1893.] Mode of Formation of Food Vacuoles in Infusoria. 469 



the firm unbroken edge of a composite solid which abuts on the fluid 

 of the vacuole in which it was formed. On these and other grounds 

 this hypothesis appears to me inadequate, and I would lay but little 

 -stress upon it. 



2. Again, it may be believed that, as in plasmolysed vegetable 

 cells, the primordial utricle shrinks centripetally, gathering up in its 

 retreat all granules which lie within or throughout its substance, so 

 here a highly elastic pellicle, living, or the product of secretion, is set free 

 from the walls of the vacuole and contracts rapidly, gathering within 

 its lessening circumference all the solid particles which were sus- 

 pended freely before. The unbroken line, which from the moment of 

 aggregation marks off the clustered particles from the fluid in which 

 they lie, does indeed suggest the presence of an enclosing film, but 

 other experimental facts are clearly out of harmony with an hypothesis 

 which postulates its existence. Thus ths final cohesion of aggregated 

 particles is at least as perfect in the centre of the composite mass as 

 round its circumference ; when by certain changes which may follow 

 aggregation any particles are set free slowly, it is from the outside of 

 the food mass only, and ejection from the body never means disinte- 

 gration of the contents of an excretory vacuole, but is rather the 

 freeing of a resistent solid. Further, there is, rarely, a want of syn- 

 chronism in the aggregation of the particles in a vacuole ; Brownian 

 movement may persist for a time towards the end of the vacuole, when 

 the majority of the granules present are quiescent ; in other rare cases 

 the aggregated mass clings to the cell substance from which it is (for 

 the most part) separated by fluid, by slender threads of almost in- 

 visible, possibly mucilaginous, substance, these threads breaking 

 presently and being dragged into the central solid ; and it is usual to 

 find that actual measurement of a food mass demonstrates the per- 

 sistence of secondary shrinking after aggregation is over. I am 

 inclined then to think that a third hypothesis meets the case more 

 fully than either of those just mentioned, and to suggest 



3. That the solid particles which undergo change of position in aggre- 

 gation are dragged together by the comparatively rapid retraction of some 

 substance contained in the vacuole ; this substance is probably viscous. 



Such an hypothesis does not, however, offer any description of the 

 mechanism of retraction, and the nature of this mechanism is certainly 

 obscure. But I may point out that there are some undoubted re- 

 semblances between this rearrangement of substance and the phe- 

 nomena which are grouped together as " clotting actions." In all 

 perfected clots we have to recognise the interaction of two bodies or 

 it may be the reconstitution of one body removed from the seat of 

 most vigorous metabolic change the cell, and a separation of solid 



relatively large particles are present scantily in a vacuole do they move markedly in 

 aggregation. 



