Unicellular, Microscopic Animals 357 



Two individuals construct a ''protoplasmic bridge'' between their oral 

 surfaces, and certain of their reorganized nuclear materials migrate to 

 the opposite paramecia (Figs. 171 and 172). Fusion of nuclear mate- 

 rials results in fertilization, which is somewhat like sexual reproduction, 

 although no sex cells are actually formed. Paramecium aurelia has been 

 carried in a culture continuously for a period of over forty years, and 

 neither conjugation nor death from age have occurred, yet fission took 

 place at a vigorous, normal rate. It has been found that there are eight 

 distinct races of paramecia, each with its inherited characteristics. Para- 

 mecium aurelia has been found to have at least two mating types called 

 types I and II. Neither of these will conjugate with one another, but 

 mating type I must conjugate with mating type II. In order to rejuve- 

 nate certain paramecia, conjugation must take place between different 

 strains, while in other species rejuvenation is accomplished by inbreeding 

 or even self-fertilization. 



It has been discovered recently that the cytoplasm contains a system 

 of particles which have the property of self-duplication, a property simi- 

 lar to that of chromosomes and genes of the nucleus. Such a system of 

 cytoplasmic genes are called plasma genes to distinguish them from nu- 

 clear genes. The plasma genes play important roles in the heredity _ of 

 paramecia. 



Another method of reproduction is known as autogamy (o -tog' a my) 

 (Gr. autos, self; gamos, marriage) and consists of a nuclear reorganiza- 

 tion of a single individual. It involves meiosis (mio' sis) (Gr. meion, 

 smaller) in which there is a reduction of nuclear materials from the dip- 

 loid (double) to the haploid (single) condition. It also includes self- 

 fertilization. This method is used by paramecia under certain conditions. 



It has been discovered recently that there are races of paramecia which 

 produce and liberate a substance in their cytoplasm which kills animals of 

 other races. The two races have been called ''killers'' and "sensitives." 

 This killer substance is called "paramecin," and one particle of it {kappa 

 particle) can kill a sensitive Paramecium. Killers may have hundreds 

 of these kappa particles in their cytoplasm, while sensitives have none. 

 Hence, these kappa particles are cytoplasmic genes called plasma genes. 



EUGLENA 



Euglena (u -gle' na) (Gr. eu, good; glene, pupil of the eye) is a com- 

 mon, fresh-water flagellated protozoan. Two common species are Eu- 

 glena viridis (vir'idiz) (L. viridis, green) 2ind Euglena gracilis (gras'il- 



