Frog Dissection Guide Name:______
During this dissection, I need to be sure that I...
1.2.
Purpose:
In this lab, you will dissect a frog in order to observe the external and internal structures of frog anatomy, and therefore learn more about human anatomy. Specifically, describe the appearance of various organs found in the frog, name the organs that make up various systems of the frog and complete a lab report that demonstrates understanding of organ systems and functions of the frog.
Read through the following information carefully BEFORE you begin the dissection. Be sure you understand what is required. Ask any questions before and along the way.
SAFETY REQUIREMENTS:
- Wear safety goggles at all times.
- Wear plastic gloves during the dissection. You will have one pair per class period.
- Wash all splashes of preservative from your skin and clothing immediately.
- Make smart choices in regards to your actions with the frogs – be respectful of the frog, your peers and the equipment.
- Always cut away from yourself and others sitting near you, in case the scalpel slips.
- When you have finished for the day, preserve your specimen as directed, clean your work area, and wash your hands thoroughly with soap and water.
· If you cannot follow these safety requirements you will lose the opportunity to participate in the lab.
· You are being marked on your participation, behaviour and completion of tasks. Your mark will also reflect your teamwork.
Materials
· Safety goggles / · Gloves (one pair/person/day) / · Scissors, scalpel, forceps and probes / · Dissecting pins / · Dissecting tray· String and ruler / · Paper towel / · Preserved frog
Frog Dissection Lab – what do I need to hand in?
Title – Be clear, specific and include more detail than you probably think you should.Problem – What are you trying to solve? What is the question you are attempting to answer?
Materials – List all items used. / Procedure – See Dissection Lab Booklet.
Observations – Drawings – there should be 3
Discussion – Questions at end of this outline.
Wait! Please read this before you begin the actual dissection:
Your frog has probably been “double injected,” which means that arteries have been injected with red plastic and veins with blue. If it was done well, blood vessels will be easy to see and internal organs will have some color to them.
Unlike humans, frogs don’t store fat next to the skin. Frogs store winter fat in fat bodies, found inside the body cavity. If your frog was collected late in the year, the body cavity might be full of orange fat bodies.
Now you’re ready to begin the actual dissection. Please be respectful of your lab partners and of the specimen you are about to explore, observe, and learn from.
Take extra care with your dissection tools. Your scissors are your most important tool, but be sure to handle them carefully and always double check before cutting.
Procedure: Check off each step as you complete them to ensure you are following the directions closely and won’t miss a step!
1. Put on safety goggles and gloves.
2. Rinse the frog with water in the sink, and then place him/her in the dissection pan.
External Examination of the Frog
3. To determine the frog’s sex, 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 however they will have larger tympana. Observe several frogs to see the difference between males and females.
Do you think your frog is male or female? ______
3. Use the diagram below to locate and identify the external features of the head. Find the mouth, external nares, tympanum, eyes, and nictitating membranes (‘third lid’).
What human structures do you think each of the following corresponds to:
a) Tympanum? ______
b) Nictating membrane? ______
c) External nares? ______
Complete an external drawing on the frog from the dorsal point of view.
4. Place the frog on its dorsal (back) side so the belly is facing up.
5. Pin each of the four legs to the dissecting pan.
6. Cut the hinges of the mouth and open it wide. Use the diagram below to locate and identify the structures inside the mouth.
Use a probe to help find each part: the vomerine teeth, the maxillary teeth, the internal nares, the tongue, the esophagus, the pharynx, and the slit-like glottis.
I-shaped incisions in the skin layer:
5. Look for the opening to the frog’s cloaca, located between the hind legs. Use forceps (tweezers) to lift the skin, and make a small cut through the lifted skin with the scalpel. Take care to cut only the skin.
6. Use scissors to cut along the center of the body from the cloaca to the chin. Turn back the skin, cut toward the side at each leg, and pin the skin flat. The diagram above shows how to make these cuts.
7. Use scissors or scalpel to make sideways incisions in the skin. The first incisions are made between the front legs, and the next incisions are made just above the rear legs.
8. Pick up the flap of skin with the forceps. Use a scalpel to help separate the skin from the muscle layer below. Do the same with the other flap.
9. Pin each flap of skin to the dissection tray. Before you move on, have your teacher initial here: ______
I-shaped incisions in the muscle layer
10. Repeat the incisions (I-shape again) through the muscle layer. You will find it easier to begin the vertical incision by lifting the muscle layer with the forceps, starting between the rear legs of the frog.
VERTICAL INCISION (from rear legs to neck):
11. Again, make a small cut with the scalpel, and continue the incision with the scissors to a point just below the front legs.
12. Be careful not to cut too deeply as the muscle is thin and it is easy to damage the organs underneath.
13. When you reach the point just below the front legs, turn the scissor blades sideways (so your hand has changed its angle and when you open the scissors they will open to the sides, and not up and down). You want to be sure that you only cut through the bones in the chest. Be careful that you don’t cut too deeply. This should prevent damage to the heart and other internal organs.
14. When the scissors reach a point just below the frog’s neck, you have cut far enough.
HORIZONTAL INCISION (between the front and rear legs):
15. Just as you did with the skin layer, make a sideways incision in the muscle layer with the scalpel. Make the first incision between the front legs and the next incision just above the rear legs.
SEPARATE MUSCLE FLAPS FROM THE ORGANS BELOW
16. Pull back and hold the muscle flaps with the forceps.
17. Use the scalpel to separate the muscle from the organ tissue.
18. Pin the muscle flaps back far enough to allow easy access to the internal organs. Pin the upper flaps back also. Before you move on, have your teacher initial here: ______
Create a drawing of what you see and label as many structures as you can. The list below will help you to identify what you are looking at.
Examining the Internal Organs:
19. If your frog is a female, the abdominal cavity may be filled with dark-colored eggs. If so, remove the eggs on one side so you can see the organs underneath them.
20. Using a probe, the descriptions below and the diagrams on the following pages, locate each of the organs described below. Check the box to indicate that you found the organs.o Fat Bodies --Spaghetti 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. /
o Peritoneum A spider web like membrane that covers many of the organs, you may have to carefully pick it off to get a clear view /
o Liver--The largest structure of 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. /
o Heart - at the top of the liver, the heart is a triangular structure. The left and right atrium can be found at the top of the heart. A single ventricle located at the bottom of the heart. The large vessel extending out from the heart is the conus arteriosis. /
o Lungs - Locate the lungs by looking underneath and behind the heart and liver. They are two spongy organs. /
o 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) /
o 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. /
o 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. /
o 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) /
o Spleen--Return to the folds of the mesentery, this dark red spherical object serves as a holding area for blood. /
o 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. /
STOP! If you have not located each of the organs above, do not continue on to the next sections!
* Have your teacher initial here when you have identified all of the above organs. _____
Create a drawing of the digestive system labeling as many structures as you can.
Removal and investigation into the stomach:
21. After identifying each of the organs on the previous page, you will now explore the contents of the stomach. Using your scalpel, cut the stomach out of the frog, place it on the dissecting tray beside your frog, and open it up. You may find what remains of the frog's last meal in there. Look at the texture of the stomach wall (on the inside).
What did you find in the stomach? ______
Measuring the small intestine:
22. Remove the small intestine from the body cavity and carefully separate the mesentery (holding the small intestine together) from it. Stretch the small intestine out and measure it.
23. Now take measurements of your frog. Record the measurements below in centimeters.
Frog length: ______cm Intestine length ______cm
Investigating the Urogenital System:
24. The frog's reproductive and excretory system is combined into one system called the urogenital system. You will need to know the structures for both the male and female frog,
25. Using a probe and the descriptions below, locate each of the organs described below. Check the box to indicate that you found the organs.
o 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.
o Testes - in male frogs, these organs are located at the top of the kidneys, they are pale colored and roundish.
o 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.
o Bladder - An empty sac located at the lowest part of the body cavity. The bladder stores urine.
o Cloaca - mentioned again as part of the urogenital system - urine, sperm and eggs exit here.
26. Remove the kidneys and look for threadlike spinal nerves that extend from the spinal cord.
27. Dissect a thigh, and trace one nerve into a leg muscle. Note the size and texture of the leg muscles
28. Dispose of your materials according to the directions from your teacher.
29. Clean up your work area and wash your hands before leaving the lab.
Diagrams to help identify internal organs:
You should use a probe to gently move various organs to the side without damaging them.