THE LIVING SUBSTANCE. 35 



complete than at others, the pelHcular substance becomes so 

 completely relaxed, so fluid, that it mingles with the interal- 

 veolar substance of the surrounding protoplasm, and the 

 vesicles which at one time in the rhythm formed a more or 

 less perfect alveolar layer about the swollen vacuole, are 

 also mingled freely amongst those of the surrounding sub- 

 stance, after the manner of endosarc. As the membrane 

 re-forms after such collapse, its thickness, which at first is 

 slight, is gradually augmented by access of interalveolar stuff ; 

 and at the same time it undergoes more or less modification of 

 its optical qualities, until near the most distended moments of 

 the vacuole these qualities are at their maximum. That 

 obliteration of the membrane is here a physical fact, and not 

 mere optical illusion, is shown by the flux of the vesicles 

 amongst each other, and also by the failure of even osmic 

 fumes at such times to show a trace of substance modification, 

 about the point of collapse ; while, on the other hand, during 

 ordinary collapses, when the pellicle becomes merely plicated 

 and difficult to detect optically, one can so preserve a very 

 definite substance structure. 



Nor is the vacuole itself a permanent organ in the strictest 

 sense, since, contrary to the accepted idea, it does not persist 

 at one spot in the endosarc throughout the life history of the 

 animal, but as I have witnessed, may fail after collapse to reap- 

 pear where it has done so for hundreds of consecutive times. 

 When this perhaps rare phenomenon occurs, the protoplasm of 

 the immediate area has a very fluid, vague, appearance, and I 

 have seen the whole area, continuous substance as well as 

 inclusions, excreted by a wave of contraction in the endosarc. 

 Outside, it seemed to be a flocculent mass, which soon disinte- 

 grated in the water and was dissipated. Later, after a consid- 

 erable interval, another contractile vacuole was formed in the 

 endosarc at another point, and the surrounding protoplasm of 

 that region soon gained the more fluid appearance which 

 usually characterizes the region of a contractile vacuole in the 

 forms which gave these facts. 



It is no uncommon thing in Amoeba protetis and in other 

 Protoplasta, notably the Heliozoa, to cast out thus portions of 



