PHYLOGENY OF THE YEASTS 



145 



Finally, E. decipiens and E. Magnusii produce numerous ascs 

 which form at the extremities of the filaments. In E. decipiens, they 

 are not preceded by a sexual act, but in E. magnusii a heterogamic 

 copulation has been established which results in the asc. (Fig. 6.) 

 This is accomplished between a male gamete and a female gamete, 

 each being at the end of a filament. The male gamete is a small, 

 short, cell, with a single nucleus which is located at the end with a 

 shape like a screw. The female gamete is a long cell which also in- 



Fig. 61. Various Stages in the Copulation and Formation 

 of Ascs in Endomyces Magnusii. 



eludes a single nucleus. The gametes unite by their -ends. (Fig. 61, 

 1 and 2.) The middle wall which separates them breaks down, 

 their contents fuse protoplasm with protoplasm, nucleus with nucleus. 

 The egg thus formed grows and is transformed into a tetrasporous 

 asc. Although, heterogamic, this copulation resembles very much 

 that of Sch. octosporous. 



It looks as if one might regard the Schizosaccharomyces as de- 

 rived from a hypothetical form analogous to E. Magnusii but more 

 advanced, in which the copulation may be isogamic. From this form 

 comes on one side E. Magnusii and its parthenogenetic form, E. 

 decipiens, and on the other part Schizosaccharomyces. 



The scheme presented below represents the different steps in the 

 phylogeny of' the yeasts according to the theory which has been out- 



