26 MORPHOLOGY AND DEVELOPMENT OF YEASTS 



Fig. 28. Parthe- 

 n 



losing their sexuality. This opinion is supported by a series of very 

 striking iacts which are quite apparent. A consideration of the 

 development of various members of this group will give the proof 

 of progressive disappearance which sexuality in yeasts has undergone. 

 In the Schizosaccharomyces, which present in this connection very 

 interesting characteristics, copulation is illustrated in three varieties: 

 Sch. odosporus, Sch. Pombe, and Sch. mellacei. In 

 the first copulation is almost universal. With the 

 other two, on the contrary, copulation is rather fre- 

 quent and a great number of cells sporulate without 

 copulation. A species of Schizosaccharomyces has 

 been transmitted from the laboratory of Professor 

 Beijerinck under the name of Sch. mellacei, which 

 neticAscsin did not present any trace of sexuality; the asco- 

 romyces go- S p Ores originated in ordinary cells which had not 

 undergone copulation. This yeast, which resembled 

 Sch. mellacei very closely, may have been another variety. 



Among the different species of the genus Zygosaccharomyces 

 numerous cases of parthenogenesis have been observed. With De- 

 baromyces globosus, however, this characteristic is more predomi- 

 nant and many ascs originate without copulation. These may be 

 formed by ordinary cells or cells with long projections giving them 

 the shape of dumb-bells. (Fig. 28.) 



Some yeasts have lost their sexual characteristics but have re- 

 tained traces. Such is the case with Schwan- 

 niomyces occidentalis which has been described 

 by Klocker. Guilliermond has shown that at the 

 moment of sporulation in this yeast, the cells 

 destined to form the ascs emit projections of 

 different length by means of which they try to 

 unite two by two. But the sexual attraction 

 seems to disappear; it is only exceptional that 

 they join. These little projections, then, may be 

 the remnants of an ancestral sexuality. 



Since then, Ludwig Rose 1 and Dombrowski 2 have observed the same 

 characteristics, one in Torulaspora Delbrucki, in a new species related 

 to this latter and in a new yeast isolated from the mucilaginous 

 secretions from an oak tree, which was provisionally named Yeast 

 F; the other in a milk yeast, S. lactis. Yeast F was studied by 



1 Ludwig Rose. Beitrage zur Kenntniss der Organismen in Eichenschleimfluss. 

 Inaugural Dissertation, University Berlin, 1910. 



2 Dombrowski, W. Die Hefen in Milch und Milchprodukten. Cent. Bakt. Abt. 

 II, 28, 1910 



.29. Formation 



of Asc in Schwan- 

 niomyces occiden- 

 talis. 



