The transition between primary cultures and established cell lines is large. Once animal tissue (often kidney cortex, as mentioned above) is broken down and the dispersed cells planted in a culture vessel, some of the cells settle to the glass or plastic surface, attach, spread, and multiply until a confluent monolayer of cells is formed. This is almost entirely fortuitous-the selection of cells that attach to the growing surface (those that do not are discarded at the first medium change) and multiply, and why they ideally stop multiplying when the monolayer is complete (the so-called contact inhibition) are beyond the control of the scientist, although the preparation of growing surfaces for this purpose is now well understood. In the case of the monkey species from which kidney cultures played such a key role in poliomyelitis research, it is said that poliovirus did not infect upon direct injection into the kidney of a living monkey. One can harvest cells from primary culture in a number of ways, dilute them in fresh growth medium, and plant a larger number of secondary cultures with a high probability of success. However, subsequent passages fail more often than not, and so the establishment of a durable cell line requires considerable tenacity. Primary cell cultures were considered better for production of vaccines, even though primary monkey kidney cells often harbored adventitious simian viruses that might threaten the vaccinee, because established lines were suspected of having undergone a malignant transformation. Primary cells were also widely used in food and environmental virology because they often had a wider virus-susceptibility spectrum and a greater tolerance for toxic substances in field samples than did available established lines. ''Diploid'' cell lines were developed which were said to preserve the karyotype of the source species: these tended to grow slowly and sometimes develop karyotypic abnormalities. Established lines, such as the venerable HeLa, have recognized identity, but they too are affected by selective pressures during repeated passage in any given laboratory, whereby they may acquire an identity that varies from what would be obtained under the same name, say, from the American Type Culture Collection.