150 



on growth substances. It has been previously reported that certain dwarf 

 races of Nctirospora tctrasperma which grow very slowly by themselves 

 seem to act in a complementary manner to stimulate growth in other rather 

 slow-growing races, and vice-versa, so that the heterocaryotic mycelia, or 

 races, grow up to two or three times as rapidly as does either of the individual 

 components A rather slow growing race C4, was crossed with a dwarf 

 race, No. 16, and many ascopores had been isolated at random. Cultures from 

 these individual ascopores showed that the factors for heterocaryotic vigor 

 seemed to be heritable. Certain questions arose, however, which indicated 

 that random selections from dispersed ascospores was not the most desirable 

 method of procedure. The present work has consisted in the isolation of the 

 four spores from individual asci, or isolation of the full complement of spores 

 whenever other than four spores were delimited. All the spores from 131 

 asci were isolated and grown in culture separately. Of these 118 asci con- 

 tained four spores, except two or three which contained five spores. Ten 

 asci contained two normal sized spores and one larger spore. Two asci con- 

 tained two abnormally large spores and one contained a single giant spore. 

 In addition, three of the four spores of 39 asci were also isolated and grown 

 in culture. It was found that 35 of the 118 asci which had four spores 

 showed that all four spores developed similar cultures which grew vigorously 

 and all had perithecia. Thirty-five others showed a two and two pattern in 

 which two grew vigorously and produced perithecia, while the other two 

 grew vigorously but very few, if any, perithecia matured. Forty asci showed 

 a two and two distribution, two cultures growing vigorously, producing an 

 abundance of ascocarps, while two were dwarfs. These were called double 

 dwarfs because so far as tested they have shown that two nuclei of both 

 sexes were present because they fruited with both of the tester strains. The 

 other asci from which the components were grown showed various sorts of 

 irregularities which have not as yet been analyzed. In some cases all the spores 

 were clearly unisexual, as shown by tests. 



The advantage in using races of Neurospora tetraspenna for this work over 

 an obligately heterothallic species such as A'', crassa or N. sitophila is that 

 in the latter forms the nuclei of the opposite sex tend to remain apart even 

 in mixed cultures so that it is difficult to obtain a heterocaryotic race by 

 growing two individual unisexual races together in a culture ; with N. 

 tetraspenna one has no difficulty at all in obtaining heterocaryotic races by 

 growing two individual races together. In this way it is possible to com- 

 pare not only the morphological characters exhibited by unisexual or com- 

 ponent races as compared with a heterocaryotic race composed of the same 

 two individual races, but also their comparative growth rates can be ac- 

 curately measured. 



In order to secure fairly accurate growth rates of a large number of in- 

 dividual races a modification of what we are calling the Beadle and Tatum 

 tubes are used. 



The individual components of 80 bisexual races representing the full in- 

 heritance of 20 asci have been obtained by plating out conidia, hyphal frag- 



