German Hans Spemann proposes nuclear transfer----

John Gurdon, a British molecular---- biologist

 conducting research on nuclear transfer produces adult frogs from tadpole intestine cells, proving that even specialized cells are totipotent- they retain the ability to produce a complete organism. Thus it suggested that all cells contain a complete set of genes and that certain genes are active only in certain cells. However, throughout the 1960's and 1970's, scientists cannot produce a cloned vertebrate that would survive to adulthood.

February 1997, ---

the cloning of Dolly is announced -
August 1997, a more efficient cloning method is developed.
June 1999, the first successful cloning of a human embryo is announced.
August 2000, cloning technique improved further.

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---- Robert Briggs and Thomas King, development biologists at what is now the Fox Chase Cancer Center in Philadelphia, develop nuclear transfer. Briggs and King use body cells from frog embryos to produce tadpoles. Researchers are unsure if specialization means that only certain cells have certain genes, or if the genes that are not used by the cell are just inactivated. Throughout the 1950's, scientists clone amphibians such as frogs and salamanders using nuclear transfer.

--- Embryo splitting developed. Embryo splitting entails the splitting of an embryo into individual cells, a process similar to what happens naturally in twins. After each cell develops into a new embryo, they are placed in the womb of a host animal to carry it to full term. This technique does not ensure that the physical characteristics of the cloned animal are known. This technique proved useful to livestock breeders and by the 1990's, using this technique, various animals such as pigs, sheep, cows and rabbits have been cloned.

---- July 1997, transgenic animals are produed, opening the possibility of using animals to treat human diseases.
July 1998, improvements are made to the cloning process. August 1999, study on Dolly sugggests that she might be prematurely aging.
September 2000, researchers find cloning does not necessarily cause premature aging in clones.