186 . PROTOZOOLOGY 



(Aa). Thus the genie agreement between two conjugants of a pair 

 and the relative frequency of various gene combinations as shown in 

 these experiments confirm definitely the occurrence of meiosis and 

 chromosomal exchange during conjugation which have hitherto been 

 known only on cytological ground. 



In Euplotes patella, Kimball (1942) found that the six mating 

 types (p. 159) are determined by six possible combinations of a series 

 of three allelic genes. There is no dominance among these alleles, the 

 three heterozygous combinations determining three mating types 

 being different from one another and from the three determined by 

 homozygous combination. Kimball (1939, 1941) had shown that 

 the fluid obtained free of Euplotes from a culture of one mating type 

 will induce conjugation among animals of certain other mating 

 types. When all possible combinations of fluids and animals are made 

 it was found that the fluid from any of the heterozygous types in- 

 duces conjugation among animals of any types other than its own 

 and the fluid from any of the homozygous types induces conjugation 

 only among animals of the types which do not have the same allele 

 as the type from which the fluid came. These reactions are explained 

 by an assumption that each of the mating type alleles is responsible 

 for the production by the animal of a specific conjugation-inducing 

 substance. Thus the two alleles in a heterozygote act independently 

 of each other; each brings about the production by the animal of a 

 substance of its own. Thus heterozygous animals are induced to con- 

 jugate only by the fluids from individuals which possess an allele 

 not present in the heterozygotes. 



The relation between the cytoplasm and nucleus in respect to in- 

 heritance has become better known in recent years in some ciliates. 

 De Garis (1935) succeeded in bringing about conjugation in Para- 

 mecium caudatum, between the members of a large clone (198m long) 

 and of a small clone (73/i long). The exconjugants of a pair are dif- 

 ferent only in the cytoplasm as the nuclei are alike through exchange 

 of a haploid set of chromosomes. The two exconjugants divide and 

 give rise to progeny which grow to size characteristic of each parent 

 clone, division continuing at the rate of once or twice a day. How- 

 ever, as division is repeated, the descendants of the large clone be- 

 come gradually smaller after successive fissions, while the descend- 

 ants of the small clone become gradually larger, until at the end of 22 

 days (in one experiment) both clones produced individuals of inter- 

 mediate size (about 135^ long) which remained in generations that 

 followed. Since the exconjugants differed in the cytoplasm only, it 

 must be considered probable that at first the cytoplasmic character 



