rise to small colonies by repeated divisions; b. Ovaries, each of 

 which contains a single ovum surrounded by a spinous membrane; 

 c. Spermaries, each of which contains many spermatozoids. After 

 a spermatozoid has entered an ovum the latter undergoes re- 

 peated divisions and gives rise to a daughter colony. 



Draiv various stages in the formation of the daughter colonies 

 from parthenogonidia and from fertilized ova. 



VIII. SACCHAROMYCES CEREVISIAE 



(A Fungus). 



A. MORPHOLOGY. 



Place some growing yeast in a drop of water on a slide and ex- 

 amine under a low power, then under the high one. Observe the 

 small oval bodies or yeast cells. Note : Size ; is it constant ? 

 Measure several. Shape; does it change? Nature of surface. 

 Mode of union. Is there any regular number or arrangement of 

 cells in the various groups ? How many cells in a complete yeast 

 plant? Structure; Observe the cell wall; contents. Is a vacuole 

 present? \Yhere is it found? Do you ever find more than one? 

 Is it contractile? A nucleus is present but it can be demonstrated 

 only by the most careful staining. Place a piece of blotting paper 

 over the cover glass and press firmly upon it ; in this way some of 

 the cells will be bursted and the sac and contents can be studied 

 separately, (a). What is the nature of the sac? Its color? Is the 

 color of the cell due to the cell wall or contents? Is there any open- 

 ing in the sac through which food can be ingested? Are there 

 any organs of locomotion? Is the wall elastic? (b). What is the 

 physical nature of the contents? Its color? 



Draw several cells to show size, mode of union and structure. 



B. CHEMICAL REACTION. 



1. Run a drop of aceto-carmine under the cover glass and ob- 

 serve which individuals stain soonest and most deeply. Do the 

 crushed cells stain as readily as the entire ones ? Does the sac 

 stain ? 



2. Treat another drop of yeast with dilute caustic potash. What 

 happens to the cells? 



Kill some yeast cells by boiling them with water in a test 

 tube. Mount some of this dead yeast and stain with aceto-carmine. 

 Does it stain differently from the living yeast? What inferences 

 may be drawn ? 



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