REPRODUCTION 171 



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. This union is known as 

 conjugation and during it a complicated interchange 

 of cellular and nuclear substance takes place. 



When this interchange is satisfied, the conjoined indi- 

 viduals separate and each again begins to multiply by 



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

 from B, Hertwig.) 



fission as though the virility of their respective strains 

 had never diminished. 



From this experiment we learn that material derived 

 from two individuals, for some unknown reason, affords 

 the cell greater vigor than that exclusively its own. 



In many cases conjugation is an occasional phenom- 

 enon in which there are no essential differences between 

 the cells participating; in a far greater number of cases 

 there are such constant differences that it is possible to 

 separate them into male and female elements or gametes. 

 The development of sexual differences may or may not 

 be incompatible with the continuance of reproduction 



