21.   Doctors have tried for at least three decades to use animal parts as replacements for failing human organs.

22.   Demand for human organs has for years far exceeded the supply.

23.   Despite the advances, xenotransplant experts expect human organs to remain the preferred choice for patients.

24.   Federal law expressly forbids trafficking in human organs but not in sperm and eggs, they said.

25.   For several months, Wells guided his new apprentices through a meticulous study of preserved human organs.

26.   For example, a single tone can represent a color, a season or even something as unusual as a human internal organ.

27.   Gears, motors, bearings and pumps with dimensions comparable to the thickness of a hair have become available for implantation into human organs.

28.   Genetically engineered pigs could be developed as a source for human organ transplants.

29.   He was credited with the discovery of isozymes, enzymes that vary in form to allow the development of different human organs and tissues.

30.   Here as elsewhere the procurement of human organs has always existed.