The Life Cycle of the Single Cell 17 



1. Pairing. The animals collide anteriorly and also become attached 

 at the region of the mouth. A bridge then forms between them. Mouths 

 and gullets regress into very tiny rudiments and much of the cell contents 

 becomes radically reorganized. 



2. Nuclear changes. First, the macronuclei disintegrate into many 

 small fragments which eventually disappear. Second, the diploid micro- 

 nuclei undergo meiosis (i.e., each pair of chromosomes in the micronu- 

 cleus duplicates to yield a quartet; the micronucleus then divides twice, 

 reducing the quartets to pairs and thence to singles ) . In this manner, each 

 micronucleus gives rise to 4 haploid nuclei, and the two micronuclei of 

 each conjugating cell together produce eight. Seven of the eight disappear. 

 Only the one nearest the bridge between the conjugant cells remains. This 

 nucleus, still haploid, divides once more and now each cell has two 

 gamete nuclei. One of these stays where it is. The other migrates through 

 the bridge to the partner cell. The result is that the conjugant Paramecia 

 have identical pairs of nuclei, each with one of its own and one of its 

 partner's. The two haploid nuclei in each cell come together and fuse to 

 yield a single diploid nucleus. 



3. Post-conjugative changes. The newly formed diploid nucleus di- 

 vides mitotically into two, and the two in turn produce four. One pair 

 takes a position at the anterior end of the cell and one pair at the posterior 

 end. The cell now constricts in the middle just as it does during asexual 

 fission and produces two daughter cells, each with two nuclei. One of the 

 two enlarges to form the new macronucleus. The other divides to yield the 

 two micronuclei. Thus, four post-conjugate daughter cells eventually 

 emerge from the two Paramecia that originally conjugated. 



AUTOGAMY 



Many organisms, particularly in the plant kingdom, are self-fertile, 

 which means that they can undergo sexual congress alone and need no 

 partner, i.e., they produce gametes that can fuse with each other in pairs 

 to yield fertile zygotes. Paramecium is capable of self-fertilization and 

 the process is designated autogamy. 



The nuclear changes in an autogamous Paramecium are the same 

 as those involved during conjugation. The macronucleus disappears and 

 the micronuclei undergo meiosis to produce a total of eight haploid nuclei. 

 Seven disappear. The eighth divides mitotically into two gamete nuclei 

 and these fuse to yield a diploid zygote. The subsequent nuclear changes 

 follow the pattern of those displayed by cells after conjugation. Figure 7 

 summarizes autogamy. 



