The method of these remarkable achievements, now often repeated, is to put tiny bits of living tissue in a plasma of blood serum that will coagulate. The blood must be deprived of its cells by the centrifugal process and must generally be taken from the animal for which the tissue is to be cultivated or, at any rate, generally from the same species, although this is not without exceptions, for chicken tissue has been grown in the blood of human beings, dogs, and rabbits; morbid tissue, perhaps, like cancer, being most indifferent. The tissue is taken from an etherized subject, with every possible precaution against bacteria, chilling, or drying, and so liable is it to be killed by exposure to air that it is best dissected in serum. Both plasma and tissue are kept in cold storage and the time during which it can be thus kept varies very greatly with different animals. The bit of tissue must be very small because only the outer edge can get the nourishment when deprived of the normal blood circulation, for when the piece of tissue is large, all but the periphery dies. To see these changes of form, small bits of tissue are grown on the inside of a coverglass of a microscope slide that has been overlain with a prepared plasma, sealed with paraffin and put into an electric incubator provided with a microscope. The period before growth begins varies but when it occurs, the microscope shows the direct division of the nuclei and the growth taking the form either of layers or of radiating chains, depending on whether epithelial or connective tissue is being developed. Each tissue, whether normal or morbid, develops very precisely tissue of its own kind, and sometimes as, for example, with cancerous tissue, the growth is so rapid that it can be observed with the naked eye. This, of course, opens an immense field of observation and experiment, for example, immunity, protection against antibodies, redintegration, regulation of growth of the whole or parts, and perhaps especially rejuvenation and senility, to say nothing of the character and the influence of the secretions from all the glands. The trouble at first was that the artificial growth was so short-lived; but by changing the medium often and by frequent washing away of the waste products in a salt solution, it was found that the life and growth of these isolated bits of tissue could be very greatly prolonged. It seemed that the process of decay was due to the inability of tissues to eliminate waste products. So in 1912 Carrel’s problem was whether these effects could be overcome.