no BIOLOGICAL LECTURES. 



once, by osmotic stimuli, by heat, by cold, by mechanical shock. 

 Its general effect is to take the organism out of the sphere of 

 operation of the agent causing the stimulus, and to prevent it 

 from reentering. The fact that certain areas are left vacant is 

 because the agencies within these areas cause this reaction ; the 

 collecting of the organisms within certain areas is due to the 

 fact that here the reaction is not produced, while it is caused, 

 by the influences active in the surrounding regions. 



Thus, chemotaxis, tonotaxis, thermotaxis, and the like are 

 unified ; they are not qualitatively different activities, but are 

 fundamentally one activity due to different causes. The tactic 

 phenomena of unicellular organisms are brought throughout 

 under the same point of view as the motor reflexes so well 

 known in the physiology of higher animals. 



We may now return to a brief consideration of the problems 

 which formed the starting point of this investigation — the 

 relation of the phenomena studied to the growth processes in 

 the protoplasmic masses of higher organisms. Do the laws 

 of the motor reactions of unicellular organisms, chemotaxis 

 and the like, really give us a basis for the understanding of 

 protoplasmic migrations and other processes in growth and 

 differentiation .-* 



We find that the tactic phenomena of these unicellular forms 

 are brought about through a refiex that is in all essential points 

 similar to the reflexes of higher animals. The nature of this 

 reflex is closely bound up with the physiological and structural 

 differentiations of the body of these organisms ; it has a 

 specially close relation to the asymmetry of the cell body in 

 these Protozoa, and to the manner of the usual forward motion. 

 These differentiations are absent in the masses of protoplasmic 

 substance that are moved about in the processes taking place 

 within the eggs and embryos of Metazoa. It is difficult to see 

 how the laws controlling the movements of such substance 

 masses can have any similarity to the laws above developed 

 for the reflexes of free unicellular organisms. Above all, it is 

 evident that the tactic movements of unicellular organisms are 

 not direct expressions of simple chemical and physical laws ; 

 chemotaxis, for example, is not a direct result of chemical 



