133 IRRITABILITY 



needle, to which fat has been previousl)' applied and subse- 

 quently the excess removed. The extension of the response 

 from one point toward the other can then be followed with 

 great ease. The pseudopod of the rhizopod has the great advan- 

 tage over the nerve that its excitation can be directly observed. 

 The excitation following weaker stimulation is manifested by 

 a wrinkling of the previously completely smooth surface ; stronger 

 stimulation produces differentiation of the hyaline protoplasm to 

 a strongly refractive strand in the axis and a turbid myelinlike 

 mass at the periphery, the pseudopod at the same time retracting 

 toward the central cell body. In spite of all these occurrences 

 being of microscopic dimensions, still with some practice it is 

 quite possible to experiment on them under the microscope. In 

 this way I found it comparatively simple to study the fundamental 

 principles of conductivity. 



All these factors, the intensity with which the excitation 

 extends from the point of stimulation, the rapidity of the exten- 

 sion, and finally the area over which conduction takes place, are 

 manifestations of the intensity of stimulus, and as such alter 

 with these in corresponding manner. If the end of a pseudopod 

 is barely touched and thereby weakly stimulated, the response is 

 restricted to a slight wrinkling of the surface, which slowly ex- 

 tends to the immediate neighborhood, whilst the more distant 

 parts of the pseudopod are not affected at all by the excitation. 

 (Figure 15, A.) On a stronger stimulation of the pseudopod by 

 slight pressure, the response is likewise stronger, and the char- 

 acteristic differentiation of the protoplasm, consisting in the 

 strongly refractive strand in the axis and the turbid myelinlike 

 outer mass, appears at the point of stimulation. From here a 

 peculiar alteration spreads gradually further over the pseudopod, 

 in that first upon its smooth surface a few myelinlike droplets 

 are seen, which become larger and with the development of the 

 strand in the axis, dissolve into a wrinkled mass on the surface. 

 The further this process extends from the point of stimulation, 

 the weaker it becomes and the more slowly it proceeds, until at 



1 Max Verworn: "Psycho-physiologische Protistenstudien. Experimentelle Unter- 

 suchungen." Jena 1889. 



