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Frogs: Princely models for science
These frog eggs have just been fertilized. The dark portion will become the back of the frog, the lighter part will become the front. These eggs are very large compared with those of many other animals, making it easy to study fertilization and development.

A froggy dance in rubber pants
Long before frog eggs were used to detect pregnancy, they helped scientists discern where new life comes from. Scientists knew centuries ago that sperm existed, but there was considerable debate about its role. In 1777, the Italian scientist Lazzaro Spallanzani (and some fashionably dressed frogs) confirmed that sperm are needed to activate the egg. Spallanzani outfitted male frogs with form-fitting rubber pants, a kind of primitive frog family planning suit. He saw that the pants-wearing frogs didn’t produce any tadpoles, but that if he then mixed the pants in with freshly-laid eggs—voilá, offspring.

While it’s easy to make light of the image of frogs in togs, the importance of Spallanzani’s experiment shouldn’t be underestimated. He put to rest the notion that new life could arise from either the egg or the sperm alone. He didn’t know, however, what the contribution of the sperm actually was. That discovery would have to wait until researchers saw fertilization, the fusing of the sperm and egg nuclei, in sea urchin eggs.

Watch the Sea Urchin Fertilization Movie

Size matters

Today, Xenopus eggs are still used for research, in part because they’re big enough to be manipulated easily. Their size—about one millimeter, huge compared to most other animals (human eggs are about 1/10 as large)—makes it relatively easy for scientists to inject genes. The egg then becomes a kind of factory, pumping out thousands of copies of the protein that that gene codes for. In many cases, the protein will function normally even though it’s being made and is functioning in a frog egg rather than a human cell. This kind of protein-manufacturing capability allows researchers to quickly test the effectiveness of a wide variety of drugs, and researchers have used this technique to assess effects on a diverse array of proteins, from those involved in brain function to ones that regulate the growth of tumors.

Next: A pondful of uses »