1910] Wilson: On the Life-Histonj of a Soil Amoeba 263 



moist chamber film side up. After seventeen honrs they were fixed 

 at intervals of fifteen minutes for more than five hours. The amoebas 

 had all come out very near the beginning of the experiment. Another 

 series was started in the same way, but fixed at the same intervals 

 beginning at the time the covers were put into the moist chamber and 

 extending over a period of five and a half hours. Here the amoebas 

 began coming out in two and a half hours. The karyosomes of the 

 emerging and emerged amoebas were compact. There was no evidence 

 of fertilization, but division stages were common soon after excystment. 



2. Encystment 



Eneystment follows a period of rapid division. The amoebas are 

 always very numerous before this tal?es place and only an occasional 

 division stage may be found in a preparation in which most of the 

 individuals are encysting. Encystment occurs on the fourth day or 

 later, depending on the heaviness of the inoculation or the relative 

 numbers of amoebas in the culture. Encystment is delayed when 

 numbers are small. 



Amoebas going into this condition have no food vacuoles. They 

 round up with a very large contractile vacuole pulsating (pi. 21, fig. 

 82). The protoplasm becomes dense and a wall is formed around the 

 amoeba. First a thin outer wall is formed in which no openings have 

 been observed. A thick wall is formed within the outer thin one, and 

 in it round openings are left (pi. 21, fig. 83). Around these the wall 

 in some instances seems slightly thickened. 



3. Excystment 



Excystment has been found in cultures which have been depopu- 

 lated by removal of individuals for study and fixation. It may also 

 occur when none have been removed, but no investigation has been 

 made to prove this because when once a cover is removed it is not put 

 back for fear of contamination. It may also be induced by the 

 addition of fresh medium and is apparently a reaction to a favorable 

 change in the medium. 



An amoeba that is coming out may be easily recognized by the 

 appearance of contractile vacuoles, which are from one to four or more 

 in number. These contract frequently and their activity may extend 

 over a period of twenty minutes or longer before a pseudopodium is 

 put out. The first appearance of the latter is a little rounded mass 

 protruding beyond the cyst wall through one of the micropyles (pi. 21, 



