Introduction: DIY Frog for Dissecting
What is one of the first things that come to mind when you think of a biology classroom? To some, it might be the dreaded frog dissection, a rite of passage for students of the life sciences the world over. However gross it might seem, this frog dissection is the starting point for understanding how our bodies work.
A friend of ours, a high school biology teacher in a small rural town in South Africa, recently lamented the fact that this iconic activity is no longer part of the school curriculum. She would have loved to give her students more practical experience with animals, but ethical considerations and limited resources make it impossible for her to expose her students to anything more hands on than a YouTube video.
As an engineer, an occupational therapist and a zoologist, we immediately put our heads together to design the perfect solution – an educational tool that would allow anyone to perform a virtual dissection of a frog! And the best part? It can all be done with bits and bobs lying around the house, with no background knowledge of frog anatomy whatsoever.
It is our hope that teachers and pupils alike will be inspired by this Instructable on how to make your own ready-to-dissect frog. We added some fun facts and useful links for the curious.
Enjoy! Simon, Renate & Anneke
- Green, beige, cream and maroon felt – the green must be at least A4 in size
- Pink foam, A4 in size
- 6 white pipe cleaners
- 1 green and 2 red pompoms
- White gauze
- Yellow yarn
- Thin yellow and red plastic-coated copper wires
- Red and white uncooked quinoa, mixed
- ScissorsCraft glue or double-sided tapeSturdy needle
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Step 1: Get Ready
We're not just going to dissect any old frog – we're going to dissect an African clawed frog!
Did you know that the African clawed frog is the world's most widely distributed amphibian? It is used in laboratories, universities and schools all over the world to teach us about the basics of biology. It was even used as a pregnancy test...
To start, you need to download and print out this life-sized template of the frog and all its organs. Cut out all the shapes in the template:
- A. Overall shape of the frog
- B. Three liver lobes –
- C. Two lung lobes – left and right
- D. Digestive tract
- E. Mesentery
- F. Ovary
- G. Kidneys
- H. Bladder
You're going to cut these shapes out of the various materials to make organ systems that you can add to your froggy one step at a time.
Step 2: Skin
Your skin might seem like nothing more than a giant sack that keeps all your bits inside, but it's actually your body's biggest organ. Frog skin is even more special and scientists regard it as one of the secrets to their success. Not only is the slimy skin the first line of defense against bacteria and other germs that can make the frog ill, but it also protects the frog against drying out. Frogs even use their skins to breathe!
Use the big frog-shaped template to cut two skin flaps out of the green felt – one for the top and one for the bottom. Lay one of them on your work surface.
Step 3: Skeleton
By just looking at a frog's skeleton, you can already guess what is its favourite way of moving around – jumping! Powerful hind legs dominate the skeleton. The bones of the second half of the spinal chord are fused together to form a rod (called the urostyle) between the sitting bones that propel the frog into the air. The frog skeleton also contains a lot more cartilage than yours to make it lighter.
The bones in the body are not only there to support the muscles when the frog is moving about. They also protect vital organs (like the brain) from mechanical shock, make blood cells and store special minerals.
We're going to use the pipe cleaners to make a simplified model of the skeleton. If you look carefully at the template, you'll see that the skeleton consists of different shades of grey, dividing it into six sections. We recommend that you use one pipe cleaner for each of these sections. It might seem a bit daunting, but just follow the steps below, have a look at the pictures and use your own creative sense, and you'll be fine...
Start with the skull: take the middle of the pipe cleaner as the tip of the nose, make two loops on either side of the nose tip to represent the eye sockets, bend into a u-shape and join together the two ends to make the back of the skull.
The shoulder girdle and the arms come next: make a small hand with four fingers at the one end by making three and a half zig-zags, secure by hooking long end of in the pipe cleaner through the first V you made, squeeze together the zig-zags to make the four fingers, make a bend at the elbow, repeat at the other end of the pipe cleaner and attach the central part to the base of the skull. You might need to play around with the length of the fingers – slightly less that 1 cm did the trick for us.
Next up is the spinal chord with the urostyle. Simply attach the third pipe cleaner to the base of the skull, make seven small zig-zags to represent the unfused vertebrae in the spine and the fold and twist the remainder of the pipe cleaner into a rod so that the whole structure is the length of the frog's body.
The pelvic girdle is even easier: fashion the fourth pipe cleaner into a shield shape based on the template and leave the two ends on either side to attach the legs. Attach the pelvis to the spinal chord in between the vertebrae and the urostyle so that the urostyle is free to do its job as a lever.
You'll need the last two pipe cleaners for each of the legs. Start with the four toes in exactly the same way as the fingers, just make them a bit longer (2 cm worked for us), make bends for the ankle and knee and attach to the pelvis.
Voilá! Your frog skeleton is complete! Place it on top of the skin layer to continue.
Step 4: Kidneys
In vertebrates, kidneys filter the blood to remove all the toxins. This is then expelled from the body in the form of urine. Frog kidneys are not as well developed as ours, but the still do the job: blood enters the cortex (represented by the two maroon lobes in our model) to be filtered, the urine collects in the collecting tubes (represented by the brown cloud) and exits through the uretra into the bladder.
Cut out the two lobes of the kidney from the maroon felt. The central collecting tubes can be cut out from light brown or cream-coloured felt. Glue it to the two maroon lobes and secure it with a short piece of red plastic-covered wire, representing the blood vessels that provide the kidneys with blood to be filtered. Glue the kidneys to the bottom half of the body, in the middle of the two legs.
Step 5: Lungs
We already know that frogs use their skins to breathe. However, all frogs also have lungs that perform the same function. In some frogs the lungs do the bulk of the work, in others they are less important. Sometimes the skin plays a bigger role in releasing carbon dioxide, whilst the lungs are optimised to capture oxygen. For this reason, the lungs consist of hundreds of small sacs called alveoli – in this way, the surface area for absorption is enlarged.
Use the pink foam and cut out the two lobes with the template as guide. Glue the top bit to the skin right below the armpits. In real life, two bronchi will branch from the throat to the lungs and the lungs will also be richly served by lung vessels from the heart, but we're going to use our imagination for that.
Step 6: Excretory Bladder
When the urine leaves the kidneys, it collects in an expandable sac called the excretory bladder. Here, the urine is stored until it can leave the body.
Cut out the bladder from cream-coloured felt. Glue the narrow end of the bladder to the skin right between the legs of the frog, leaving the rest of the bladder loose.
Step 7: Reproductive System
The male reproductive system in the frog is quite boring. Two small, cream-coloured, bean-shaped testes attached to the top of the kidneys. When the male clasps the female during mating (in a position called amplexus), he squeezes her sides to stimulate her to release her eggs into the water. Sperm from his testes are released too and the eggs are fertilised outside their bodies.
The female reproductive system is much more exciting, which is why we decided that you'll be making frog princesses and not princes. When you open a female frog up during dissection, the first thing you'll probably see would be masses of tiny yellow and brown eggs. These can make up around 10% of her body mass in breeding season!
In the place of the testes, the female frog has two giant ovarian lobes. These are filled with eggs in various stages of maturation. When the eggs are ready, they leave the body through the oviducts. Fat bodies usually accompany the reproductive system to provide the growing eggs with energy.
Cut out the ovary in light brown or yellow felt. Cover the felt in glue or double-sided tape and sprinkle it with the red and white quinoa. Allow the glue to dry. For the oviducts, coil the yellow plastic-covered wire around a tooth pick or the sturdy needle. Pull from the tooth pick or needle and bend in a C that is long enough the go around one lobe of the ovary. Attach to the top left of the ovary. The loose end exits the body to release eggs. Make another for the right lobe. The fat bodies are represented by four or five short pieces of yarn, tied together at their one end and attached to the ovaries. Make two clusters of yarn, one for each lobe of the ovary.
The ovaries are usually attached to the kidneys with a thin membrane. Simply glue the middle of the ovary to the middle of the kidneys to continue.
Step 8: Digestive System
As both predators and prey, frogs are a vital component of food chains. The insects, aquatic invertebrates and small fish that form their diet all pass through the digestive system so that energy can be extracted for the frog to live and reproduce. Prey enter through the mouth, often with the help of the tongue, but, unlike in humans, frogs cannot use their tongues to maneuver the food down their throats. Instead, they squeeze their eyes shut, pushing the balls of their eyes into their mouth cavity, forcing the food down the esophagus, the first part of the digestive system. From here, the food moves down to the stomach, where the breakdown of the food into smaller pieces can start. Remember, a frog cannot chew, so it relies heavily on special molecules in the stomach acid called enzymes that break down the food. Through the process of peristalsis (or the rhythmic squeezing of the stomach wall) the food moves down into the intestine. It is here, in this long, winding tube, that the absorption of food molecules takes place. Eventually, what is left of the food ends up in the rectum, where the waste is collected and dried out. The last step in the process is the cloaca, a common opening for everything that exits the body of the frog, be it pee, poop or soon to be tadpoles. If this sounds a bit yucky to you, you should know that it was also the case for the guy that named it. Cloaca literally translates to sewer in Greek.
Holding together this whole system is the mesentery, a thin membrane filled with blood vessels. These blood vessels supply the digestive system with oxygen-rich blood and also carry away the absorbed nutrients to the areas in the body where they can be processed.
Attached to the mesentery, but totally separate from the digestive system, is the spleen. Here, the red blood cells, the oxygen carriers of the body, are made.
Cut out the digestive system in light brown felt. Next, cut out the mesentery from the gauze. Glue the gauze to the back of the intestine and rectum. Intertwine a bit of red wire with the gauze to represent blood vessels. Finally, glue a red pompom, representing the spleen, to the mesentery near the top bottom end of the stomach. Place the digestive system on top of all the other assembled organs and glue the top part to the head area and the bottom part to the area between the legs.
Step 9: Liver and Gall Bladder
When reading about the digestive system, you might have wondered where in the body those enzymes in the stomach acid come from. Well, the answer lies in the big, dark red organ that dominates the top half of the body cavity – the liver. In frogs, it is divided into three lobes and together they are responsible for manufacturing bile. Bile is a bitter, green substance that contains all the enzymes necessary for the breakdown of food (also known as digestion). From the liver, the bile travels to the gall bladder, where it is stored until the frog has a meal. During mealtime, the bile ducts transport bile from the gall bladder to the stomach, where digestion takes place.
Cut out the three lobes of the liver in dark red felt. Arrange the three lobes, starting with the middle one, then the right one and lastly the left one. Glue them together at their top edges. Finally, glue the green pompom to the left lobe to represent the gall bladder. Glue the top edge of the liver to the top of the stomach (the liver doesn't really attach here, but the bile ducts do, so let's allow ourselves some creative freedom).
Step 10: Heart
So, what is still missing? We left one of the most important organs for last – the heart! When you dissect a frog, the creepiest part will probably be the fact that the heart will still be beating. This is not because the frog is still alive, rather a result of tiny electrical impulses that are still running on the energy left in the frog's blood. These impulses will fire for as long as there is blood sugar available, even when you remove the heart from the body...
Even though a beating heart doesn't necessarily mean life, there can also be no life without a beating heart. With each beat, the heart pumps blood through the lungs and skin, where it picks up oxygen. The blood then returns to the heart, where it is pumped out by another artery to the body, where the oxygen is delivered to the cells. Because frogs have a three-chambered heart, not one with four chambers like us humans, there is a bit of mixing of oxygen-rich and oxygen-poor blood in the biggest heart chamber, namely the ventricle. To counter this, there is a structure known as a spiral valve, which aids in directing the right blood to the right organs. Luckily, frogs have much lower energy demands than us, so the slight mix-up is not a big problem.
The heart is surrounded by a transparent, silver-specked sac called the pericardium. It is filled with fluid that energises the heart and protects it from mechanical shock.
We're going to simplify the three chambers of the heart to a single red pompom. Cut out a piece of the gauze, large enough to wrap around the pompom, to represent the pericardium. Wrap the pompom in the gauze, gather the ends of the gauze together and tie it together with a piece of red wire. Glue the heart to the space right above the liver and arrange the wire so that it looks like two blood vessels carrying blood away from the heart to the rest of the organs in the body.
Step 11: Wrapping It Up
Under the skin, fleshing out the bones and surrounding the body cavity, lie several muscles that allow the frog to move about, whether by jumping or swimming. The whole musculature is a complex, intertwined system of pushing and pulling forces, perfectly suited to their task. However, when you dissect a frog, you'll probably just cut through them all, from the throat to the belly and pull them aside to reveal the organs below.
Cut out the shape of the frog from the pink foam. Place it on top of the organs, followed by the second layer of green skin. Stitch together all three layers – the back skin, muscle layer and belly skin – at the sides of the body between the legs and arms using the sturdy needle and the yellow yarn. This won't only serve to hold your frog together, but the stitches also represent the lateral line organs.
Lateral line organs are unique to clawed frogs. These bumps in the skin (that look a lot like little stiches) pick up waves in the water caused by other animals. The African clawed frog uses this stimulus, rather than its eyes, to locate underwater prey and predators, leaving the eyes free to watch the skies above.
Now you're finally ready to dissect your frog! Will you be brave enough to cut open the belly to reveal the little beating heart below?
Judges Prize in the