534 THE NATURAL HISTORY OF 



state, as a contractile cell or as muscular substance, some- 

 times amorphous, as in the bodies of the Infusoria, 

 Rhizopoda, and Hydroida. Kolliker confirmed this view, 

 and carried it out particularly in the case of the Infusoria, 

 which he declared to be unicellular animals with a con- 

 tractile cell-membrane and contents. 



The contractile substance is characterised by the fol- 

 lowing attributes : — it is homogeneous, or finely granular, 

 transparent, of the consistence of albumen, gelatiniform, 

 soft, more refractive than water, but less so than oil ; 

 insoluble in water, but gradually decomposed ; destroyed 

 by caustic potass ; coagulated and contracted by carbonate 

 of potass, as well as by alcohol and nitric acid ; having 

 the power of forming aqueous cavities, which originate 

 either by the separation of the water contained in it, or 

 by its reception from without ; owing to which the 

 remainder becomes denser and more granular; and, 

 lastly, it presents the appearance, in water, of contractile 

 drops, which move like an Amceba. 



All these properties had already been observed by 

 Dujardin, in a substance of which the Infusoria and 

 Rhizopoda are principally composed, and which he 

 termed " Sarcode ;" the aqueous spaces or hollows he 

 named " Vacuoles," regarding them as the most charac- 

 teristic feature of the substance ; these spaces had been 

 erroneously regarded by Ehrenberg as stomachs. 



All these properties, however, are possessed by that 

 substance in the plant-cell, which must be regarded as 

 the prime seat of almost all vital activity, but especially 

 of all the motile phenomena in its interior — the proto- 

 plasm. Not only do its optical, chemical, and physical 

 relations coincide with those of the " Sarcode," or con- 

 tractile substance, but it also possesses the faculty of 

 forming " vacuoles," at all times, and even externally to 

 the cell ; a property, it is true, which has for the most 

 part been hitherto overlooked or misinterpreted.* 



* Sometimes, as in tlie zoospores of Volvox, these vacuoles exhibit 

 rhythmical ooutractions. — [Ed.] 



