SEXUALITY 



691 



combinations nothing happens — each individual moves about inde- 

 pendently of the others; but in other mixtures the animals quickly unite 

 in large clusters. The animals stick together as they collide in their ran- 



» ^ 



I . ;^ 





s - 







% 



^ ^^ 



r t 



• / 





S 'Jt 



^ i." 



'-'/- 



"/ • 



' • 



.S'^ ' 





^M 



^ 9 



♦ ^ 

 •> 



•; 



t \ 



«• 





Figure 166. The mating reaction in Paramecium bursaria. Upper left, single mating 

 type with individuals scattered singly. Upper right, the clusters formed six minutes after 

 mixture of cultures of two different mating types. Lower left, a later stage of the mating 

 reaction (after five hours). Lower right, the final conjugating pairs as they appeared 

 twenty-four hours later. (From Jennings, 1939.) 



dom movements. Animals not in contact do not attract each other; nor 

 are they in a specially sticky condition, as has been so often maintained, 

 for neither caryonide shows the least trace of stickiness until the animals 

 are mixed, and then only when animals of different caryonides collide. 

 The clusters begin with just two individuals and build up into larger 



