372 The Animal Kingdom 



sion of the micronucleus of each conjugant. A second division follows 

 this, resulting in each conjugant's possessing four micronuclei and but 

 a single macronucleus. Three of the micronuclei degenerate in each 

 conjugant, and the remaining one divides unequally into a large station- 

 ary "female" and a smaller motile "male." The "male" micronucleus 

 of each conjugant crosses the protoplasmic bridge and fuses with the 

 "female" micronucleus of the other individual. Following this, the in- 

 dividuals separate. During the next series of micronuclear divisions, 

 the macronucleus of each exconjugant gradually disintegrates. The 

 single micronucleus resulting from the fusion of the "male" and "fe- 

 male" micronuclei next undergoes a series of divisions which result 

 in eight micronuclei. Four of these enlarge to become macronuclei 

 and four remain as micronuclei. Three of these latter degenerate, 

 leaving but a single micronucleus and four macronuclei. Next each 

 exconjugant undergoes two divisions in which a macronucleus is dis- 

 tributed to each of the resulting four cells; the single micronucleus 

 divides mitotically at each division. This results in four new individuals 

 from each exconjugant, each having a single macronucleus and a single 

 micronucleus. The eight individuals resulting from the two exconju- 

 gants thus have identical chromosomal constitution, but the cytoplasm 

 of each group of four will be different from that of the other. 



Cytoplasmic Inheritance. — This natural experimental situation 

 has been utilized by many workers, culminating in the work of Son- 

 neborn for the study of cytoplasmic inheritance. While many of the 

 characters of paramecium are controlled by the genes contained within 

 the micronucleus, it has been possible to identify some characters 

 controlled by cytoplasmic inclusions. 



One of these cytoplasmic controlled characters is the production 

 of mating strains. Mating or conjugation can occur only between two 

 different strains. Each exconjugant produces individuals of its own 

 mating strain. As the nuclear material of two exconjugants is the 

 same, this can be understood only by assuming that the character for 

 the mating strain is controlled by materials in the cytoplasm. These 

 materials are known as "plasmagenes." 



Other Free-Living Ciliates. — Numerous other genera and species 

 of ciliates occur in fresh and salt waters. Some of these move freely 

 about in a manner similar to that of paramecium, while others are at- 

 tached by means of stalks. Some of the nonstalked forms include such 

 genera as Euplotes, Spirostomum, Lionotus, Lacrymaria, and some 



