Role of Preformed Structure in Cell Heredity 173 



the right now mean? focusing up to the right edge of Fig. 3A, where 

 the dorsal field and contractile vacuole pores appear; the focus is too 

 deep to show the rest of the dorsal field ; but, at the end of the circuit, 

 the anterior left kincty field is in focus beside the starting point, the 

 first oral meridian. 



Internally, there is a common endoplasm without partition and 

 usually only one macronucleus. In newly formed doublets, which 

 arise from a fusion of conjugants, there are two macronuclei. After 

 a relatively short time, these give way to a single macronucleus in the 

 later progeny. Normally, the macronucleus of a singlet occupies a 

 position close to the gullet with its major axis extending longitudi- 

 nally; in doublets, the macronucleus usually lies close to both gullets, 

 making its major axis transverse (Fig. SA) . 



III. The Basis of the Hereditary Difference 

 between Singlet and Doublet P. aurolia 



As has long been known, singlets and doublets reproduce true to 

 type through fissions (with relatively rare exceptions that will be men- 

 tioned later). Both kinds of cells also reproduce true to type through 

 repeated autogamies and through repeated conjugations, each with its 

 own kind. Thus, the singlet and doublet are not merely hereditarily 

 diverse; they show a high degree of stability through sexual as well 

 as asexual reproduction. 



In order to discover the basis of this hereditary difference, the 

 obvious thing to do is to cross the two types of cells and carry out a 

 typical Mendelian analysis. As is well known ( Sonneborn, 1947 1 , 

 conjugation in P. aurelia is a process of reciprocal cross-fertilization 

 leading normally to genotypic identity between mates; but there are 

 occasional failures leading instead to double self-fertilization ( cytog- 

 amy). Therefore, in critical genetic work, the mates must be marked 

 with different alleles in order to know whether reciprocal cross-ferti- 

 lization actually occurred. Thus in our work on the breeding analysis 

 of doublets and singlets, several unlinked pairs of alleles were always 

 employed. The following account is confined to exconjugant clones in 

 which the phenotypes, with respect to the marker genes, demonstrated 

 that normal cross-fertilization had occurred. 



Crosses of doublets by singlets regularly yielded clones of singlets 

 from the singlet conjugants of each mating and a clone of doublets 

 from the doublet conjugant of each mating, in spite of the fact that 

 both kinds of Fl clones had identical genotypes. These results indi- 

 cate that the difference between singlets and doublets is not due to a 



