Download Study and Removal of the Frog¹s Brain

Frog Dissection Lab
Objectives:


Describe the adaptive value of various organs found in the frog.
Name, locate, and identify the organs that make up various systems of the frog.
Materials:
Safety goggles, gloves, dissecting pins, forceps, scissors, paper towel, dissecting probe, preserved frog,
dissection tray.
Purpose:
In this lab, you will dissect an adult frog in order to observe the external and internal structures of frog anatomy
that make the frog adapted to its environment.
Background:
Frogs are classified as amphibians meaning "live a double life.” Frogs are part of the amphibian order, Anura
who are adapted to live in water and on land. Tadpoles are aquatic and herbivores while adult frogs can live on
land and in water and are carnivores. Strong muscles and special fused bones help frogs be powerful swimmers
and jumpers. As young tadpoles, they breathe through gills, but on land, frogs breathe though underdeveloped
lungs and their skin. Adult frogs have loose, mucus lined skin to help them escape from predators, and keep
them wet which aides in cutaneous respiration (breathing through the skin). Cutaneous respiration limits the
frog’s body size. The backs of frogs are dark, while their undersides are light, to camouflage them on land and
water. The organization of an adult frog’s internal organs is similar to the internal organization of other
vertebrates that live on land.
Problem
What are some features of a frog’s anatomy that help it adapt to its environment?
Pre-lab: Getting to know your Frog Prince or Frog Princess!
Answer the following questions in your lab notebook (loose leaf paper). Use the internet to assist you.
1. What problems did terrestrial vertebrates face when they came out of the water?
2. What structures on early amphibians evolved in order to survive out of water?
3. Are amphibian eggs fertilized internally or externally before they are abandoned?
4. Some amphibians do not abandon their eggs. What is one way they incubate their eggs?
5. Why do most amphibians live in moist environments?
6. How do amphibians contribute to the ecosystem in which they live?
Procedure Part 1: External Anatomy and Mouth Observation
Locate each of the structures below on your frog. Make observations about the structure, including
measurements, if they apply. Then, describe the adaptive value of the trait.
Structure/Feature
Observations
Measurement
Dorsal color
N/A
Ventral color
N/A
Location of eyes
Diameter of each eye
L: ____________ mm
R: ____________ mm
Location and shape of
tympanic membrane
(ear)
Diameter of each
membrane
L: ____________ mm
R: ____________ mm
Location of nostrils
N/A
Adaptive Value
Shape of head
Length of head nose to
neck
____________ cm
Length of each leg
Hind legs (form)
L: ____________ cm
R: ____________ cm
Length of each foot
from ankle to longest
toe
Feet (form)
L: ____________ cm
R: ____________ cm
For the following structures, you will need to see inside the frog’s mouth. Pry the frog's mouth open and
use scissors to cut the angles of the frog's jaws open. Cut deeply enough, through the jaw joints, so that
the frog's mouth opens wide enough to view the structures inside.
Tongue
Point of attachment:
Length of tongue
______________
____________ cm
Vomerine teeth
N/A
Maxillary teeth
N/A
In the diagram below, label the head of the
frog. Include all the parts you have looked
at.
In the diagram below, label the frog’s
mouth. Include all the parts you have
looked at.
Procedure Part 2
Thoracic and Abdominal Cavities
1. Place the frog in the dissecting pan ventral (bottom) side up.
2. Secure the frog to the pan by pinning its legs through the feet.
3. Use scissors to lift the skin away from the body cavity. Cut
along the midline of the body from the pelvic to the c
4. Make transverse (horizontal) cuts near the arms and legs.
5. Lift the flaps and pin back to the pan.
6. Repeat Steps 3 – 5 for the abdominal muscles.
*If your specimen is a female, the body may be filled with eggs and an
enlarged ovary. Ask your teacher to remove these eggs to view the
organs. Please do not remove them yourself!
Make observations about the structures, including measurements, if they apply. Then, describe the adaptive value
of the trait.
Structure/Feature
Observations
Measurement
Fat bodies
N/A
Make drawing with dimensions
Size of liver
Heart
Length: ___________ cm
Width: ___________ cm
Lungs
cm
L:
Left
Right
R:
Make drawing with dimensions
Shape, texture of stomach
Small intestine
Length: ___________ cm
Circumference:
___________ cm
Adaptive Value
Length and
circumference of large
intestine
Length: ___________ cm
Circumference:
___________ cm
Length: ___________ cm
Body cavity
Width : ___________ cm
Depth: ___________ cm
Fat Bodies Linguini 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.
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.
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.
Lungs Locate the lungs by looking underneath and behind the heart and liver. They are two spongy organs.
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.
Removal of the Stomach: Cut the stomach out of the 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 on the inside. What
contents did you find in the stomach? Describe the texture of the stomach on the inside.
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.
Measuring the small intestine: Remove the small intestine from the body cavity and carefully
separate the mesentery from it. Stretch the small intestine out and measure it. Add the numbers
together to get the total frog length. Record the measurements below in centimeters.
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)
In the diagram below, label the head of the frog. Include all the parts you have looked at.
Procedure Part 3
The Urogenital System
The urogenital system of the frog consists of organs that function in reproduction and excretion. Check the box
when you have located the structure. The descriptions of their locations and appearance will help you label the
diagrams.
Male Frog
Female Frog
1. Locate the kidneys. They are reddish brown
organs located to the back of the frog and close
to the spine.
1. Locate the kidneys. They are reddish brown
organs located to the back of the frog and close
to the spine.
2. Find the vessels attached to the kidneys. These
are the renal vessels (it may be difficult to
determine which is an artery and which is a
vein).
2. Find the vessels attached to the kidneys. These
are the renal vessels (it may be difficult to
determine which is an artery and which is a
vein).
3. The orangish-yellow hair like structures
attached to the top of the kidney are the fat
bodies.
3. The orangish-yellow hair like structures
attached to the top of the kidney are the fat
bodies.
4. Locate the testes which are light colored
spherical objects at the top of the kidney.
4. The tube like structures to the side of the
kidney are oviducts. They may not be visible if
your frog has eggs. Eggs can be seen as black
speckled masses in the frog's body cavity!
5. Small tube like structures to the side of the
kidney are vestigial oviducts. In the male frog,
they have no function.
6. The tubes that extend from the kidney and
enter the cloaca are the ureters - they carry
urine.
7. The flap like structure is the bladder, it is
empty in a preserved frogs
5. At the top of the oviducts is the ostium. This
is where the eggs in the body cavity enter the
oviducts to be excreted. You may not be able to
find it on your frog.
6. The oviducts lead to the cloaca, eggs and urine
will exit the body through the cloaca.
7. The flap like structure is the bladder, it is
empty in a preserved frogs
8. The tube extends from the kidneys to the
cloaca is the ureter - it carries urine.
male
female
Procedure Part 4
Study and Removal of the Frog¹s Brain
Turn the frog dorsal side up. Cut away the skin and flesh on the head from the nose to the base of the
skull. With a scalpel, scrape the top of the skull until the bone is thin and flexible. Be sure to scrape AWAY
from you. With your scalpel held almost horizontally, carefully chip away the roof of the skull to expose the
brain. Use scissors to cut away the heavier bone along the sides of the brain.
Find the nasal pits at the anterior end of the brain. The olfactory nerves leave these structures and connect to
the most anterior lobes of the brain, the olfactory lobes (A).
Just posterior to the olfactory lobes are two elongate bodies with rounded bases, this is the cerebrum (B), and it
is the frog¹s thinking center. The cerebrum is the part of the brain that helps the frog respond to its
environment. Posterior to the cerebrum are the optic lobes (C), which function in vision. The ridge just
behind the optic lobes is the cerebellum (D), it is used to coordinate the frog¹s muscles and maintain balance.
Posterior to the cerebellum is the medulla oblongata (E) which connects the brain to the spinal cord (F).
Brain Part
Cerebellum
Cerebrum
Olfactory Lobe
Optic Lobe
Medulla Oblongata
Function
Letter
Analysis Questions Complete in your lab notebook.
1. Explain how the coloration on the dorsal and ventral sides of a frog helps it to adapt to its environment.
2. What percentage of the total length of the frog (body + leg) are legs? How does this large percentage
help a frog adapt to its environment?
3. Describe the anatomy of a frog’s eye (you can draw a diagram if you’d like!). How does the presence of
the nictitating membrane help a frog adapt to its environment?
4. Explain how the anterior attachment of a frog’s tongue helps it adapt to its environment.
5. If you were to dissect a frog at the end of hibernation, you would observe the fat bodies looking similar
to strands of yellow thread. Why would they be so small?
6. Why are the lungs of a frog so tiny compared to its body size?
7. Explain how laying hundreds of eggs helps a frog adapt to its environment.