YEASTS AND MOLDS 



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spores, appear to be homogeneous with the exception of a comparatively 

 small number of large spherical 0.5 micron granules. The plasmic gran- 

 ules are especially prominent and numerous in the arthrospores and 

 in the arthrospore sphaerocytes. They are highly refractive and stain 

 readily. They are actively motile, especially in the sphserocytes' where 

 they also show remarkable Brownian vibration. 



Pig. 83. — ^Arthrospore formation and ascospore development of Nematospora. 

 Ordinary vegetative (somatic) cells are gradually transformed into spores; A, arthro- 

 spores derived from vegetative cells; B, an ascospore entering upon a new vegetative 

 cycle; C, detailed structure of a mature ascospore more highly magnified; a, vacuolesin 

 the chromatin-bearing cell of the spore; b, chromatin substance; c, transverse septum; 

 d, plasmic masses in the achromatin cell of the spore; e, achromatin;/, the portion of the 

 ligule which stains very heavily; g, the greatly elongated ligule by means of which the 

 spore attaches itself to various substances with which it is brought in contact. The 

 ascospore is distinctly two-celled. The spore wall as well as the septum^are thin. 

 There is a distinct widening of the spore at'and near the^transverse septum. 



Ascospore formation is generally the result of the gametic union 

 (isogamous) of two elliptical vegetative cells. The apical cell membranes 

 which are in contact dissolve. The plasmic contents of the two cells fuse. 

 The complete changes are shown in Fig. 81. Eight spores are formed in 

 each spore sac. At an early period in the development of the spore 

 sac (ascus) the associated vegetative cells become separated and the spore 

 sac exists as an independent cell structure. There are indications that a 

 spore sac may be derived from a single vegetative ceU, especially when 

 spore formation becomes very active. 



As a rule active ascospore formation is accompanied by active arthro- 



