CONDUCTIVITY 131 



although they are often of microscopic dimensions, they possess 

 elongated fingerlike or threadlike pseudopods. 



Indeed, a rhizopod cell, with its straight, elongated pseudopods, 

 is preeminently fitted as an object of comparison with a neuron. 

 Although the difference in respect to the individual points is' so 

 far-reaching, still, based on their outward morphological similarity 

 various physiological parallels in both are forced on our observa- 

 tion. A comparison of the rhizopod cell with the neuron can 

 consequently guard us from many erroneous generalizations 

 which we might be inclined to deduce from a one-sided investi- 

 gation of the nerve. This is especially the case in regard to the 

 conductivity of excitation, which was formerly studied almost 

 exclusively on the nerve and only occasionally on the muscle, 

 which offers similar conditions. The nerve, in which the func- 

 tion of the conductivity of excitation is particularly highly devel- 

 oped, was considered at the same time as the type in which this 

 process could be most readily analyzed, and from which it was 

 believed general information of the process of the conductivity of 

 excitation could first be gained. This view has led to serious 

 errors, as the nerve, resulting from the high development of its 

 conductive capability, shows quite one-sided specialized conditions, 

 which can by no means be transferred to other forms of living 

 substance. 



A very suitable object among rhizopods for the study of con- 

 ductivity, and which is everywhere easily procured, is Difflugia. 

 This species living in small pools has a delicate urn-shaped, pear- 

 shaped or flask-shaped capsule built up of sand grains, diatomes 

 or material produced by the organism itself. From the opening 

 the protoplasm extends often to a considerable length its finger- 

 shaped hyaline pseudopods. When Difflugia is placed in a flat 

 dish in water and observed under the microscope, it is frequently 

 seen to extend from the opening long pseudopods in exactly oppo- 

 site directions, which reach for a considerable distance on the 

 bottom. These offer particularly favorable conditions for the 

 study of the conduction of excitation. When this animal is 

 placed under a microscope, the pseudopods are very readily stim- 

 ulated at any position to a desired extent by means of a sharp 



