SACCHAROMYCETACEAE 



303 



the beer yeast, S. ellipsoideus, the wine yeast, etc. A few species have been re- 

 ported as pathogenic, but generally they are rather imperfectly described. 



The heterogamic series producing rough ascospores is less known and has 

 not degenerated quite so far. In Nadsonia fulvescens ascospore formation does 

 not generally occur within the larger fusion cell; on the side opposite the 

 copulatorj^ canal, a sprout cell receives the united contents of both fusion 

 cells and develops 1 or 2 brown ascospores (Fig. 61). Thus we find a trace 

 of ascus formation characteristic of the Eremascaceae, as also cell division 

 intermediate between that of the Schizosaccharomyceteae and normal sprout- 



Fig. 61. — Nad^miia fulvescens. 1-3, copulation of mother and daughter cell; i, 5. development 

 of ascus; 6, ascus set free with ascospore. (After Guilliermond 1928.) 



Fig. 62. — Debaryomyces Kloeckeii, development of asci. (After Guilliermond & P6ju 1920.) 



ing. In N. Richteri occasionally the male cells are not present on liquid media. 

 In this case long copulation tubes are formed. 



In Deharyomyces, traces of a separate ascus formation have disappeared. 

 While copulation is typically heterogamous, we find increasing parthenogenesis. 

 In D. glohosus, from earth in the Virgin Islands, about 75% of the asci are 

 parthenogenetic. Occasionally copulatory processes are formed, but they 

 grow past each other, apparently for lack of sexual attraction. Copulation 

 occurs in about one-fourth of the cases. This may take place between two 

 mature individuals, where the zygote nucleus either divides in the bridge and 

 returns a daughter nucleus to each fusion cell or migrates before division into 

 one fusion cell and there forms a spore (Fig. 62, 3, 4). Copulation may also 



