SEVENTH LECTURE. 



THE BEHAVIOR OF UNICELLULAR ORGANISMS. 



HERBERT S. JENNINGS. 



In recent biological writings there is manifest a growing 

 tendency to interpret the processes taking place within the 

 bodies of higher animals — especially the developmental proc- 

 esses — as a series of responses to stimuli. In the Qgg and 

 the developing embryo, masses of protoplasm migrate from one 

 position to another, cells and cell masses alter in form, changes 

 of the most varied character are continually occurring. To 

 explain such changes it is becoming usual to call upon chemo- 

 taxis, geotaxis, phototaxis, thigmotaxis, and other motor reac- 

 tions of similar character. The prevalent ideas of these 

 reactions, known usually under names terminating in -faxts 

 or -tropism, have been derived to a large extent from the phe- 

 nomena shown by the movements of unicellular organisms ; the 

 classic experiments of Pfeffer on the chemotaxis of bacteria and 

 flagellates and of Strasburger on the phototaxis of swarm spores 

 having opened a fountain from which all have felt entitled to 

 draw. To understand the migration of a cell or mass of cells 

 in the embryo we are referred back to experiments on unicellu- 

 lar organisms, wherein it is shown that the movements of the 

 latter are controlled by chemical agents, by heat, by light, and 

 the like. Here the vital processes are seemingly brought into 

 the closest relation with chemical and physical ones ; chemo- 

 taxis, for example, is frequently interpreted as the direct expres- 

 sion of chemical affinity or chemical repulsion between the 

 substance of the protoplasmic mass and some other substance, 

 or between two protoplasmic masses. There is thus established 



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