Nuclear transplantation is a technique that has enormously facilitated the analysis of these interactions between nucleus and cytoplasm. Such facts have justified the belief that the early events in cell differentiation depend on an interaction between the nucleus and the cytoplasm. It is also evident in the effect of egg cytoplasm on the behavior of chromosomes. The importance of the egg's non-nuclear material-the cytoplasm-in early development is apparent in the consistent relation that is seen to exist between certain regions in the cytoplasm of a fertilized egg and certain kinds or directions of cell differentiation. If gene activity were not required for gastrulation and further development, the hybrids should survive as well as nonhybrids. Yet the hybrids differ from nonhybrid embryos only by the substitution of some of the nuclear genes. Such hybrids typically die before they reach the gastrula stage, the point in embryonic development at which major cell differences first become obvious. This is clearly shown by the nonsurvival of hybrid embryos produced by fertilizing the egg of one species (after removal of the egg's nucleus) with the sperm of another species. Several different kinds of experiment have revealed the dependence of cell differentiation on the activity of the genes in the cell's nucleus. As with most animal eggs, the early events of amphibian development are largely independent of the environment, and the processes leading to cell differentiation must involve a redistribution and interaction of constituents already present in the fertilized egg. Much of the experimental work designed to investigate the problem has been done with amphibians such as frogs and salamanders because their eggs and embryos are comparatively large and are remarkably resistant to microsurgery. The means by which cells first come to differ from one another during animal development has interested humans for nearly 2,000 years, and it still constitutes one of the major unsolved problems of biology. The full article with images, which was published in the December 1968 issue, is available for institutional users only at this time ( pdf). Gurdon, is one of the winners of the 2012 Nobel Prize in Physiology or Medicine. Editor’s note (): We are making the text of this article freely available for 30 days because the author, Sir John B.
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