THE SIMPLER ORGANISMS in 



it is well nigh universal) another phenomenon known 

 as conjugation or sexual reproduction enters in 

 periodically, alternating with long periods of cell 

 division. This process consists in the fusion of two cells, 

 and so, is the reverse of cell multiplication. Like cell 

 division, it is an automatic spontaneous act of the cells 

 themselves, that cannnot be artificially performed. It oc- 

 curs after a long period of cell division (after more than TOO 

 generations of cells produced by fission in Paramoecium, 

 according to Maupas), and it seems to follow a decline in 

 vegetative vigor. This is indicated by the loose amoeboid 

 form taken on by certain shapely flagellates (fig. 66) just 

 previous to conjugation. It is followed by restoration of 

 the normal form, and by renewed activity in cell division, 

 therefore, it seems to be a sort of rejuvenating process. 



It introduces into cell lineage new influences from with- 

 out, by commingling the substance of two cells that are of 

 diverse ancestry and that have lived apart under different 

 influences. 



This fusion may occur among the simpler organisms in a 



great variety of ways. It may be temporary and partial as 



in Paramoecium. Two individuals come together and 



partially fuse at the anterior 



FIG. 67. Diagram il- " , , . ., . -. 



lustrating conjuga- end. Their meganuclei degen- 



tion between two 1 . 1 1 



ceils of ciosterium; erate ] tneir micronuclei di- 

 discarded ei cen y waiis; vide ; a part of each micronu- 



z, the zygospore, or , . ,, , , 



zygote, resulting cleus passes over into the other 



from conjugation. ,-. . f , i 



Paramoecium to fuse with a cor- 

 responding part of the micro- 

 nucleus of that animal, the two together forming the new 

 micronucleus. The two paramoecia, after this exchange 

 of substance separate, and enter upon another period of in- 

 crease by fission. 



More often the fusion is total and permanent. It may be be- 

 tween cells that are of normal size and equal activity, (fig. 67) 



