1. Running lengthwise; in the direction of the long axis of the body or any of its parts. 2. Studied over a period of time, diachronic; contrast with cross-sectional or synchronic, which give equivalent results only under certain strict conditions of stability and equilibrium. Such strict attention to these conditions is of the greatest importance in the study of survivorship either in demographics or in cell economy (survival pattern of erythrocytes and platelets).