46 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



poured into the Petri dish until it reaches half the height 

 of the block, when the yeast mass to be studied is spread 

 over the surface. This is placed in a thermostat at the 

 right temperature, usually about 25 C. The cells will 

 usually develop spores in from twenty-four to forty-eight 

 hours. Some species of yeast which form films over the 

 surface of their nutrient solutions may develop the spores 

 in such films. 



Yeast spores, like the spores of bacteria, are always 

 formed inside the mother cell, but differ in that usually 

 more than one spore is formed within a single cell. 



The first evidence of sporulation in yeasts is the division 

 of the nucleus first into two, then into four, six, or eight 

 nuclei, by a process of mitosis. Each one of the nuclei then 

 surrounds itself by a bit of protoplasm and finally by a cell 

 wall or membrane. The spores grow in size by absorption 

 of the cytoplasm from the mother cell until they come to 

 occupy practically the entire interior. The number of 

 divisions of the nucleus determines the number of spores 

 which will develop. This is usually fairly constant in a 

 given species, although variations may be found. Yeasts 

 are known which typically produce one, two, four and eight 

 spores to a cell. 



The mother cell with its spores shows such close rela- 

 tionship to the spore-bearing sacs produced by certain of 

 the fungi that it is usually given the name ascus, and the 

 spores are known as ascospores. 



The spores vary not only in number but in shape and 

 size as well. Some are spherical, some elliptical, others are 

 hat-shaped, or even elongate. Some have a single mem- 

 brane and others apparently a double membrane. 



Sexual reproduction of a primitive type may be observed 

 in some species of yeasts, though not with the most common 

 types. In a few species cells which are about to form 

 spores fuse in pairs, the nuclei also fuse and then divide 



