CAPE CANAVERAL, JULY 11 -- The space shuttle Columbia's payload commander Richard Hieb is growing in orbit, and now exceeds NASA's height limit for astronauts.

Hieb started the two-week laboratory mission Friday at 6 feet 3 inches. Today, he topped 6 feet 4, the limit for someone on a space shuttle.

"According to my quick calculations here, I seem to have grown about an inch or so. So I'm now too tall to fly in space," Hieb informed payload controllers after measuring himself as part of a medical experiment. "And that's without slipper-socks."

Astronauts -- tall and short alike -- tend to grow two inches or more in space because of spinal elongation, a phenomenon caused by the absence of gravity and often accompanied by back pain. They revert to their normal heights once back on Earth.

Hieb and Japanese astronaut Chiaki Mukai, Columbia's tallest and shortest astronauts (Mukai is 5 feet 2 inches), are measuring themselves each morning so researchers can compare growth to soreness. The two also are recording the curvature of their spines via stereo photography and sending tiny electric impulses through their spinal column nerves to see if the nerves function properly when stretched.

Alan Mortimer, project scientist for the Canadian Space Agency, said the findings should contribute to the treatment of back pain on Earth as well as in space.

Aspiring astronauts must be at least 4 feet 10 1/2 inches to be accepted into the program and at least 5 feet 4 inches to be a shuttle pilot. The seven astronauts aren't the only ones growing aboard Columbia.

The 144 newt eggs sent into space by developmental biologists already are sprouting tails. A few even are developing gills.

Astronaut Donald Thomas injected hormones into two of the four adult female newts today to induce egg laying. The two others received enough hormone before liftoff. Altogether, the newts have laid at least 15 eggs.

There are a few dead flies but Columbia's other animals, fish, were reported in good shape. Scientists want to see how the creatures develop, behave and, for Japanese Medaka fish, mate in weightlessness.

Japanese project scientist Shunji Nagaoka said the four Medaka -- two male and two female -- are mating in space just as they would on Earth. At least 10 eggs have been produced.