238 THE LOWER FUNGI— PHYCOMYCETES 



(+) and ( — ) character is especially instructive. The conditions 

 presented in the germ sporangium as determined for the three 

 species, Mucor mucedo, Sporodinia grandis, and Phycomyces nitens 

 represent three distinct types. The conditions are summarized 

 by Blakeslee as follows: 



1. The germination of the zygospores of the homothallic species, Sporo- 

 dinia grandis, is pure homothalHc, all the spores of the germ sporangium on 

 germination developing homothallic mycelia. 



2. The germination of the zygospores of the heterothallic species, Mucor 

 jnucedo, gives rise to germ sporangia in which all the spores are aUke, either 

 ( + ) or (— ). Segregation of sex is accomplished at some point before the 

 formation of the sporangiospores, and a single zygospore produces a germ 

 sporangium containing spores of but one sort. 



3. In the germination of the zygospores of the heterothallic species, 

 Phycomyces nilens, segregation of se.x occurs preceding the formation of the 

 sporangiospores but is only partial. As a result the germ sporangium con- 

 tains in addition to (+) and ( — ) spores, a few spores which on germination 

 give rise to homothallic mycelia characterized by the production of con- 

 torted aerial outgrowths termed pseudophores and the occasional forma- 

 tion of homothallic zygospores. The sexual character in these homothallic 

 mycelia is unstable and in their sporangia segregation again occurs and 

 (+) and ( — ) spores and others which develop homothallic mycelia are 

 produced. Finally the homothallic condition disappears. It is interesting 

 to note that there is apparently no definite numerical ratio between the (-)-) 

 and ( — ) spores formed in the germ sporangium, and it sometimes even 

 happens that all the spores are of one sort. 



If from analogy with other fungi segregation of sex may be 

 assumed to occur in these moulds during the heterotypic mitosis, 

 it is evident that this division does not occur at the same point 

 in the life cycle of every species; nor does it occur in all of the 

 diploid nuclei of a given species at the same time. This is 

 evidenced by the presence of ( + ), ( — ), and "bisexual" spores in 

 the same germ sporangium in Phycomyces. If a number of pairs 

 of nuclei fuse in the mature zygospore of Phycomyces as described 

 by Keene (1919) the presence of "bisexual" spores in the germ 

 sporangium might be explained as due to the fact that certain 

 of the nuclei retain their diploid character longer than others. 

 This assumption has not as yet been sustantiated by cytological 

 observations. 



Blackman (1906) objects to the use of the term sexual in 

 connection with the fusion of the gametangia in the Mucorales. 

 He states that the term sex as applied to organisms in general 

 cannot be used where the fusing cells are morphologically 



