140 



ORIGIN OF THE YEASTS 



are generally mononuclear. It never produces conidia but, on the con- 

 trary, forms a rather large number of ascs. These are derived from 

 an isogamic copulation which is accomplished, usually, between two 

 contiguous cells in the same filament. The two cells unite by means 

 of little canals playing the role of gametes, which anastomose, form- 

 ing in this way, a sort of bridge between the two cells. (Fig. 54.) 

 The wall which separates the two cells at the middle of the copula- 

 tion canal is not slow to break down. Part of the cytoplasm enters 



the canal from each cell and forms 

 a swelling at the middle of the 

 copulation canal which becomes 

 the zygospore. At this moment each 

 of the cells divides its nucleus. 

 One of the daughter nuclei thus 

 formed remains in the cell and the 

 other passes into the zygospore. 

 (Fig. 54.) There the two sexual 

 nuclei fuse and develop into a single 

 large one. As this proceeds the 

 zygospore forms a wall which sepa- 

 rates it from the two threads which 

 formed it. From this the zygo- 

 spore grows and develops into an 

 octosporous asc quite similar to that 

 of a yeast. The ascospores are 

 enveloped as those of Saccharomyces 

 guttulatus by a double membrane 

 in which the external one breaks 

 at the moment of germination. 

 They germinate directly into a 

 mycelium. It cannot be refuted that Eremascus resembles the yeasts; 

 its ascs present the same characteristics as those of the yeasts and 

 result from a copulation which is able to be approached by cells 

 which one sees in the Zygosaccharomyces and Schizosaccharomyces. 

 By the copulation which precedes the formation of the ascs, many 

 yeasts are similar. In most of the yeasts, it is true, copulation differs 

 from that of E. fertilis in that it is incomplete and ends in the for- 

 mation of an asc having the form of a dumb-bell; Schizosaccharomyces 

 octosporus offers an intermediate stage between the copulation of Ere- 

 mascus and that of the yeasts. In this yeast, copulation is more 

 often complete and produces a large oval cell which is transformed 

 into an asc. In this case, copulation is absolutely homologous to 

 that of E. fertilis. In reality, E. fertilis differs especially from the 



Fi 



_ 54. Different Stages in the 

 Copulation and Formation of Ascs 

 in Eremascus fertilis; 1 and 2: Be- 

 ginning of Copulation between Two 

 Contiguous Cells; 3 and 4: Copula- 

 tion; 5: Demarcation of the Asc; 

 6 and 7: Formation of the Asco- 

 spores. 



