Materials

■ prepared slides of sea-star development, including unfertilized egg zygote 2-cell stage 4-cell stage 8-cell stage 16-cell stage 32-cell stage 64-cell stage blastula early gastrula middle gastrula late gastrula young sea-star larva

■ compound light microscope

■ paper and pencil

Background

1. Most members of the animal kingdom (including sea stars and humans) begin life as a single cell—the fertilized egg, or zygote.

2. The early stages of development are quite similar in different species. Cleavage follows fertilization. During cleavage, the zygote divides many times without growing. The new cells migrate and form a hollow ball of cells called a blastula. The cells then begin to organize into the three primary germ layers: endoderm, mesoderm, and ectoderm. During this process, the developing organism is called a gastrula.

3. The early stages of mammalian development are difficult to study because mammalian eggs are tiny and are not produced in great numbers. In addition, mammalian embryos develop within the mother's body. In the laboratory, it is difficult to replicate the internal conditions of the mother's body. Because the early stages of echinoderm development are similar to those of human development, and because echinoderm development is easier to study in the laboratory than human development, you will observe the early developmental stages of an echinoderm—the sea star—in this investigation.

2-cell stage

4-cell stage

2-cell stage

4-cell stage

8-cell stage

64-cell stage

8-cell stage

64-cell stage

4. As development continues, the cells continue to specialize as they become part of specific tissues and complex structures. Ectoderm forms the epidermis and nerve tissue. Mesoderm forms muscle, connective tissue, and vascular organs. Endoderm forms the lining of the digestive, urinary, and respiratory tracts.

5. Similarities and differences in early stages of development reflect evolutionary relationships between species.

Procedure

1. Obtain a set of prepared slides that show sea-star eggs at different stages of development. Choose slides labeled unfertilized egg, zygote, 2-cell stage, 4-cell stage, 8-cell stage, 16-cell stage, 32-cell stage, 64-cell stage, blastula, early gastrula, middle gastrula, late gastrula, and young sea-star larva. (Note: Blastula is the general term for the embryonic stage that results from cleavage. In mammals, a blastocyst is a modified form of the blastula.)

2. Examine each slide using a compound light microscope. Using the microscope's low-power objective first, focus on one good example of the developmental stage listed on the slide's label. Then switch to the high-power objective, and focus on the image with the fine adjustment.

3. In your lab report, draw a diagram of each developmental stage that you examine (in chronological order). Label each diagram with the name of the stage it represents and the magnification used. Record your observations as soon as they are made. Do not redraw your diagrams. Draw only what you see; lab drawings do not need to be artistic or elaborate. They should be well organized and include specific details.

4. Compare your diagrams with the diagrams of human embryonic stages shown on previous page.

5. ijjK Clean up your materials and wash your ^w hands before leaving the lab.

Analysis and Conclusions

1. Compare the size of the sea-star zygote with that of the blastula. At what stage does the embryo become larger than the zygote?

2. At what stage do all of the cells in the embryo not look exactly like each other?

3. How do cell shape and size change during successive stages of development?

4. Do the cell nuclei stay the same size, get larger, or get smaller as the stages progress?

5. Compare the number of chromosomes in a fertilized sea-star egg with the number of chromosomes in one cell of each of the following phases: 2-cell stage, blastula, gastrula, and adult stage.

6. From your observations of changes in cellular organization, why do you think the blastocoel (the space in the center of the hollow sphere of cells of a blastula) is important during embryonic development?

7. Label the endoderm and ectoderm in your drawing of the late gastrula stage. What do these two tissue types eventually develop into?

8. How are the symmetries of a sea-star embryo and a sea-star larva different from the symmetry of an adult sea star? Would you expect to see a similar change in human development?

9. What must happen to the sea-star gastrula before it becomes a mature sea star?

10. How do your drawings of sea-star embryonic development compare with those of human embryonic development? Based on your observations, in what ways do you think sea-star embryos could be used to study early human development?

11. Describe one way that the cleavage of echinoderms and mammals is alike.

12. Describe two ways in which cleavage in echinoderms differs from cleavage in mammals.

13. Why are sea-star eggs a good choice for the study of embryonic development in humans?

Further Inquiry

Using the procedure that you followed in this investigation, compare embryonic development in other organisms with embryonic development in sea stars. Which types of organisms would you expect to develop similarly to sea stars? Which types of organisms would you expect to develop differently from sea stars?

Laboratory

Skills SAFE LABORATORY PRACTICES

It is your responsibility to protect yourself and other students by conducting yourself in a safe manner while in the laboratory You will avoid accidents in the laboratory by following directions, handling materials carefully, and taking your work seriously Read the following general safety guidelines before attempting to work in the laboratory Make sure that you understand all safety guidelines before entering the laboratory If necessary, ask your teacher for clarification of laboratory rules and procedures.

General Guidelines for Laboratory Safety

Do not perform experiments not specifically assigned by your teacher. Do not attempt any laboratory procedure without your teacher's direction, and do not work in the laboratory by yourself.

Familiarize yourself with the experiment and all safety precautions before entering the lab. Be aware of the potential hazards of the required materials and procedures. Ask your teacher to explain any confusing parts of an experiment before you begin.

Before beginning work, tie back long hair, roll up loose sleeves, and put on any required personal protective equipment as directed by your teacher. Avoid or confine loose clothing that could knock things over, catch on fire, or absorb chemical solutions. Nylon and polyester fabrics burn and melt more readily than cotton fabrics do. Do not wear open-toed shoes, sandals, or canvas shoes in the lab.

Always wear a lab apron and safety goggles in the lab. Wear this equipment at all times, even if you are not working on an experiment at the time. Laboratories contain chemicals that can damage your clothing, skin, and eyes. If your safety goggles cloud up or are uncomfortable, ask your teacher for help. Lengthening the strap slightly, washing the goggles with soap and warm water, or using an anti-fog spray may help alleviate the problems.

Do not wear contact lenses in the lab. Even if you are wearing safety goggles, chemicals could get between contact lenses and your eyes and cause irreparable eye damage. If your doctor requires that you wear contact lenses instead of glasses, then you should wear eye-cup safety goggles—similar to goggles that are worn for underwater swimming— in the lab. Ask your doctor or your teacher how to use eye-cup safety goggles to protect your eyes.

Know the location of all safety and emergency equipment used in the laboratory before you begin a lab. Ask your teacher where the nearest eyewash stations, safety blankets, safety shower, fire extinguisher, first-aid kit, and chemical spill kit are located.

Immediately report any accident, incident, or hazard—no matter how trivial—to your teacher. Any incident involving bleeding, burns, fainting, chemical exposure, or ingestion should also be reported immediately to the school nurse or to a physician.

In case of fire, alert the teacher and leave the laboratory. Follow standard school fire safety procedures.

Do not fool around in the lab.

Take your lab work seriously, and behave appropriately in the lab. Be aware of your classmates' safety as well as your own at all times.

Do not have or consume food, chew gum, or drink in the laboratory. Do not store food in the laboratory. Keep your hands away from your face. Do not apply cosmetics in the laboratory. Some hair-care products and nail polish are highly flammable.

Keep your work area neat and uncluttered. Have only books and other materials that are needed to conduct the experiment in the laboratory.

Clean your work area at the conclusion of each lab period as your teacher directs. Dispose of broken glass, chemicals, and other laboratory waste products in separate, special containers. Dispose of waste materials as directed by your teacher.

Wash your hands with soap and water after each lab period. Also wash your hands before each lab period to avoid contamination.

Key to Safety Symbols and Their Precautions

Before you begin working in the laboratory, familiarize yourself with the following safety symbols that are used in this textbook and the guidelines that you should follow when you see these symbols.

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