REPRODUCTION 185 



when the organisms were becoming inactive, the contents 

 of two aquaria were poured together, a phenomenon 

 known as conjugation quickly takes place. Two organ- 

 isms, presumably one from each stock, become attached 

 to one another by what might be called their ventral 

 surfaces i.e., the surfaces containing the oral apertures 

 undergo a partial fusion of the surface, adjust their cilia 

 so that they move synchronously, and remain united for 

 some time, during which a complicated interchange of 

 cellular and nuclear substance takes place. 



FIG. 71. Epistylus umbellaria. Showing conjugating cells. (After Graeff 

 from R. Hertwig.) 



When this interchange is satisfied, the conjoined indi- 

 viduals separate and each again begins to multiply by 

 fission as though the virility of their respective strains 

 had never diminished. 



From this experiment we learn that material derived 

 from two individuals that have for some time been 

 accustomed to a somewhat different environment, for 

 some unknown reason affords the cell greater vigor than 

 that exclusively its own. 



In many cases conjugation appears to be an occasional 

 phenomenon in which there are no essential differences 

 between the cells participating; in a far greater number of 



