294 



GENERAL ZOOLOGY 



dorsal surface. These are contractile vacuoles (Fig. 152, 7). 

 It is supposed that the liquid waste, when formed, flows 

 through the protoplasm along somewhat definite channels 

 to the point where it collects in a gradually increasing vacu- 

 ole. Soon the maximum size is attained, and the vacuole 



bursts at the surface, the waste pour- 

 ing into the water outside. The two 

 contractile vacuoles alternate in con- 

 tracting. 



Paramecium has two nuclei ; the large 

 one is called the macronucleus (Fig. 152, 

 8), and the small one the micronucleus 

 (Fig. 152, 9). The macronucleus is 

 thought to be the seat of control of the 

 general activities of the cell, while the 

 micronucleus is the seat of control of 

 the important process of reproduction. 

 Paramecium divides into two equiv- 

 alent cells (Fig. 153), with half the 

 macronucleus and half the micronu- 

 cleus in each, probably because the 

 volume of the cell becomes too great 

 to be kept alive by the protoplasm 

 of the relatively decreasing surface. 

 As many as three or four generations 

 of paramecia may be produced in a 

 single day. The frequency of division of the cell depends 

 upon the kind and abundance of food obtainable, upon 

 temperature, and also upon a process known as conjuga- 

 tion (Fig. 154). This complicated process was worked out 

 in great detail in 1888 by Maupas, a French librarian, who 

 in his spare time studied the life history of Paramecium 

 and other unicellular organisms. 



1 From Sedgwick and Wilson's General Biology. 



Fig. 153. Paramecium 

 dividing. (Much en- 

 larged) 



1, mouth and gullet; 2, 

 macronucleus dividing ; 

 3, micronucleus dividing x 



