18 MORPHOLOGY AND DEVELOPMENT OF YEASTS 



17. 



Population and Formation of the 

 Asc in Schizosaccharomyces Pombe. 



a daughter cell which separates from the mother cell. This may 

 unite with another daughter cell from the same parent to form an 

 asc. However, as we shall see further on, the ascospores of the same 

 asc, when in an environment unfavorable to multiplication, fuse two 

 by two and are transformed into ascs without preliminary biparti- 

 tion. In this case, as the asc always contains 4 or 8 ascospores, the 



gametes will not be separated by 

 more than 3 or 4 generations. 



Copulation may also be brought 

 about between cells of very different 

 parentage. As we shall see further 

 on, Beijerinck has shown that Sch. 

 octosporus, in continued cultivation 

 in artificial media, may lose its 

 properties of forming ascospores 



Different Stages in the and becomes, after a long time, an 

 asporogenous organism. In cultures 

 undergoing this change, the num- 

 ber of asporogenous cells becomes greater and greater at the expense of 

 the sporogenous cells. It happens that these latter cells 

 are isolated in colonies in which all of the other cells 

 have lost the sporogenic function. These, then, have 

 to go to other colonies in their vicinity for sporogenous 

 cells with which they are able to anastomose. They 

 send out long tubes. These often go astray, form a 

 wall across themselves and dissociate. From these 

 facts, it is apparent that the parentage of the gametes 

 is of little importance; as a rule gametes more or less 

 closely situated fuse and copulation follows the law of Fig. 18. Dif- 

 least resistance. 



In Sch. Pombe and Sch. mellacei, two related species, 

 copulation takes place in the same manner as in Sch. 

 octosporus, with the only difference that fusion remains 

 almost always incomplete. The cells destined to copu- 

 late are generally united in colonies of 4 to 20 cells 

 situated in chains and presenting the form of small rods. Copula- 

 tion is accomplished ordinarily between two cells adjacent in the 

 same colony. (Figs. 17 and 18.) The gametes are united through a 

 canal through which nuclear and protoplasmic fusion takes place. 

 (Fig. 19.) The nucleus resulting from this fusion rather quickly 

 divides, and the two nuclei thus formed emigrate to both enlarge- 

 ments of the zj'gospore where they undergo a second division neces- 

 sary to the formation of ascospores. (Fig. 19.) The zygospore then is 



ferent Stages 

 of Copulation 

 in Schizosac- 

 charomyces 

 Pombe as Ob- 

 served in Bot- 

 tcher's Moist 

 Chamber. 



