In September 1946, Levi-Montalcini was granted a one-semester research fellowship in the laboratory of Professor Viktor Hamburger at Washington University in St. Louis; he was interested in two of the articles Levi-Montalcini had published in foreign scientific journals. After she duplicated the results of her home laboratory experiments, Hamburger offered her a research associate position, which she held for 30 years. It was there that, in 1952, she did her most important work: isolating nerve growth factor (NGF) from observations of certain cancerous tissues that cause extremely rapid growth of nerve cells. By transferring pieces of tumors to chick embryos, Montalcini established a mass of cells that was full of nerve fibers. The discovery of nerves growing everywhere like a halo around the tumor cells was surprising. When describing it, Montalcini said it is: "like rivulets of water flowing steadily over a bed of stones." The nerve growth produced by the tumor was unlike anything she had seen before – the nerves took over areas that would become other tissues and even entered veins in the embryo. But nerves did not grow into the arteries, which would flow from the embryo back to the tumor. This suggested to Montalcini that the tumor itself was releasing a substance that was stimulating the growth of nerves.