of the frog in the chest. Yes, it will be tough and will
crack when broken.
32. Using dissection pins, pin the two "flaps" of skin and muscle down to the tray. Using your thumbs,
pull open the sternum. You may have to pull vigorously. After you can see the organs in the upper
body cavity, pin back the sternum so that you can have an unobstructed view. This will be
difficult. The bones of the chest and shoulders do not naturally lay back in such a way. Cutting
muscle will ease this process. Be careful not to cut any organs other than muscle in this procedure.
Digestive and Associated organs:
33. One of the most prominent structures in the body cavity is the liver. It is a bluish brown organ with
three distinct lobes centered in the body cavity. It is an associated organ for several systems, but
for our purposes, it will be said that it makes bile. Make sure that you and your partners can
recognize and identify all three of the lobes. You may have to move them around a bit to see all the
lobes. Refer to diagram 6
34. Between two of the lobes of the liver, and partially imbedded in one of them is a small greenish sac
that resembles an English pea. This is the gall bladder. It stores bile made by the liver. This is
used in the digestion of fatty foods. Refer to diagram 6
35. Using great care, find to point where the liver lobes are attached to the body and remove them
using the scissors. Lay them out to the side on your paper towel so they can be saved for later
identification.
36. {Note: If you have a female frog, you may have to remove the egg sacs (salt and pepper sacs) to be
able to continue with the dissection. Ask your instructor how this should be done.} With the liver
removed, the stomach is visible as a light gray to tan "banana shaped" organ that is dorsal and
posterior to the liver's position. Touch it with your finger. It is hard. Using your fingers, on either
side of the stomach, move to the posterior end of the organ and feel for a small knot that separates
the stomach form the lower digestive system. This knot is the pyloric sphincter. It closes off the
stomach for full digestion to occur.
37. There is a small bluish red ribbon organ that is on the inside curve of the stomach and attaches just
posterior to the pyloric sphincter. This is the pancreas. It is an associated organ that is responsible
for creating insulin used in regulating sugar.
38. Following the digestive system down, we find the tube of the small intestines. They coil around
several times and are held by a thin plastic-like connective tissue called the mesentery. The
mesentery holds internal organs in place and the small intestines absorb nutrients.
39. Posterior to the small intestines there is a noticeable junction between the small intestines and the
large intestines. They absorb excess water so that the frog does not dehydrate. Refer to diagram
6.
40. In the muscle tissue of the legs is another structure called the cloaca. It is a place where digestive,
excretory, and reproductive systems find an exit from the body. We will not be able to find it due
to the muscle tissue being preserved.
Circulatory System:
41. Just anterior to the stomach is the heart. Refer to diagram 7. The heart is broken down for our
purposes into the ventricle and two atria. Collectively they pump blood. Moving us from
between the two atria, you should see the aorta, the main blood artery of the body.
42. As carefully as you can, cut the aorta and associated veins that hold the heart in place and remove it
from the body. Make sure that you remember the direction it was laying in the body. Cut the heart
from anterior to posterior along the lateral line so that you have a front half and a back half. Notice
that the two atria have a smaller muscle wall than does the ventricle.
43. Another organ that is associated with the circulatory system is the spleen. It is usually blue or
brown. It is located inside the tissue of the mesentery. It filters blood.
Digestive continued:
44. As high as you can, cut the esophagus leading to the stomach.
45. Cut the large intestines as posterior as you can. To do this, you will have to cut through a clear sac.
This is the urinary bladder. Make sure that all members of your group can identify this, as it will
be difficult to identify in the excretory system.
46. Gently remove the entire digestive tract. Make sure not to disturb any of the other organs of the
body.
47. Cut the tissue of the mesentery and stretch the digestive tract out. Measure the stomach, small
intestines, and large intestines (metric) and record on your lab write up. Why would the small
intestines length be so much longer than the large intestines length?
48. Cut the stomach along a line from the esophagus to the pyloric sphincter. Describe the contents of
the stomach in your lab write up.
49. Notice the ridges that run lengthwise in the stomach. It looks like a miniature washboard. These
are called rugae. What would be their purpose? Do not cut open the small or large intestines.
There is no reason to do that and it will become messy. Go to other groups and see the internal
lining and the contents of the stomach. Stomach size and structure vary widely in all frogs, so you
need to get an idea of the range of size.
50. Lay the digestive tract beside the liver for later identification.
Respiratory system:
51. In the anterior dorsal part of the body cavity, you will find two spongy structures on either side.
These are the lungs. They are responsible for part of the respiration of the frog. The skin is
responsible for the rest.
52. Remove one of the lungs. Cut is anterior to posterior laterally (as the heart) and look at the
consistency with the hand lens. Describe what you see in the lab write up. Discard the lung in the
trash.
Reproductive System:
Female Frog:
53. If your specimen happens to be a female, the body cavity was probably be nearly filled with eggs.
You possibly have removed the egg sacs in step 36. The eggs sacs were attached near the kidneys
(brown structures along the dorsal side). You will find also near the kidney, two long coiled, white
oviducts leading from the anterior end of the body cavity to the posterior end. Remove the
oviducts but carefully cutting the mesentery that holds them in place. Dispose of them in the trash.
Now you have a clear view of the kidneys. Refer to diagram 8.
54. At the anterior end of the kidney, you should see the ovaries (one per kidney). They are creamwhite and are oval shaped and make all the eggs for the frog. Some species of frog lay over one
thousand eggs in a season.
Male Frog:
55. Without the overly large parts of the female frog anatomy, it is relatively easy to see the parts of the
reproductive system of the male frog. At the anterior end of the kidney you should see the testis
(one per kidney). They are cream-white and are oval shaped. They make the sperm for fertilizing
the eggs. Refer to diagram 8.
56. Lateral to the kidneys you may (not always) see a white wavy structure that seems to be buried in
the dorsal wall of the body cavity. It is vestigial oviducts. The male frog has these, and quoting
from Jurassic Park:
"Now, some west African frogs have been known to
spontaneously change sex from male to female in a
single sex environment."
Dr. Alan Grant
Both Sexes:
57. Both sexes of frog send their respective sexual material to the cloaca where it exits the body.
Fertilization takes place externally.
Excretory system:
58. Along the dorsal of the body cavity, you should see two long bluish brown structures on either side
of the now visible spine. These structures are the kidneys. They are responsible for filtering the
blood and removing nitrogen waste in the form of urea. Refer to diagram 8 for both male and
female.
59. Visible only in certain specimens is the ureter that caries the urine from the kidneys to the urinary
bladder. You identified and cut through the urinary bladder in step 45.
60. While not associated directly with the excretory system, the fat bodies are easy to identify here.
They are yellow "fingers" that are attached to the lateral side of each kidney. Refer to diagram 8.
This is the only place that the frog stores fat. Why would it not store fat all over the body, as does a
human? Answer in your lab write up.
Endocrine System: (hormones and signaling molecules)
61. At the most anterior point of the kidney, you should be able to see a cap of white tissue smaller
than the ovary or testis. Identify the structure.
62. This is the adrenal gland responsible for producing adrenaline for the frog
ad
is Latin for near
renal is Latin for kidney
Body cavity clean up:
63 The frog now has little in the body cavity, and most of the organs which did occupy it are either
laying in the tray, or are thrown away. By this time you should have been able to see other groups
specimen and noted any unusual characteristics. At this time, take another piece of paper towel
and wrap up the liver, heart, and digestive tract. Take the package and throw it away.
Eyes:
64. Using your scissors, cut through the clear cornea of the eye. Be careful not to push too hard. The
fluid may squirt out.
65. Using your scissors or a probe, remove the lens from the eye. It is a small yellowish ball no larger
than the testis or ovaries.
66. Place it on the dissection tray and cut it in half with the scissors. The teacher has placed one of the
lenses on a dissection microscope for viewing. Describe what you observe in your lab write up.
Remember that taking any part of a dissection specimen out of class is a major infraction of the lab
rules.
Nervous system:
67. Lay your frog dorsal side up.
68. Note the position of the external nares. Using your scissors (or provided bone cutters), make an
incision through the upper jaw until you reach a point between the nares. One tip of the scissors
will be above the skull and the other will be in the mouth.
69. You will now continue this incision by cutting toward the tympanic membrane on either side of the
head. This should make a "V" shape at the front of the head. Refer to diagram 9.
70. Using a probe and your scissors, remove all the skin and muscle from the skull.
71. From this point, you must be very careful. The skull is the densest bone in the frog and the brain is
one of the most delicate organs and is found directly in the center of your "V"
72. Your scissors or bone cutters will be used from this point. You must carefully cut away at the
anterior tip of the "V" until you see a small cavity begin to appear right in the center or the portion
of bone. This is the anterior of the brain case.
73. Using as much care as you can, begin to remove the top bone that covers the brain case. This
should be done by placing the tip of the scissors into the opened case and cutting outward so as not
to damage the tissue of the brain. You may partially or totally destroy the brain lobes in this
process. Usually a few of the groups are lucky to get the entire brain exposed for the class viewing.
74. You should be able to peel the top layers of bone away from the brain. The brain is pinkish cream
colored (usually). Refer to diagram 10.
75. The most anterior end of the brain is a pair small thin lobes tapering to a point in front. The most
anterior ends of these lobes are the olfactory lobes. They process signals from the nose.
76. At the swelling at the posterior of these lobes are the auditory lobes. They process signals from
the tympanic membrane.
77. Centered in the brain, posterior to the auditory lobes, are tow large round lobes. They resemble
eyes, and that is where they get their information for processing. They are the visual lobes. Refer
to diagram 10.
78. Just posterior to the visual lobes you will see a small triangle that is inset in the brain. This is the
medulla. It is responsible for base functions of the body.
79. One either side of it is tissue resembling the sides of the triangle and is the cerebellum. It controls
respiration, heart rate, and other vital functions.
80. The triangle tapers down to a swelling that is the brain stem, and leads in a posterior direction to
the spinal cord with the job of carrying signals to the body.
Lab Take Down
81. Take all the pins out of your bullfrog and lay the following materials on your lab table:
Blunt ended dissection probes
Dissection pins
Hand lens
Scissors
82. Wrap up the bullfrog in the paper towel and throw the paper towel and the frog in the trash. Do not
worry about the chemicals. The dissection fluid will break down in the trash and the frog will
decompose.
83. Place all the materials listed above into the lab tray and return it to the designated spot.
84. Wash your hands, clean your area, and sit with your partner at your lab station.
85. Return the dissection guide to the designated area.