Dennis Smith pointed out that in South African frogs, the species Gurdon used in his cloning experiment, undifferentiated sex cells were present in small numbers mixed in with the intestinal tissue. Smith speculated that the few tadpoles Gurdon had successfully cloned were cloned using the nucleus of one of these undifferentiated sex cells. As Briggs and King proved in 1952, cloning could be accomplished with the use of nuclei from undifferentiated cells. If Smith was correct, Gurdon's findings were actually nothing new at all.
Additional criticism of Gurdon's work came from Marie A. Di Berardino. Di Berardino was unable to clone tadpoles using the nucleus of sperm cells. Because sperm cells obviously contain the genetic information necessary to direct an organism's development, Di Berardino concluded that the method of nuclear transfer damaged the transferred nucleus, and therefore was a bad way to determine if a specialized cell's genetic information retained the potential to direct a complete organism's development.
It was never been decisively determined in Gurdon actually succeeded in cloning from differentiated cells in 1962. Gurdon's work, did however, trigger the beginning of mainstream public debate on the cloning topic.
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