1, 11 I CYCli S, SIXUAl.n Y, AND SFXUAL AlKCHANISMS 51 



clone. A new rcriii, secaiidiVy bo/z/othnllis///, is applied to self-fertile 

 hctcrocarvons. These will he discussed in detail later. 



Inevitably, there exist a number of forms which fit uneasily into 

 a simple breakdown of this sort; in a group of organisms as varied as 

 the fungi, this situation almost necessarily follows any attempt at 

 categorization in respect to characteristics of the mature thalli. A 

 somewhat less ambio;uous system could be erected on the distinction 

 between phenotypic and genotvpic determination of sexual or mating 

 behavior or both. The two major groupings here would be based 

 upon the ability or inability of genetically identical nuclei (sister 

 nuclei, daughters of a single primary meiotic product) to participate 

 in sexual fusion. Such a distinction would roughly parallel that be- 

 tween homo- and heterothallism. \\'ide acceptance and common usage 

 of the homo-heterothallism concept, however, dictate its perpetuation 

 without radical change in spite of its intrinsic shortcomings. Recog- 

 nition of the pattern of segregation at meiosis as the chief, and fre- 

 quently the sole, factor in determining the ultimate sexual character 

 or mating behavior, or both, of the thallus, however, results in a far 

 clearer understanding of the homo-heterothallism concept. 



Each mature thallus, at the stage in its development at which 

 sexual fusions occur, commonly contains nuclei of only a single kind; 

 that is, they are hovwcaryotic (a number of important exceptions to 

 this generalization will be considered later). These sexually mature 

 thalli thus represent the expressed potentialities determined at meiosis 

 and imparted to the spores that constitute the immediate products 

 of this process. Spores, and the thalli into which they develop, may 

 be divided into four types in respect to the segregation of determining 

 sexual or mating capacities; (1) segregation of sexual factors, (2) 

 segregation of incompatibility factors, (3) segregation of sexual and 

 incompatibility factors, and (4) segregation of neither sexual nor 

 incompatibility factors. In the simple cases under consideration, spores 

 of types 1, 2, and 3 give rise to thalli which are self-sterile but which 

 are cross-fertile in those combinations bringing together complemen- 

 tary sexual or incompatibility factors. Such forms are clearly hetero- 

 thallic. Spores of type 4, on the other hand, produce thalli of only 

 a single kind, all of which are self-fertile; such forms are homothallic. 



A number of complicating phenomena tend to mitigate some- 

 what the simplicity of this picture. Foremost among these is the reg- 



