UNICELLULAR ORGANISMS 83 



in the state of division may readily be found. Conjugation, however, is 

 rare and cannot be studied by a class. 



The other unicellular animals mentioned in Chapter II may be commonly 

 found with Amoeba and Paramedum. They cannot always be obtained, 

 however, and the student will often be obliged to omit them. Euglena 

 should not be omitted, however, if any appear in the dishes of decaying 

 pond weeds. 



Pleurococcus. The best method of obtaining this for study is to find some 

 fence post or log which is covered with a green growth. This material 

 scraped from the wood will usually prove to be a mass of Pleurococci. No 

 special method of study is needed except to place a small quantity in a 

 drop of water and study with a 1/6 inch objective. The structure can be 

 readily seen and cells may be found showing division by fission. 



Yeast . A cake of ordinary compressed yeast furnishes excellent material. 

 A small quantity should be rubbed with a little water in a watch glass. A 

 minute drop of this material diluted still further in water, and studied with 

 a 1/6 inch, will show the structure of the yeast except the nucleus, which 

 can only be made out by special methods. Many cells showing buds may 

 be found in a fresh yeast cake. Such a yeast preparation usually contains 

 grains of starch, which may be distinguished from the yeast by running 

 a little iodine solution under the cover glass, which will turn the starch blue. 

 The starch has nothing to do with the yeast, being added to the cake to 

 give it body. A few drops of the yeast emulsion should be planted in several 

 large test tubes containing a fermentable liquid. Pasteur's solution is best, 

 but a little diluted molasses will serve. Pasteur's solution contains the 

 following ingredients: 



Water 837.60 c. c. 



Grape sugar 150 gms. 



Ammonium tartrate . 



Potassium phosphate 



Calcium* phosphate 



Magnesium sulphate 



If these tubes are placed in a warm place, 80 to 90 F., fermentation will 

 soon begin, and after a few hours bubbles of CO2 may be seen rising through 

 the liquid. After 12 hours a little of the scum or the sediment will show the 

 actively growing yeast. This growing yeast should be carefully compared 

 with the fresh, dormant yeast in the yeast cake. 



Bacteria. Only a little work can be done without special methods which 

 are complicated and difficult. Bacteria may be shown, however, as follows: 



