Name: ________________________________ Hour: _______ Date Due: _______________
HONORS BIOLOGY B
Animal Development Lab
Most animals develop from a single fertilized egg (zygote). How does a zygote develop into
the many different kinds of cells that make up the entire animal? This question can be answered by
studying the process of embryological development. In this lab, you will observe the various stages of
embryological development in the starfish. Starfish belong to a group of organisms called
Echinoderms (ee-kine’-o-derms). Their development is very much like that of humans, and they
belong to a branch of the Kingdom Animalia called deuterostomes, as do all primates (humans are
primates). Deuterostomes are distinguished by how their embryos develop; in deuterostomes, the first
opening (the blastopore) becomes the anus, while in protostomes (simpler animals) it becomes the
mouth.
Materials:
 Microscope
 Prepared slide of mixed stages of starfish development
Procedure:
After getting your equipment together, place the prepared slide on the stage of your microscope
under low power (40X); bring the stage up while looking through the eyepiece until you see the
specimens. Focus carefully, then take a few minutes to scan the slide (move the slide around), and
locate a portion of the slide showing many different types of embryos (some may be colored
differently). Then move your objective lens to medium (100X) and follow the steps below. Be very
careful that you do not crush the slide with the objective lens; these specimens are thicker than usual.
1. After fertilization by a single sperm, the egg is fertilized and is now known as a zygote. This
zygote then divides into two cells by the process of mitosis. This new two-celled organism is
now referred to as an embryo:
egg
zygote
embryo
On your slide, find a two-celled embryo under 100X magnification. Being careful to draw to scale,
sketch the embryo here, using the appropriate colors if you have colored pencils.
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2. Following the division into two cells that continue to be joined together, the embryo further
divides into four cells:
2 cells
4 cells
Find the four celled stage and sketch it here. In addition, be sure that the scale and color are
correct.
3. The embryo continues to divide, going from four cells to eight cells.
4 cells
eight cells
Locate the eight cell stage and sketch it here:
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4. The embryo continues to divide by the process of mitosis. After many cell divisions, the
embryo appears as a hollow ball of cells. This hollow ball is called the blastula.
blastula
Find this stage under low power (40X), then view it under medium power (100X). After being certain
that the embryo you are viewing is exactly in the center of the field of view, carefully swing the high
power (400X) objective into view (do not lower the stage when you swing the high power objective
into view). Using only the fine focus, look at the embryo carefully, and sketch it here:
Estimate the number of cells in this embryo: ______________ How does the size of these cells
compare with the size of the original zygote?
__________________________________________________________________________________
__________________________________________________________________________
At this point in the embryo’s development, all cells are nearly the same. They have not yet begun the
process of differentiation that will result in cells specializing to carry out different functions.
Once the hollow ball of cells (called the ____________________________) is complete, some of its
cells begin to move, or migrate, in a very precise way. The beginning of this movement can be seen in
the image below, and is called the blastopore.
blastopore
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Some cells move towards the interior of the ball of cells, and this stage is called the gastrula.
The blastopore is much larger now, and some cells that were on the outside of the embryo are now on
the inside.
blastopore
Find this form on your slide, first under low power, then under medium (100X) power. Sketch it here:
The outer layer of this form is called the ectoderm, and develops into the skin and nervous system of
the animal. Label this layer on your sketch.
The inner layer of this form is called the endoderm, and develops into the lining of the digestive tract
and organs that are part of the digestive process. Label this layer on your sketch.
In many animals, like humans, a third layer of cells forms between the outer and inner layer. This layer
is called the mesoderm and develops into the muscles, circulatory system, excretory system, and in
some animals, the respiratory system. In deuterostomes the mesoderm forms as evaginations of the
developed gut that pinch off, forming the coelom (sea’-lum). The coelem is a body cavity, and will be
completely lined with mesoderm in the embryo. Label this layer (mesoderm) in the following
diagram:
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5. Starfish larval forms are called bipinnaria. They are significantly more developed than the
previous stages. They move using cilia, and you can see a mouth, a digestive system, and an
anus. This larva will metamorphose into an adult starfish.
Locate a larva under low power, then view under medium power and sketch the larva here:
Estimate how many cells this larva contains: _____________ cells
8. You have observed many stages of starfish embryo development. How does cell size change
from the zygote to the larval forms?
____________________________________________________________________________
____________________________________________________________________________
9. Observe the drawing of a blastula below. What might happen to the adult starfish that grows
from this blastula if the cell marked A was destroyed at the blastula stage?
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
A
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Extra Credit: All animals begin life as a single fertilized egg, yet they grow to become a complex
organism consisting of billions of cells. Yet, even though every cell in an organism contains the same
DNA, not all cells are the same. Keeping in mind that all cell activities are controlled by DNA,
suggest a reason that different types of cells are different, even though they have the same DNA.
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