THE MECHANISM OF LIFE 503 



movement on the two sides of the body, and, according to known 

 principles, this must cause a rotation upon the axis until the 

 difference is balanced. 



If we add to our first scheme the various axial positions that a 

 cell -body, differentiated as regards its poles either temporarily or 

 permanently, may assume by reason of an excitation or depression of 

 contraction or expansion at one pole, we obtain the following cases, 

 in which the points of the arrows indicate the position of the 

 anterior end of the body : 



c, c-\-n c c 



e 

 n c 



en 



fcf Expressed in words, this means : the anterior pole of the body 

 turns away from the source of the stimulus with excitation of 

 contraction or depression of expansion upon one side ; and toward 

 the source of the stimulus with depression of contraction or 

 excitation of expansion upon one side. 



Whether the cell in axial orientation moves forward or back- 

 ward, or stands still, depends in. a given case upon the relation, as 

 regards intensity, of the phase of contraction to that of expansion 

 in the whole cell. 



Thus the phenomena of positive and negative chemotaxis, 

 barotaxis, thermotaxis, phototaxis and galvanotaxis which are so 

 highly interesting and important in all organic life, follow with 

 mechanical necessity as the simple results of differences in biotonus, 

 which are produced by the action of stimuli at two different poles 

 of the free-living cell. 



We recognised with Pfliiger as the essential part of metabolism 

 the continual construction and destruction of certain proteid- 

 like compounds of very labile constitution. Although at present 

 any chemical characterisation of these is unsatisfactory, we termed 

 them, in brief, biogens because of their great significance for life, 

 and we defined the vital process in the simple schematic form : 

 the sum of all the processes that are associated with the construction 

 and destruction of biogens. 



The non-living matter that enters into the living substance from 

 outside is manufactured continually into living matter by complex 

 transformations in the living substance ; it also dies continually and 

 is excreted as non-living matter. Thus, life consists of an eternal 

 process of becoming alive and dying, which go on in all living sub- 

 stance at every moment side by side and uninterruptedly. 



The sum of all processes associated with the construction of living 

 substance forms the phase of assimilation, the sum of all processes 



