370 The Animal Kingdom 



Autogamy. — At times paramecia undergo a process of self-fertiliza- 

 tion known as autogamy. This has been best described in Paramecium 

 aurelia, a species with two micronuclei and one macronucleus. In 

 the process of autogamy first the two micronuclei form long threadlike 

 crescents, then they divide into four micronuclei, each of which divides 

 again to produce eight. Eventually all but two of these eight micro- 

 nuclei degenerate. The two that are left come to lie in a newly formed 

 bulge near the mouth of the animal. Here they soon fuse into a single 

 nucleus. This fused nucleus divides twice to produce four nuclei. Two 

 of these latter remain as micronuclei, and two become macronuclei. 

 When the first division of the animal occurs, the two micronuclei divide 

 so that each resulting cell has the usual complement of one macronucleus 

 and two micronuclei. During all these micronuclear changes the origi- 

 nal macronucleus fragments and degenerates. 



Fig. 119. — Autogamy in Paramecium aurelia. A, Individual with normal num- 

 ber of one macronucleus and two micronuclei; B, micronuclei become threadlike; 

 C, micronuclei divide to form four; D, each micronucleus divides again to produce 

 a total of eight, macronucleus degenerating; E, two micronuclei move toward a 

 newly formed bulge near the mouth; F, two micronuclei are in bulge, others de- 

 generating; G, these two micronuclei in the bulge fuse, others degenerate; H, 

 fused nucleus divides to produce two; /, each divides again to produce four; 

 J, two remain as micronuclei and two become macronuclei; K, first division in 

 which each micronucleus divides so that each resulting cell now has one macronu- 

 cleus and two micronuclei. 



Other nuclear changes known as endoiiiixis and hemixis have 

 been described for paramecia ; however, these rare phenomena are poorly 

 understood. 



In addition to these processes, paramecia also undergo a type of 

 bisexual reproduction termed conjugation. 



Conjugation. — Individuals ready to undergo conjugation be- 

 come sticky in the region of the oral groove and are soon seen to adhere 

 in pairs. An actual protoplasmic fusion takes place and may last for 

 several hours. During this period the nuclear changes take place. 



In Paramecium caudatum, a species with but a single macronucleus 

 and a single micronucleus, the first change to be observed is the divi- 



