AMCEBA AND SOME ALLIES: PROTOZOA 289 



volume of the cell becomes too great to be kept alive by the 

 organs of the relatively decreasing surface. As many as 

 three or four generations of Paramoe- 

 cia may be produced in a single day. 

 The frequency of division of the cell 

 depends upon the kind and abun- 

 dance of food obtainable, and also upon 

 a process known as conjugation (Fig. 

 145). 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 

 Paramoecium and other unicellular 

 organisms. 



In conjugation two Paramoecia unite 

 temporarily in the manner indicated in 

 the figure. A fraction of the micro- 

 nucleus of each passes through the 

 two contiguous layers of ectoplasm 

 and unites with a similar fraction in 

 the other animal. When this and cer- 

 tain other less essential phenomena 

 have taken place, the individuals sep- 

 arate and continue the process of transverse division, but 

 with greater frequency than before. Maupas interpreted 

 conjugation as a process of rejuvenescence, or renewing the 

 youth, through which these organisms, being exhausted by a 

 long series of divisions, could regain their vitality, thus pre- 

 venting the extermination of the race. It has been pointed 

 out by some zoologists that conjugation in unicellular organ- 

 isms has many points of resemblance with the union of egg 

 and spermatozoon in the higher animals. We may therefore 

 speak of conjugation as sexual reproduction, to distinguish it 

 from the non-sexual reproduction by division. 



FIG. 145. Paramoecia con- 

 jugating. Much en- 

 larged. (After Saville 

 Kent) 



