PROTOPLASM A VISCID FLUID 225 



substances." Hence, under certain circumstances, a framework 

 of a firmer nature can make its appearance. Since Schwarz, on 

 p. 139, declares that his results agree with those of Berthold, 

 I must certainly assume that he conceives of this "mixture," 

 which the cytoplasm is in his opinion, as a fluid one. Such 

 expressions as " semi-fluid " are too vague to be taken definitely 

 into account. 



I had myself in 1887 advocated the fluid nature of the 

 endoplasm of the Infusoria, and, in this connection, also that 

 of the greater part of other protoplasm, which throughout 

 behaves in a similar manner (p. 1392). I drew attention 

 more especially to the constantly spherical form of the 

 vacuoles that appear in protoplasm, a fact which would prove 

 that both the contents of the vacuole and the surrounding 

 protoplasm are fluid throughout. In Protozoa there is 

 such frequent opportunity for observing vacuoles of different 

 kinds — food vacuoles, contractile vacuoles, and ordinary 

 fluid vacuoles — that no one can be in doubt as to their 

 regular appearance in normal protoplasm. It is, however, 

 just as certain, and as plain, that all these vacuoles assume 

 the form of a spherical drop when they are not hindered by 

 solid bodies to which they adhere, or by mutual pressure, 

 streamings, or any other kind of special force which is act- 

 ing upon them. From this positive experience, which is 

 just as capable of proof as any other physical fact, there is 

 only one conclusion to be drawn, namely, that expressed 

 above, that the contents of the vacuoles, as well as the 

 protoplasm surrounding them, must be fluid. In addition 

 we know definitely with regard to the contents of the 

 food vacuoles that it is water. But all other vacuoles exhibit 

 just the same appearance and behaviour, for which reason 

 their contents must in like manner be a watery, very dilute 

 solution. We know further that vacuoles may flow together, 

 and in this process behave in exactly the same way as any 

 two drops of water in viscid oil. The study of the contrac- 

 tile vacuoles, which unfortunately have received very little 

 attention from many investigators who have given vent to 

 opinions upon such things, gives us the most beautiful 

 instances of such fusions, and the consequent rounding off 



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