1913.] 



Colpoda cucullus from its Resting Cysts. 



433 



finally, it becomes invisible, at which time the Colpoda swims away. The 

 whole process only occupies from 5 to 10 minutes, when incubation has 



taken place at 30° C. It seems as if the endocyst is gradually dissolved or 

 digested so that the enclosed organism may be liberated. 



In order to determine as far as possible the nature of the process, the 

 following methods were adopted : — 



Killing Excysting Colpoda with Different Reagents. — Hanging-drop cultures 

 were made in the usual way, and when there were several Colpoda rotating 

 within their endocysts the cover-slips were lifted and the cultures exposed 

 for a short time to the action of the vapour of some particular reagent 

 capable of killing the organisms already liberated and those still excysting. 

 The cover-slips were then waxed to the slides again. When volatile anti- 

 septics were used, such as xylene, toluene, chloroform, and carbon disulphide, 

 the organisms were killed within 30 seconds, and then it was observed that 

 the endocyst still continued to increase in diameter, the wall becoming 

 gradually thinner and thinner until finally it disappeared. 



This was determined accurately by taking measurements of the endocyst 

 at different times with an eye-piece micrometer. In some cases the endocyst 

 disappeared in the course of 10 to 15 minutes, and in other cases after the 

 lapse of a few hours. 



When osmic vapour and vapour of 40 per cent, formalin were used the 

 Colpoda, were killed in a few seconds. It was then noticed that the endo- 

 cysts did not continue to swell up and disappear, but remained exactly as 

 they were, in diameter of cyst and in thickness of wall, at the moment of 

 killing. There was no alteration even after 24 hours. 



This interesting difference between the effect of volatile antiseptics and 

 osmic acid and formalin vapour indicates that the process of excystation 

 from the endocyst is normally effected by the secretion of a solvent or 

 digestive ferment. The antiseptics, toluene, chloroform, etc., kill the 

 excysting organisms but not the enzyme which it is secreting, and thus the 



