A fertilized egg experiment aboard the recent Discovery flight raised more questions than it answered after half of them returned dead, the shuttle's pilot says.

The experiment included 16 chicken eggs fertilized two days before the March 13 blastoff and another 16 fertilized nine days prior to launch.Although all of the older eggs hatched Saturday, the others were found dead when the shuttle returned March 18. Both the hatched chicks and the dead ones are being studied to determine how weightlessness affected them.

"What looked like a very simple experiment may turn out to have generated thousands of questions now instead of providing any answers to anything," Discovery pilot John Blaha said Tuesday at a news conference. "It's certainly a great mystery as to why this happened."

Dr. James Bagian, another astronaut on the flight, said the experiment raised questions as to whether plant or animal life can begin in zero gravity.

Bagian said the chicken eggs and another experiment in which he observed plant growth were designed to determine if cell division is somehow "impeded or mutated" by a lack of gravity.