38 SEX IN AIICROORGANISMS 



serum from humans. It must be admitted that the time of samphng 

 may influence these results. Secondary colonies were formed on 

 media containing all of Cohn's blood fractions, but conjugation was 

 observed only with the combined fractions (I, II, III-O, III-l, 2, IV, 

 V, VI) and the fraction PGP (I, II, III), both of which contain 

 fraction III. 



The occurrence of conjugation tubes in B. megaterhnn has been 

 challenged by Bisset (1952, 1953). It is pointed out that the rods of 

 this organism occur in chains and that shrunken rods are frequently 

 found joined at each end to their neighbors. These shrunken cells, 

 Bisset claims, have been mistaken for conjugation tubes whose in- 

 terior is presumed to be continuous with the two attached cells 

 because DeLamater's preparations fail to show the cross walls that 

 would be present. 



As yet, serial photographs of the formation and behavior of con- 

 jugation tubes in living material have not been presented. If sufficient 

 optical path differences exist between the migrating nuclei and the 

 cytoplasm, it is theoretically possible to observe and photograph nu- 

 clear migration. 



The genetic evidence of Hayes (1952) and of Lederberg and 

 co-workers (1952) implies that cytoplasmic fusion is probably not 

 involved when recombination occurs between certain mutants of the 

 K-12 strain of E. coli. Hayes showed that cells "killed" by strepto- 

 mycin were still capable of participating in recombination. He con- 

 sidered it probable that the viable cell extrudes genetic elements 

 which adhere to the cell wall. The genetic elements, being unaffected 

 by the streptomycin, can serve in recombination, whereas the killed 

 cell acts as a passive carrier. 



CONCLUSIONS 



It has been demonstrated beyond question from continuous ob- 

 servation of living material that cells of Proteus vulgaris OX- 19 may 

 fuse together in pairs. There is no final demonstration as yet that this 

 is a sexual process involving fusion and segregation of nuclear mate- 

 rial. 



Less convincing are the numerous reports of cell and nuclear 

 fusion and of conjugation in several bacterial species, because the 

 reports are based upon fixed and stained material in which any se- 



