-210 Nottingham - Nottingham eTheses - The University of Nottingham

mosaic after a few cell divisions.
Subsequently to the studies of Wilhelm Roux, rather than killing a single blastomere,
Hans Adolf Eduard Driesch separated the blastomeres at the 2-cell stage and found
that each developed into a complete sea urchin (Driesch, 1892). These results were
later confirmed by Hans Spemann. In 1894, Jacques Loeb performed his experiment
on artificial parthenogenesis, by which the eggs of sea urchins started embryonic
development without sperm. This was achieved by slight chemical modifications of
the seawater in which the eggs were kept (Loeb, 1914). Driesch and Loeb's separate
experiments demonstrated that nuclei had not lost any genetic material at the cleavage
stages of development.
In 1895, Yves Delage published a book on general biology and wrote a short
paragraph, which is translated as follows: `Every nucleus, at least at the beginning of
ontogenesis, is a sex cell nucleus and if, without deterioration, the egg nucleus could
be replaced by the nucleus of an ordinary embryonic cell, we should probably see this
egg developing without changes', which indicated a proposal for performing a
nuclear transfer into an enucleated egg. (Delage, 1895, p. 738; Beetschen and Fischer,
2004).
Cloning of vertebrates traces back to 1914 (Appendix 1; Gurdon and Byrne, 2002;
Solter, 2000; Zhu and Sun, 2000). Hans Spemann proposed a set of experiments to
determine when the first irreversible differentiation events, or unequal inheritance,
occurred. He tied off a small part of the cytoplasm (without nuclei) of a fertilised
salamander egg using a thin hair so that the enucleated position could not develop.
After several cell divisions (approximately 16 cell stage), he loosened the loop and
allowed a nucleus from one of the developing cells to migrate into the cytoplasmic
compartment. He then retightened the loop so that the small part with the migrated
nucleus remained separated from the rest of the embryo. The small piece of
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