Many species of invertebrates -- and, among the vertebrates, several species of frogs and at least one reptile -- use dissolved chemicals of small molecular size such as glycerol and glucose to survive repeated freezing and thawing of their natural, Arctic habitats.

We use similar substances in the laboratory to enable embryos to be cooled to extremely low temperatures and 'cryostored'. The antifreeze chemicals are called cryoprotectants. The trick is to get them to replace the water that's in the cells, otherwise when the water freezes it forms expanding ice crystals that crush the delicate cellular machinery.

There are two main ways the cryoprotectants work.

With conventional cryostorage of human embryos, we use a mixture of propanediol and sucrose. The propanediol penetrates into the cells, pushing water out, while the sucrose stays outside in high concentration, pulling water out. The embryo in its containing "straw" is slowly cooled in the mixture, using a computer to gradually super-cool it to -7 degrees C. Then, like throwing a stone into a calm, sub-zero lake to instantly freeze it over, the embryo is "seeded" (by grasping it with even colder forceps), the remaining water forms microcrystals too small to cause trouble, and the embryo is plunged into liquid nitrogen.

A newer, more experimental technique, goes further. Called vitrification, the penetrant is dispensed with, the sole chemical remaining outside the cell, but in even higher concentration than the sucrose used for conventional cryostorage. Still as much an art as a science, vitrification may work better for unfertilized eggs and blastocysts.

Either way, the embryos' environment is at the low temperature of liquid nitrogen, -192 degrees C, which is two thirds of the way down to the virtually absolute zero of the furthest reaches of outer space.