Disorientations of Development in Crepidula Plana Produced by Low Temperatures

The orientations of the developing egg are caused by vortical movements (plasma flowing) which bring about segregation of structures and substances in the chief axis of the egg and later in the cells into which the egg divides. In normal development these vortical currents run in preëstablished directions and lead to the typical localization of substances and cells in development. If the developing eggs of the marine gasteropod, Crepidula, are kept at a temperature near freezing for a few hours, plasma flowing and cell divisions are stopped though dividing nuclei may finish division and resting nuclei may grow and prepare to divide. After such eggs are returned to temperatures of about 20°C., plasma flowing is resumed but often in abnormal directions with the result that the directions of division walls and the positions and contents of the cells are abnormal. Sometimes the plasma flowing is resumed at a later stage than that at which it was stopped, with the result that certain early cell divisions are entirely omitted whereas later divisions are normal, except for the reduced number of cells. In short all developmental processes are not stopped at the same time by cold and even though certain stages of development are omitted altogether the development may go forward from a later stage, as if the factors of development were active during refrigeration although the typical expression of those factors is suppressed.