Phylum Inophyta [127 



fuse to form a binucleate cell which may become an additional ascus, or else may 

 grow forth and give rise to more asci than one. 



The stage consisting of cells with two nuclei of different origin is called the 

 dikaryophase. It is characteristic of Ascomycetes ,and also of Basdiomycetes: among 

 Inophyta, it is a normal and familiar thing. To a concept of cytology founded on 

 studies overlooking the Inophyta, it would appear an extreme anomaly, almost an 

 impossibility. It has the appearance of a rather awkward device for making cells 

 genetically and physiologically diploid while the nuclei remain haploid. In most 

 Ascomycetes it is a brief stage, but there are some, as Taphrina, whose mycelium 

 consists prevalently of binucleate cells. 



The detailed behavior of nuclei in the ascus was first described by Harper (1895, 

 1897, 1900) from studies of Peziza, Sphacrotheca, Erysiphe, and Pyronema. The two 

 nuclei in the primordium of the ascus unite into one. The fusion nucleus divides 

 three tim.es, each time in much the same manner. A centrosome with astral rays is 

 present at the nuclear membrane, apparently outside. It divides, and a spindle forms, 

 inside the intact nuclear membrane, between the daughter centrosomes. The chromo- 

 somes appear and divide. As they move toward the poles of the spindle, the nuclear 

 membrane collapses or dissolves, leaving the spindle free in the cytoplasm. The 

 mass of chromatin at each pole of the spindle shreds out into a nuclear network, 

 duly surrounded by a nuclear membrane and usually containing a nucleolus. 



Haploid chromosome numbers of Ascomycetes (all of which have been observed in 

 the ascus) include the following: 



Ascoidea rubescens, fide Walker (1935) 2 



Eremascus alhus, fide De Lamater et al. (1953) 6 



G/om^r(?//<z, fide Lucas (1946) 4 



HypornycesSolaniv:xr.Cucurbitae,fi.dtY{.\r?,ch.{\9'^9) .... 4 



Lachnea scutellata,^dt^ro\\'n {\9\\) 5 



Neurospora crassa, fide McClintock (1945) 7 



Peziza do miciliana,^dt ?)c\\n\iz {\921) 8 



Phyllactinia corylea, fide Colson (1938) 10 



Pyronema confiuens\?Lr. igiieum, ^dtV>ro\vn {\9\b) 5 



Taphrina deformans, fide Martin (1940) 4 



According to Harper, when the third division in the ascus is complete, each of 

 the eight nuclei produced by it thrusts forths its centrosome upon a beak. The astral 

 rays of the centrosomes become recurved in the cytoplasm about the nucleus, and 

 grow and multiply until they are converted into a smooth membrane, outside of 

 which a wall is deposited. Most observers have not seen so much detail. Brown (1911) 

 and Dodge (1937) describe the cell membrane of the ascus, apparently under the 

 influence of the centrosome of each nucleus, as cutting into the cytoplasm in an ellip- 

 soid pattern. In Taphrina (Martin, 1940), the cytoplasm of the spores is delimited 

 simply by accumulation about the nuclei. By whatever process the ascospores are cut 

 out. some of the cytoplasm of the ascus is excluded and left without nuclei. Harper 

 (1899) proposed to limit the older term free cell formation to processes which have 

 this effect; he observed that the occurrence of such processes distinguishes asci from 

 the sporangia of Oomycetes and Zygomycetes, in which spores are cut out by cleavage. 

 Harper believed that a fusion of nuclei follows immediately the fusion of gametes; 

 that the karyogamy observed in the ascus is a uniting of diploid nuclei, producing 

 tetraploid nuclei; and that the characteristic three nuclear divisions in the ascus are 

 necessary for reduction of the chromosome number from tetraploid to haploid. These 



