THE PHYSIOLOGY OF EXCRETION. 83 



absence of a better term, the tendency towards isolation, or, 

 shortly, isolability of the bioplasm. This means that bioplasm 

 tries to isolate solid and dry foreign matters, if it comes into 

 contact with such. This end may be attained in different 

 ways. Protozoa, for instance, surround themselves with a thick 

 membrane, if the ditch in which they live dries out. In other 

 cases where small solid particles get into the cytoplasm of the 

 cell, they are surrounded by a fluid which the cytoplasm 

 secretes. In this case we get two different results, depending 

 upon the nature of the foreign particles. If these particles are 

 soluble and nutritive, they are dissolved by the fluid which sur- 

 rounds them, the secretion of which was in the first instance 

 only a manifestation of the negative sclerotropism of the bio- 

 plasm, and the solution is used up for the regeneration of the 

 plasma, a process which we call assimilation. The formation 

 of food vacuoles in Amoeba, Infusoria, Helizoa, etc., is evi- 

 dently only a consequence of the tendency towards isolation 

 from the side of the bioplasm, and it is my conviction that the 

 mechanism of nutrition throughout the organic world is based 

 upon this property of the protoplasm. 



If, on the other hand, the foreign particles are not soluble, 

 the fluid drops which are secreted around them serve only to 

 isolate them from the irritable part of the bioplasm, and we 

 might term them isolating vacuoles. 



Whoever has watched an Amoeba in life has seen that it con- 

 tains a surprising quantity of solid particles, and has also seen 

 that most of these particles are surrounded by vacuoles. It is 

 my opinion that the honeycomb structure which Biitschli claims 

 for protoplasm and supports by physico-chemical reasons is only 

 a secondary structure. I do not doubt that in Amoeba (which 

 is his chief subject of investigation) such a structure is present, 

 but I claim that protoplasm does not ordinarily possess this 

 structure. An Amoeba creeps around and becomes crowded 

 with solid foreign particles of all sizes. The greater number 

 of these particles will scarcely exceed a microsome in size. It 

 is very probable that every one of these minute granules is 

 surrounded by a special minute drop of secretion, and thus the 

 cytoplasm must present a vacuolated appearance under a high 



