HOUSTON, MARCH 14 -- On their second day in orbit, the crew of the shuttle Discovery played mother hen to chicken eggs and other embryonic research projects, took movies of Earth and helped NASA engineers cope with gremlins in the hardware. A mysterious reading in one of three tanks that fuel the cells that power the orbiter prompted the astronauts to turn off some lights, a computer and other unneeded equipment, but there was "no danger" to crew safety, flight director Granvil Pennington said. If the power supply is affected, he said, Discovery "could land a day early." Landing currently is scheduled for about 9:30 EST Saturday at Edwards Air Force Base in California. After four postponements, Discovery was launched smoothly Monday morning. Problems and delays are, as National Aeronautics and Space Administration space flight chief Richard H. Truly said after the launch, "part of space biz." But this has been a particularly trying lesson for two students whose experiments are on Discovery. The two have matured from junior high to college age as they waited eight years for their projects to move from conception to earth orbit. Both experiments are designed to study the impact of gravity, or the lack of it, to aid future space travelers and to increase understanding of how organisms develop. As a ninth-grader, John Vellinger, now a 23-year-old senior majoring in mechanical engineering at Purdue, designed an experiment to study for the first time how an embryo develops in zero gravity. His first version was lost aboard Challenger when it exploded in 1986. Named "Chix in Space," the experiment consists of 32 fertilized eggs in an incubator on the orbiter and 32 eggs in a control group on Earth. The Earth eggs will be rotated periodically as a mother hen would do. To research the critical period, the space eggs had to be laid exactly the right number of days before launch. This meant keeping some hens busy laying contingency eggs for potential launch delays. After the Discovery lands, half the eggs from the two groups will be compared for differences in tissue, muscle and organ development. The balance will be hatched. "We should have some baby space chicks by Easter," said a somewhat frazzled-looking Vellinger, who said he had slept only an hour in two days. Pilot John Blaha's job on the Discovery is to report on temperature and vibrations, open the incubator door seven times during the mission, let in fresh air, and regularly replace a device that maintains the "barnyard moisture" inside. Andrew Fras, 22, had the same long wait for his bone-healing experiment involving four male rats. Fras is in his second year at Brown University's medical school, studying to be an orthopedic surgeon. The experiences of cosmonauts and astronauts as well as ground studies have shown that bone calcium and other minerals are lost in the weightlessness of space. In each of Fras' rats, a fine bit on a tiny pneumatic drill was used to cut the fibula, a nonweight-bearing leg bone no larger than the point of a sharpened pencil, Fras said. As with the eggs, the operation had to be precisely timed, exactly five days before launch. This meant staggering the operation for several groups of rats. Animal rights groups have raised questions about the experiment, but Fras and NASA spokesman Charles Redmond emphasized that the rats were anesthetized for the surgery and were treated humanely in keeping with scientific practice. The space rats, and a control group on the ground, will be killed after landing so their healing processes can be compared.