Abstract
The work of Jennings 1 affords much evidence that, in Paramecium, morphological differentiations, which are hereditary, do not arise in a line reproducing vegetatively. As a result, selection is powerless to isolate hereditarily different strains among the descendants of a single cell. On the other hand, the same author 2 has shown that in the uniparental reproduction of a rhizopod, Difflugia corona, the number and length of the spines on the test, as well as other morphological characters, become differentiated in the descendants of a single individual; and that by selection it is possible to isolate many hereditarily different strains.
The effectiveness of selection in regard to what may be termed a physiological character has not been tested in Paramecium, but Middleton 3 has undertaken this in a study of the fission rate of another ciliate, Stylonychia pustulata. This author found that “by opposite selection, through more than 150 generations, of small individual variations occurring among the progeny of a single individual, it was possible to produce two sets differing hereditarily in rate of fission.” The hereditary difference persisted through periods of balanced selection, and through conjugation, and disappeared only under the influence of reversed selection.
In view of the above results a series of experiments were carried on which involved the selection of a physiological character in Paramecium.
Although there is a remarkable synchronism of fission in closely related cells of a race of P. awelia, under identical conditions, frequently one cell will divide slightly earlier than the others. This variation in the rate of division of individual cells is the physiological character which was selected in our experiments.
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