REPRODUCTION AND ENCYSTMENT 49 



temporary structures, but the vacuole which collects 

 the liquid waste is always present. When this vacuole 

 reaches full size, it suddenly contracts and throws the 

 waste into the water. This excretory vacuole is therefore 

 called the contractile vacuole. 1 



29. Respiration. — The amoeba respires. From the air 

 dissolved in the water, it obtains by diffusion the oxygen 

 necessary to its life, and it gives off carbon dioxide from 

 the cell. 



30. Reproduction and Encystment. — The chief method of 

 reproduction in the amoeba is simple (Figure 47 J. The 

 living cell divides into two 

 equal parts, forming two new 

 cells. This process is known 

 as fission (fish'un : Latin, fissus, 

 cleft). 



When the food or water be- 

 comes unsuited to supply the _ A m A □ 



rr J Figure 47. — Amceba Repro- 



needs of the cell, in order to ducing by Fission. 



live the amoeba often secretes 



(makes for itself) a thick wall completely surrounding 



the protoplasm. This process is termed encystment (en- 



sist'ment: Greek, en, in ; kystis, bladder). After the wall 



has been formed, the amoeba is able, for a long period, to 



resist cold, the drying up of the pond, or the lack of food. 



1 No suggestion can be made which will always enable the teacher to secure 

 amoebae. They are more frequently found in the slime and mud of Stagnant 

 water than anywhere else. Paramoecia and other infusoria can usually be 

 secured in abundance by placing a handful of hay or leaves in a jar and cov- 

 ering them with the ordinary water used in the laboratory. This is called a 

 protozoan culture, and should be started about four weeks before the material 

 is wanted for class study. The length of time that the culture should stand 

 can be lessened by adding a little beef-extract and by keeping the jar near a 

 radiator. Water sufficient to keep the hay or leaves covered must be added 

 from time to time. When a good culture of paramoecia is once secured, the 

 jar should be kept from year to year, simply adding water to the dried hay 

 left in the jar wheu infusoria are desired. 



