THE LARGE GROl P OF THE SMALLEST ()U(;.\M.sM> 



IM 



amecia in his laboratory at Yalo University lor thirty yoars and (hir- 

 ing that period over twelve thousand generations vv.t,. I,,-,,! I.y fissi,,,, 

 It has been observed in these cultures, however, (haf after 4() or inor.- 

 divisions have occurred, a i)rocess called cndomixis takes plac.-. in 

 which the old active niacronucleus is replaced by a new one made 



12- 



Endomixis in Paramecium aurelia. The normal condition of Parainociiiin is 

 shown in I showing niacronucleus and two niicronuclei. Follow throufrli the 

 series pictured. What happens to the niacronucleus? How many iniiTonuclci 

 are formed? What, happens next? Note in IV that only one daughler cell is 

 shown. How does this cell obtain the normal number of niicronuclei? Where 

 does the new niacronucleus come from? This rhythm of cell actixity seems to 

 occur with considerable regularity every 10 to .^0 generations and it gi\ es the new 

 macronucleus chromatin from the reserve sujiply held in the micronucleiis. This 

 process does not appear in all ciliates and is not beliexed to be necessary for 

 normal growth. (After Hegner.) 



from chromatin of the reserve micronucleus. This process is similar 

 in many respects to conjugation, except that no foreign chromatin 

 is added. 



Under normal conditions, another process known as amphimixis or 

 conjugation takes place somewhat resembling the sexual procc^^ses of 

 higher animals. Two cells come to lie with their gullet surfaces next 

 to each other and a bridge of protoplasm forms between them, \\hile 

 this is going on the micronucleus in each cell moves away from the 



