40 VETERINARY BACTERIOLOGY 



protoplasm lying in contact with it, becomes surrounded by a 

 membrane. A cell of the yeast (and certain other fungi) when 

 filled with spores is called the ascus (pi. asci) or sac. The spores 

 are called ascospores (Fig. 20). This method of spore production 

 relates the yeasts quite definitely to some of the higher fungi. 

 In some cases there is a primitive type of sexual reproduction 

 or fertilization associated with the development of the spores. 

 The spores are more resistant to an unfavorable environment than 

 the vegetative cells. When brought under favorable conditions 

 they germinate and develop into the typical yeast plant. In 

 old yeast cultures some cells develop heavy cell walls, are filled 

 with granular reserve food materials, and become potentially 

 spores. Such cells are likewise probably resistant to unfavorable 

 conditions, and serve to tide the yeast over periods of desiccation 

 or poor food supply. They resemble the chlamydospores produced 

 by many molds. 



MORPHOLOGY OF THE HYPHOMYCETES OR MOLDS 



The molds or hyphomycetes do not constitute a homogeneous 

 group in the eyes of the systematic botanist, but belong to vari- 

 ous subdivisions of the group of fungi. Some are related to 

 the algae and are grouped under the Phy corny cetes, others belong 

 to the sac fungi or Ascomy cetes, others are related to the smuts, 

 rusts, and toadstools, or Basidiomy cetes, and the largest number 

 belong to the group of imperfect fungi or Fungi Imperfecti. From 

 the viewpoint of the bacteriologist these botanic relationships 

 are not significant; all the fungi, regardless of kinship, that agree 

 in having the plant body made up of threads usually more or 

 less branched, and forming more or less loose or cottony masses, 

 in short, those that answer to the popular conception of molds, 

 are grouped together as Hyphomycetes. Such a classification is 

 scientifically justifiable only because of the great complexity of 

 the various members of the family of fungi, and the fact that 

 it is not the systematic position but the economic importance of 

 the forms that is of significance. 



Form and Size of Hyphomycetes or Molds. A mold may be 

 differentiated from the yeasts and bacteria in that it is multi- 

 cellular, with the cells united to form more or less bnmrhcd 



