50 TYPICAL ANIMAL CELL 



is formed. The macronuclei disappear and the micronucleus goes 

 through a series of compHcated divisions. Finally one part of the 

 nucleus of each cell passes over the cytoplasmic bridge and unites 

 with a part of the micronucleus in the opposite cell. The animals 

 then separate and there is a reorganization of parts to restore the 

 structure to the normal state. This exchange and union of nuclei 

 is called conjugation. The process seems to give the Paramecia 

 renewed vigor and vitality. The cells may now go on dividing for 

 hundreds of generations before conjugation again takes place. 



It has been shown bv one scientist that the lowered vitality of 

 a colony of certain Paramecia may be caused by the surrounding 

 media of their environment. If this becomes concentrated with 

 wastes or materials that are unfavorable, conjugation may occur. 

 On the other hand, if careful control is kept of this material in which 

 the creatures live, and if abundant food is supplied and wastes 

 removed, certain Paramecia will divide generation after generation 

 without resorting to conjugation. L. L. Woodruff, a professor at 

 Yale University, has kept a strain of Paramecia active for several 

 years. Thousands of generations have been produced from the 

 creature with which he started. By exercising great care to make 



the surroundings most health- 

 y^^^ j ful for Paramecia, he has pro- 



/\ .• / Pellicle - duced these thousands of gen- 



/S"^^"^! / ' erations without having them 



// .-^^ conjugate or die. This biolo- 



r.^^^^^^^oagcciatecL Protoplasm, gist believes that if conditions 



^f ^ X. ^■ AA A. r, .u . src carcfullv safeguarded and 



If alcohol IS added to a Paramecium the proto- ^ ^ 



plasm is dehydrated, the strands of cytoplasm, controlled, thcSC tiuv maSSCS of 

 cilia, are pulled through the cell membrane or ^ 



peUicle leaving the rows of openings in the nrotoplasm kllOWU aS Para- 

 peUicle. ^ . ^ . 



mecia will live forever. 

 Irritability. The reaction of Paramecia to external stimuli is 

 much the same as in amoebas. Because of their cilia, they can 

 respond more quickly toward or away from stimuli. They have a 



