296 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS 



The systole of a pulsating vacuole may either drive out the contents into the 

 surrounding water or merely into the surrounding protoplasm. The former is 

 usually the case in such Infusoria as the Acinetarias and Vorticellidae, which have 

 special exit channels leading from the vacuole to the exterior \ The latter appear, 

 however, to be absent from all other animals, and from plants also if we except 

 the Flagellatae. Transition forms occur, however, for in Paramaecium and other 

 organisms the radiating channels from the vacuole do not always appear at the 

 same point, and do not lead to the exterior. In many Amoebae, again, the peripheral 

 vacuoles bulge out externally and rupture when the wall has become very thin, so 

 that the escape of their contents may take place at any point on the surface 2 . The 

 emptying of a number of the peripheral non-pulsating as well as of the pulsating 

 vacuoles of Myxomycetes is effected in this way. The coalescence of small vacuoles 

 with one another or with a large one also involves a gradual approach of the vacuoles 

 and a thinning of the dividing membrane until the point of rupture is reached 3 . 

 Deep-seated vacuoles, however, can only empty their contents into the surrounding 

 protoplasm when they are not connected with any actual or potential channels to the 

 exterior. The extruded fluid may, however, either be imbibed by the protoplasm or 

 exude outwardly through it. In the latter case a corresponding diminution of the 

 total volume must ensue. The existence of organisms with a single vacuole or with 

 two synchronous ones shows that the vacuolar fluid is not always driven from one 

 vacuole to another. 



The escape from deep-seated vacuoles without special affluent channels takes 

 place by nitration under pressure through the vacuolar membrane, since, owing to 

 the plastic nature of the vacuolar membrane and of the surrounding protoplasm the 

 former cannot be ruptured under the conditions existent in the cell. In all cases 

 the centrally-directed pressure exercised by the vacuolar membrane partially 

 antagonizes the internal osmotic pressure required for the maintenance of the vacuole, 

 and any change in either of these factors is bound to influence the size of the vacuole. 

 It is, however, only when the latter is very minute that the centrally-directed pressure 

 attains relatively high values 4 . Under ordinary circumstances the diminution or 

 collapse of the vacuole can only result from a decrease or removal of its internal 

 osmotic pressure, produced either by the exosmosis of the dissolved materials or 

 by their conversion into larger or insoluble molecules. According to Cohn, just 

 before the systole of the vacuole of Gonium pectorale the vacuolar fluid becomes 

 turbid, possibly owing to the precipitation of the dissolved materials 5 , but it is also 

 possible that the phenomenon may have a different origin and not be directly 

 connected with the vacuolar contraction. 



When the vacuole is small, very rapid nitration under pressure through its rela- 

 tively large surface is possible, so that the vacuole may disappear instantaneously. If 

 the protoplasm is not at once able to absorb all the extruded water, radiating channels 



1 Cf. Biitschli, Protozoen, 1 880-8 ; Hertwig, Zelle und Gewebe, 1893. 

 8 See Rhumbler, Archiv f. Entwickehmgsmechanik, 1898, Bd. vn, p. 257. 



3 Cf. Pfeffer, Aufnahme u. Ausgabe ungeloster Korper, 1890, p. 159. 



4 Pfeffer, Plasmahaut u. Vacuolen, 1890, p. 298. 



5 Cohn, Nova Acta Acad. Caesar. Leopold., 1854, Bd. xxiv, i, p. 194. 



