Carrel's work consisted in extending Harrison's method to apply to warm-blooded animals, including, of course, mammals; he having primarily in view at this time a more precise knowledge of the laws governing the restoration of tissues, for example, after serious surgical wounds. He and his assistant worked steadily to this end, and succeeded. The tissues of the higher animals, including man, can now be developed in a culture, and such development can be made to correspond to a rigidly precise technique. The feat is accomplished by putting minute pieces of living tissue into a plasmatic (blood) medium which will coagulate. So complicated is this apparently simple matter in its application that only the most exquisite surgical skill is proof against incalculable modifications in results. Having obtained evidence that tissue can be cultivated in accordance with a formula that may be relied upon to give definite results, the effort was made to grow artificially the various malignant (cancerous) tissues, in turn, of chicken, rat, dog, and human being. Cancerous tissue invariably developed cancer, and so rapidly and extensively that the growth could be observed with the naked eye. It now became evident that, under the right circumstances, the artificial growth of tissues could be utilized in the study of many problems; such as malignant growth of tissue; certain problems in immunity, as, for example, the production of antitoxins of certain organisms; the regulation of the growth of the organism, or of different parts of the organism; rejuvenation and senility; and the character of the internal secretions of the glands, such as the thyroid which plays a role most important in physical and mental development. The difficulty lay in the fact that the artificial growth was so very short-lived. It was found that by passing the growth into a new medium, and repeating the process, the tissues would begin to grow again; but their life even under these circumstances was limited at the most to twenty days. This was manifestly too short a time in which to study the fundamental questions to which the researchers had addressed themselves. Thereupon, study was taken up to determine the question as to _what made these tissues die_. It was found that, apparently as incidental to growth, there was the process of decay, due to an _inability of the tissues to eliminate waste products._