Modeling Fertilization and Early Development
1. Take your lump of clay and roll it into a ball approximately 6 cm in diameter. Set
aside a small portion of clay for the next step. The ball you have just created will
represent an egg.
2. Using your knowledge of cell sizes, use some of the remaining clay to create a sperm
cell of appropriate size.
3. The first step in sea urchin reproduction is fertilization. In this event, the egg and
sperm fuse to form a zygote. Using your clay cells, model this event.
4. Following fertilization, the single-celled zygote must develop into a multicellular
adult. This development begins with a process called cleavage. In cleavage, the
fertilized egg goes through several divisions without increasing in size. Using the piece of
string provided, take your fertilized egg through its first division.
5. Repeat step four three times; your teacher will demonstrate where the plane of
division is. How many cells are now present?
6. At this stage, the embryo is called a morula. It is a solid ball of cells, equal in size to
the original fertilized egg. In the next set of divisions, the embryo will change from a
solid to a hollow ball of cells. During these divisions, the embryo will also begin to
increase in size. The cells of the morula will move outward to form the hollow sphere
called the blastula.
7. As best you can, convert your model from the morula stage to the blastula stage.
This will require that you remove tissue from the interior of your embryo and add tissue
to the outer layer. Obviously, the process by which blastula formation occurs cannot be
accurately replicated in this exercise; however, we can create something similar to the
final product.
8. So far, our embryo has maintained a spherical shape as it has increased its total
number of cells. In the next stage of development, the overall shape of the embryo
begins to change. To mimic this next stage, you should place your finger on the surface
and gently press inward. The pore you are creating is called a blastopore. Continue to
push in at the blastopore until a long tube is formed. This process is called gastrulation.
The tube you have created is called the archenteron; ultimately this tube will become the
gut of the animal.
9. The embryo at this stage is called a gastrula. Up until this point, all of the cells in
the embryo have been identical. Once gastrulation begins, cell differentation takes place.
When this process occurs, distinct cell layers begin to form. The embryo also begins to
lose its spherical shape. To model this in your clay embryo, you should roll the embryo
between your hands so it elongates.
10. Once it has developed into a gastrula, the different layers of cell begin to form.
Using your embryo and the colored clay, make the three germ layers, Ectoderm,
Mesoderm, and Endoderm. Have your teacher check your model.
11. Draw each stage of embryonic development, beginning with egg and sperm and
ending with the formation of Ectoderm, Mesoderm, and Endoderm in your notebook.
Next to each illustration, describe what is happening.