PULSATING VACUOLES 297 



filled with sap may appear around it, or the vacuole may appear to have undergone 

 fine fragmentation \ 



No increase of external pressure could produce a complete collapse of the 

 vacuole so long as it retained its dissolved substances, for any diminution of size 

 involves a corresponding increase of concentration and of osmotic pressure. For 

 this reason moderate changes in the external pressure are readily balanced without 

 appreciably affecting the size of the vacuoles, and the same applies to the protoplasm 

 in general 2 . The fact that neighbouring vacuoles may expand and contract at 

 different times, and that isolated fragments with single vacuoles may show pulsation 

 for some time, afford sufficient evidence that the systole and diastole are not produced 

 by local or general changes of pressure in the protoplasm. Nor can the pulsation be 

 due to changes in the percentage of osmotic substances in the protoplasm. 



It does not, however, follow that the mechanism is alike in all cases 3 , and in fact 

 the position of the vacuole in various Amoebae may determine whether it bursts on 

 the surface or allows its contents to escape into the surrounding protoplasm by filtra- 

 tion under pressure 4 . The latter always occurs when only a diminution in size is 

 shown, for an actual rupture of the vacuolar membrane would presumably involve an 

 escape of the whole of its contents. Under special conditions the vacuoles of most 

 organisms do not empty completely 5 , but this does not necessarily show that the complete 

 collapse is also merely due to filtration under pressure, however probable this assump- 

 tion may be. Vacuoles of Myxomycetes which have absorbed aniline blue by passive 

 secretion retain it during partial pulsations 6 , whereas the selective permeability of the 

 vacuolar membrane enables it to allow the diosmotic excretion of other dissolved 

 materials. The addition of non-exosmosing dissolved substances to a vacuole must 

 necessarily convert a previous total pulsation into a partial one, and possibly this is 

 why the union of a pulsating vacuole with a non-pulsating one produces in the 

 plasmodium of Chondrioderma only a feebly pulsating vacuole 7 . 



The continuance of rhythmic pulsation in isolated vacuoles shows that the thinnest 

 protoplasmic layers may develop the required self-regulatory activity. Although the 

 systole ensues when a definite size is reached, other inactive vacuoles may surpass 

 this size without ever pulsating. Hence the pulsation is the result of some specific 

 peculiarity, and this holds good even when pulsation may be induced under special 

 circumstances in previously inactive vacuoles 8 . It is not easy to say whether a 

 vacuole entirely disappears at the close of the systole or merely decreases to sub- 

 microscopic dimensions. In the former case the vacuolar membrane would be 

 reconverted into ordinary protoplasm, but it is also possible that special factors 

 might prevent this happening, in which case the potential walls at least of the new 

 vacuole would be retained. The reproduction of a new contractile vacuole would, 



1 See Rhumbler, Archiv f. Entwickelungsmechanik, 1878, Bd. vil, p. 289 ; Biitscbli, 1. c., &c. 



2 See Pfeffer, Plasmahaut u. Vacuolen, 1890, p. 337. 



3 A summary of the views of different authors is given by Butschli, I.e., pp. 1433, 1458, 1452. 



4 Rhumbler, I.e., pp. 257, 271. 



5 See Butschli, 1. c., p. 1457 ; Cohn, 1. c., p. 200. 



6 Pfeffer, I.e., 1890, pp. 219, 337. 7 PfefTer, I.e., 1890, p. 219. 

 8 Cf. Rhnmbler, 1. c., p. 263. 



