John Gurdon of Oxford University announced in 1962 that he cloned South African frogs with fully differentiated adult intestinal cells.  His experiment proved that cells retain the ability to form different tissues even after they specialize.  However, certain flaws were found in his work.

            Dennis Smith pointed out that there were some undifferentiated sex cells in the intestines of South African frogs, so the tadpoles Gurdon cloned may very well be cloned from those undifferentiated cells. 

            Marie A. Di Berardino also criticized Gurdon’s experiment.  She could not clone tadpoles using the nucleus of sperm cells.  Since sperm cells contained genetic information necessary to direct an organism’s development, she speculated that nuclear transfer damaged the nucleus and was a poor way to find out if a specialized cell’s genetic information can develop an entire organism.  

See also: 

History of Cloning: Timeline