LIFE CYCLFS, SFXUALH V, AND SEXUAL MECHANISAIS 49 



1942; Emerson, 1941; Harder and Sorgel, 1938; Kniep, 1929, 1930). 

 The vegetative nivcclia of the t\\o generations are identical except 

 for the specialized reproductive organs which they bear. 



Diploid Cycle 



The cycle that is typical for the animal kingdom, completely 

 diploid except for the immediate products of meiosis, is found among 

 the fungi in a number of yeasts (Guillermond, 1940; Winge, 1935) 

 and perhaps in some members of the phycomycetous order, the Blas- 

 tocladiales (Couch, 1942; McCranie, 1942). The latter case consti- 

 tutes a slight variation of this cycle, as Wilson (1952) reports a single 

 mitotic division of the meiotic products in AUoviyces cystogemis 

 prior to gamete differentiation. A regular haploid phase of two 

 nuclear generations is unique among the fungi and, strictly, should 

 be considered yet another type of life cycle. 



BASIC PATTERNS OF SEXUALITY 



Blakeslee, in 1904, in the course of an investigation of zygospore 

 formation in the common "black bread mold," Rhizopiis nigricans, 

 first demonstrated "bisexuality" in the fungi. The term hetevothallisin 

 was introduced to designate the occurrence, within a given species, 

 of two kinds of individuals, each self-sterile and presumably differ- 

 ing in sexual sign, and the necessity of interaction between mycelia 

 of the two kinds to accomplish sexual reproduction. The term hovio- 

 thallisin was coined for the antithetical condition, the occurrence of 

 only a single kind of individual, self-fertile and sexually self-sufficient. 



The original definitions of homothallism and heterothallism, un- 

 fortunately, however, were somewhat ambiguous. The derivation of 

 the term heterothallism implies differences of any sort between the 

 individuals required for sexual interaction, whereas the original defi- 

 nition strongly implied differences in sexual sign. That Blakeslee was 

 convinced of the sexual nature of the race differences, in spite of the 

 slight and inconstant morphological differences, is strongly indicated 

 by the work done by him and his associates during three decades 

 toward the definite identification of ( + ) and ( — ) as ? and $ re- 

 spectively (Satina and Blakeslee, 1928, 1929). 



Blakeslee and other workers (1920) determined the pattern of 



