defined from one another, and further as flagella pass over imper- 

 ceptibly at their base into the protoplasm of the cell. 



Therefore I agree with Schwalbe (III. 21) and with Engelmann, 

 that the vacuoles possess contractile walls although they are not 

 clearly denned from the rest of the protoplasm. In addition, it is 

 well known that delicate membranes are often imperceptible with 

 the microscope although they are undoubtedly present. In many 

 plant cells it is impossible to see the so-called primordial utricle 

 as long as it adheres closely to the cellulose membrane ; its exist- 

 ence, however, cannot be doubted, as its presence can be proved by 

 plasmolysing it. 



In this opinion, however, I find myself in opposition to Biitschli 

 (III. 3). He considers that the contractile vesicle is simply a drop 

 of water in the plasma. "Each vacuole after evacuation ceases 

 to exist as such. The one that takes its place is a new formation, 

 a newly created drop, which in its turn only exists until it has 

 discharged itself." In his opinion they are due to the flowing to- 

 gether of several formative vacuoles, which separate out as small 

 drops in the plasma, where they increase in size until, by break- 

 ing down the partition walls, they coalesce. However, the exist- 

 ence of the conducting and afferent canals, described by Biitschli 

 himself, the fact that the number of vacuoles present remains 

 constant, and the circumstance that during the diastole the vacuole 

 is seen to occupy the same position as during the systole, and 

 moreover, that the frequency of contraction bears a fixed relation 

 to changes of temperature, all appear to me to support the former 

 view, and to be opposed to Biitschli's theory. The fact that at the 

 end of the systole the vacuole, having evacuated its contents, is 

 for a moment invisible, does not seem to weigh much against the 

 theory of its constancy, especially if one considers that even large 

 lymph spaces and capillary blood vessels in vertebrates elude per- 

 ception in an uninjected condition. 



IV. Changes in the Cell during passive movement. In 

 order to complete the subject of the movements of protoplasm, it 

 is necessary to consider finally the changes of form which, to a 

 certain extent, the cell may experience in consequence of passive 

 movements. Under these circumstances, the cell is in the same 

 condition as a muscle which, being excited by an external stimulus, 

 becomes extended and then contracted again. 



In this manner the cells of an animal body may become con- 

 siderably .altered in form, in adapting themselves to all the 



