Today, in a complex of laboratories built within unassuming red-roofed barns, Wildt is summoning the tools of cryo-preservation — the storage of living tissue in extreme cold — to amass a frozen zoo, a 21st-century ark that offers hope of survival to species on the brink of extinction.

At the San Diego Zoo, cells have been stored in the deep suspending chill of liquid nitrogen - cells of hundreds of other endangered animals. In this ''frozen zoo,'' plastic vials await the development of the new technology that promises to preserve a substantial number of cell lines and fertilized eggs until a future science may awaken them. ''These techniques have not yet been fully explored or exploited,'' says Dr. Kirk Benirschke, the zoo's research director, ''but we must move on these fronts very rapidly. In eight to 10 years, it will be possible to freeze and preserve almost everything,'' he says, for future generations.

Currently, zoos follow existing livestock breeding techniques fairly closely, but scientists such as Dr. Benirschke and Dr. Conway see the day when sperm and tissue banks challenge extinction in special cases. According to researchers, the day will come when the embryo of, say, a snow leopard may be implanted into the surrogate womb of a lion, or even transported to another country in the body of a rabbit.

''The concept is simple,'' Dr. Benirschke says. ''When the time comes the cells of an extinct female can be thawed, and perhaps generations from now a new baby will enter the world.''

But, for the immediate future, the significance of a frozen zoo lies in its promise to overcome the most severe drawback of captive breeding — one that threatens the very identity of the species. Inbreeding from repeated use of limited stock can create unwanted, harmful and sometimes lethal genetic abnormalities. The only safeguard is a broad and variable gene pool — access to the genes of hundreds of specimens from each species.