VARIOUS SHOTS from a pond in the forest. A man in a white lab coat enters the pond while fully dressed. He empties a liquid containing mosquitoes mixed with radioactive phosphorus into the pond. Then he plunges a large sieve into the pond and collects tadpoles. He returns to the edge of the pond, where a female colleague stands, holding a receptacle. They place a tadpole on a tree stump and measure its radioactivity with a Geiger counter.