CHAPTER LVI 

 SEX 



344. Conjugation in Paramecium. After reaching a certain 

 size the Paramecium will divide into two cells, and the new cells 

 will again divide, and so on for hundreds of times, when the 

 conditions are favorable. Under ordinary conditions, however, 

 this successive division does not continue indefinitely. After 

 a number of divisions the cells usually become smaller and less 

 vigorous, and then the strain is likely to die out. 



Yet ordinarily the species does not die out. Sometimes two 

 individual cells come in direct contact, side by side, and stop 

 swimming. The two animals then exchange nuclear matter 

 (see Fig. 129). This process of exchanging nuclear material 

 and combining the nuclear material from two different cells is 

 called conjugation (that is, a yoking, or joining, together). 



After the conjugation the Paramecium cells seem to be 

 changed, since now they are better able than before to carry on 

 their growth and cell division. How the conjugation causes the 

 animal to take on its new lease of life is not understood, although 

 there are several theories offered to explain what happens. 



345. Conjugation in Spirogyra. In the Spirogyra all the 

 cells that make up the threads may take part in conjugation, 

 when the conditions are right. The protoplasm of two cells 

 combines into a single cell that is capable of surviving condi- 

 tions unfavorable to growth (see Fig. 130). 



346. Zygotes. In certain ways the cell resulting from the 

 conjugation of two cells is different from a spore. The im- 

 portant difference is in the way it originated. All the cells 

 which we have studied heretofore originated by the splitting, 

 or dividing, of some preexisting cell. The spores of yeast and 



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