Experimental Studies on Cellular Multiplication
Thorburn Brailsford Robertson · Biochemical Journal · 1921
Statement of the Problem.THOSE unicellular organisms which multiply by fission are, as Wassermann and others have pointed out, potentially immortal, since the protoplasm of the parent is contained in and shared between the daughter-cells.Further- more, in the presence of unlimited food and environmental conditions which are otherwise suitable they are possessed of conceivably unlimited capacity for reproduction.The actual conditions which attend the life of these organisms, however, never permit the full realisation of their capacity for multiplication, for even in the absence of any other check to unlimited reproduction, the ratio of population to food supply must sooner or later attain a point at which the further synthesis of protoplasm is an impossibility, and all obtainable food materials are requisite, or perhaps insufficient, to maintain the existing population.Notwithstanding the imaginable potentialities of a Protozoon, therefore, the actual phenomena which are displayed by a culture in a limited amount of culture medium are not unlike those which characterise the life-cycle of the interdependent cell-masses which compose the Metazoa.In the one case, as in the other, the synthesis of protoplasm, starting from a single cell, becomes more and more rapid as the number of cells increases, until a maximal repro- ductive rate, or rate of growth, is attained.From this point onwards, for reasons which are not as yet perfectly understood, the cellular reproductive capacity progressively wanes until the population has attained a maximum.It will subsequently be shown that a Protozoon culture exhibits youth, age, and the phenomenon of senescence, that is, not merely the'slowing down of multiplication which is age, but that diminution of capacity to multiply, even under suitable conditions and in the presence of abundance of foodstuffs, which is senescence.The salient characteristics of the life-cycle of the higher animals are therefore also displayed by a culture of infusoria, and although the individual Protozoon is potentially immortal, the society comprised in a Bioch.xv 38