Download Biology Chapter 9 Starfish development LAB 2009

Name: ________________________________ Hour: _______ Date Due: _______________
BIOLOGY A
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.
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:
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.
Find this stage under low power (40X), then view it under medium power (100X). 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?
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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 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. Label this layer in the following diagram:
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5. Starfish larval forms are called bipinnaria. They are significantly more developed than
the previous stages. It moves 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?
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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?
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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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