Pre A
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 Dissection pins (optional)
Blunt ended dissection probe (2) colored pencils - student
Scissors pencil - student
Hand lens, or Stereomicroscope paper for lab write up- student
Bone cutters (last day only) 2-gallon "zip lock" bag -student, or provided
Paper towels permanent marker - student
Dissection ruler
Optional:
A pair of gloves.
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 mid-dorsal 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 cream-white 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.
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