Part B

Observing the Effects of Thyroxine on Tadpoles

13. Feed each beaker of tadpoles 1 mL of spinach every other day. Be careful not to overfeed the tadpoles. Change the water in the beakers every 4 days, adding thyroxine solution in the original amounts to beakers A and B. Label the beakers, and do not mix up the tadpoles during the water changes.

14. Measure the tadpoles once a week for 3 weeks, and average the length of the tadpoles in each beaker. Record the average lengths in your data table.

15. Calculate the average growth per week for each set of three tadpoles. For example, the average growth in total length during the second week is equal to the average total length at the end of week 2 minus the average total length at the end of week 1. Record your values in the appropriate spaces of your data table.

16. Graph your data using a different colored pencil for the tadpoles in each beaker. Label the horizontal axis "Time in weeks," and label the vertical axis "Length in centimeters." You should have three graphs. Each graph should include the changes in average total length, average tail length, and average body length for one group of tadpoles.

17. Jitv jiK Clean up your materials, and wash your ^^ ^r hands before leaving the lab.

Analysis and Conclusions

1. Did this investigation include a control group? If so, describe it. If not, suggest a possible control that you could have used.

2. Why are three tadpoles used for each beaker rather than just one?

3. According to the data that you collected in this investigation, what is the effect of thyroxine on tadpole metamorphosis?

4. Which concentration of thyroxine solution caused the greatest visible change in the tadpoles?

5. How do average body length and tail length change during metamorphosis?

Further Inquiry

Iodine, typically in the form of iodide, is needed to make thyroxine. Design—but do not conduct—an experiment that shows the effect of adding iodine to water that contains tadpoles. CAUTION: Iodine is a highly poisonous substance.

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