Amoeba Scale

A standard chore in biological laboratories is weighing the experimental animals (e.g., mice and guinea pigs) to record their rate of growth. Biologist David Marshall Prescott, 27, of the University of California does this chore too, but his experimental animals are amoebas, and they weigh only ten billionths of a gram each.

The weighing is done in a complicated apparatus whose essential part is a tiny glass tube closed at one end and trapping a bubble of air. When this "Cartesian diver" is properly weighted, it floats midway in a column of water, and slight changes in water pressure can make it sink or rise by compressing its bubble.

To put an amoeba on his scale, Dr. Prescott waits until one of his stock has just divided into two new individuals. Working with a powerful microscope, he picks one of them up with a hairlike suction tube and delicately transfers it to a minute cup on top of the Cartesian diver. It cannot escape, but it thrives slimily on a broth of smaller protozoa. By measuring the pressure that will keep the underwater Cartesian diver on the zero line, Dr. Prescott can weigh his captive amoeba at all stages of growth. When the amoeba has doubled in weight, it generally divides into two "daughters."

Dr. Prescott is the first to weigh the growth of individual amoebas, but he is not tending his microscopic livestock as a mere stunt. He has already discovered something new: that amoebas generally grow for about 20 hours. Then for the next four hours before they divide, their weight does not increase. This scrap of information, gathered so laboriously, is considered important in the study of cellular growth, including the growth of cancer cells in the human body.

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