Frog Dissection Lab Key

Pre-Lab Research:

• Study this website. It has useful videos of a frog dissection.
• Study the directions, diagrams and pictures provided on this lab key.

External Anatomy:             Dorsal View         Ventral View

	(1) Verify that your frog has both a nictitating membrane and tympanic membrane. (2) Verify that only the toes on the back feet are webbed. (3) Verify that teeth are not found on the lower jaw. Find the vomerine teeth, eustachian tubes and glottis. (4) Where is the tongue attached in the mouth?
	The sex of a frog may be determined externally by examining the thumb pads on the front feet. The thumb pads of males are enlarged at the base as in the drawing on the right. (5) Determine the sex of your frog.

Internal Anatomy:

Your frog is probably "double injected". That means that the arteries have been injected with a red plastic and the veins have been injected with blue. If the injection has been done well, blood vessels will be easily seen and the internal organs will have some color to them.

Unlike humans, frogs do not store fat next to the skin. Frogs store fat for winter in fat bodies inside the body cavity. If your frog was collected late in the year, the body cavity may be full of these orange fat bodies.

(6) Does your frog have much stored fat?

Scissors will be your most used tool during this dissection. As you begin to open the frog, you will find the skin is paper thin and not tightly attached to the muscle underneath. Immediately under the skin you will find the hard abdominal muscles. The skin and abdominal muscle of your frog have been cut. This was done to allow the inside of the frog to be fully preserved.

(7) Use scissors to make the cuts shown below through the thin belly skin.

(8) Pin back (or cut away) the skin and open the body cavity the same way you opened the skin.

(9) Pin back (or cut away) the abdominal muscles to expose the internal organs.

(10) Push the 3-lobed liver to the left and expose the esophagus running back from the mouth to the large J-shaped stomach.

(11) Cut the esophagus as close to the mouth as possible.

The stomach consists of a large anterior cardiac portion and smaller posterior pyloric portion which ends at the pyloric sphincter. This circular muscle opens and closes the bottom of the stomach.

The first portion of the small intestine, the duodenum is directly below the pyloric sphincter. Posterior to the duodenum lies the elongated and coiled ileum, which in turn, connects with the large intestine. The large intestine is easily identifiable as a marked expansion of the alimentary canal in the posterior region of the body cavity.

The gall bladder is located on the dorsal surface of the right lobe of the liver. The bile duct carries bile from the liver to the duodenum. The spleen is a roughly spherical dark organ which lies in the intestinal mesentery ventral to the kidneys.

(12) Cut the large intestine as close to the anus as possible. Both ends of the canal have now been cut. Carefully remove the alimentary canal in one piece. Measure the length of each organ of the alimentary canal in centimeters and record them on the lab report form.

(13) After taking the measurements, you may want to cut the stomach open and examine the contents.

Although your preserved frog has no fresh blood, these pictures show that frog red blood cells have a nucleus and human red blood cells do not.

The adult frog has a heart with three chambers.

While it is more efficient than the two chambered heart of a fish or the immature frog tadpole, it is not as efficient as the four chambered heart of warm-blooded vertebrates.

(14) Verify that your frog's heart has three chambers.

The blood returning from the body that is full of carbon dioxide is pumbed by the same chamber as oxygen-rich blood coming from the lungs. This means that blood leaving the heart for the body has carbon dioxide diluting the oxygen.

The small, spongy lungs of the frog can be seen on each side of the heart. There is no diaphragm (breathing muscle) dividing the coelom of the frog. (15) Locate the heart and diaphragm of your frog. Once the digestive, circulatory, and respiratory systems have been removed, the kidneys and reproductive system remain. The left side of this drawing shows the testes of a male just below a kidney. The right side of the drawing shows the dark ovaries and coiled oviduct of a female covering the kidney. If your frog is a female collected in the spring, the body cavity may be full of ovaries and oviducts. (16) Did you correctly identify the sex of your frog in step #5?

The brain of a frog is located in the top center of the skull. The dorsal view on the right shows the two bones that make up the top of the skull. If the skin and muscle are removed, a scalpel can be used to shave thin layers of these two bones, eventually exposing the brain. (17) Start at the front of the bones and carefully shave backward removing thin layers of bone to expose the brain.

Lab Clean-up:

• If the dissection is continued to a second day, save the frog in a plastic bag.
• When the dissection is finished, ALL parts of the frog go into the provided trash can.
• Clean and dry the tray with paper towels - DO NOT WASH.
• Clean and dry all utensils with paper towels - DO NOT WASH.
• Return tray and utensils to lab table.