14 MORPHOLOGY AND DEVELOPMENT OF YEASTS 



With the exception of the case, which we shall consider a little 

 later, in which the asc results from a copulation, the ascs retain 

 generally the form and dimensions of ordinary 

 cells. (Fig. 12.) However, in Nematospora 

 coryli and Monospora cuspidata the ascs are 

 rectangular cells more elongated and larger than 

 Fig. 12. Ascs of Sac- the vegetative cells. 



charomyces cerevisiae Often the ascg are derived from cells which 



(according to Hansen). 



have not ceased to bud. There are no clear- 

 cut limits between budding and sporulation, for both are able to be 

 carried on at the same time. Budding continues and slows up only 

 at the time when sporulation begins. This explains why one often 

 sees cells with ascospores and buds, the bud remaining attached to 

 the mother cell and developing. 



The number of ascospores contained in an asc is variable. It 

 may vary between 1 and 12. However, it usually becomes fixed 

 as in many of the industrial yeasts where a certain number usually 

 predominates. 



The number of ascospores in S. cerevisiae varies 'between 1 and 

 5, but 4 are more frequent. With S. Pastorianus the same variation 

 obtains, but 2 ascospores are more common. With other yeasts, the 

 number varies less widely and is more constant. With Saccharomyces 

 Ludwigii and the yeast Johanisberg II, it is almost always 4. With 

 Sch. octosporus, sometimes 4 and sometimes 8 ascospores may be 

 counted. With Sch. mellacei and Pombe the number of ascospores 

 is invariably 4. The same number obtains constantly with Nema- 

 tospora coryli. Monospora cuspidata contains only a single spore in 

 the asc. With Debaryomyces globosus and Schivanniomyces occiden- 

 talis the number varies between 1 and 2. Thus, with each yeast the 

 asc tends to form a constant number of ascospores. This number 

 varies, depending upon the species, from 1 to 2, 4, and rarely 8 or 

 more. 



The ascospores have dimensions between 1.5 and 5 ju. Usually 

 they are spherical or oval (S. cerevisiae, S. Pastorianus, S. ellip- 

 soideus). Sometimes they possess a globule of fat. The ascospores 

 of some yeasts have characteristic forms. In Willia anomala, also in 

 the genus Hansenia, the ascospores present a form quite similar to 

 the cells of lower Ascomycetes (Ascoidea rubescens, Endomyces deci- 

 piens, and Endomyces fibuliger)', they are hemispherical and their 

 adjacent planes are provided with a projecting border which gives 

 them the appearance of a hat. The ascospores of Willia Saturnus 

 have the shape of a lemon and are girdled with a projecting ring. 

 (Fig. 13, 3.) Cells of Pichia membranaefaciens have irregular shapes. 



