IntroductoryIn the cultivation of isolated tissues, whether plant or animal, it is necessary to distinguish between growth at the expense of the culture medium and survival, in which the medium plays only an inert or, at most, a secondary role. In the first case the nutrient is itself the limiting factor in determining the degree and to some extent the character of the increment obtained. Given a satisfactory nutrient, adequate replacement of exhausted nutrient, and adequate removal of excretory products, growth should continue indefinitely. In the second case increment may also take place for a time at the expense of materials reabsorbed from the older portions of the original explant, the medium acting essentially as an inert substratum only. As these reabsorbed materials are exhausted, the culture will gradually succumb, no matter how often the medium is renewed. An extended period of growth ordinarily implies the maintenance of a high level of metabolism. Survival, on the other hand, is ordinarily dependent on minimal metabolic rates, and is hence prolonged by depressant conditions such, for example, as moderate cold, low oxygen supply, etc.
If, on the other hand, there has been a true segregation and loss of function so that the cells are, in the mature organism, no longer totipotent, * Italic numbers refer to bibliography.
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