1913.] Colpoda cucullus from its Resting Cysts. 435 



ferment, but in no case was the endocyst digested either by pepsin or trypsin 

 even after remaining for several days at 40° C. Pepsin had the effect of 

 turning the Colpoda almost black, whether they were free or enclosed within 

 the endocysts. 



Trypsin on the other hand gradually digested the Colpoda already free and 

 those within the endocysts, rendering them very transparent and causing 

 complete solution and digestion of all the organism except certain very 

 refractive granules. These two digestive ferments were tried on many 

 occasions but at no time were they found to attack the endocyst. 



Diastase. — The diastase used was prepared from fresh pale-barley malt by 

 the method described by O'Sullivan.* According to Brown and Escombe,t 

 diastase acts best in very dilute acid solutions and they recommend 0006-per- 

 cent, formic acid. The diastase used was therefore dissolved in formic acid of 

 this strength. One lot was heated up in a water bath at 62-65° C. for 

 about 20 minutes in order to kill off any cytase which might be present. 



Two solutions of diastase were thus obtained, one heated and the other 

 unheated. Hanging-drop cultures were made and the excysting Colpoda 

 killed with osmic vapour. One or two platinum loops of the diastase 

 solutions were added and then the cover-slips were waxed down again and the 

 culture put into the incubator at 40° C. 



Twenty-four hours after the addition of the diastase no endocysts could be 

 found even after careful search ; they had all been digested, both by the 

 heated and the unheated diastase. Similar results were obtained by 

 repeating the experiment on other occasions. 



Ptyalin. — Saliva was collected and diluted slightly with distilled water. 

 It was then filtered and a small crystal or two of thymol added to prevent 

 putrefaction. A platinum loop or two of the filtered liquid was added to 

 hanging-drop cultures, which were then allowed to incubate at 40° C. When 

 perfectly fresh saliva was used the endocysts were digested somewhat slowly, 

 i.e. in 48 hours. 



Some of the cultures made did not show the digestive action, whilst others 

 did. It is possible that a certain amount of change must first be effected by 

 the enzyme secreted by the Colpoda within, before ptyalin can act on the 

 endocyst. The digestive powers of this ferment on the endocyst are 

 therefore uncertain. 



From these experiments it is evident that the endocyst consists of a 

 substance which is not digested by pepsin or trypsin, but which is 



* O'Sullivan, C, ' Trans. Chem. Soc.,' 1884, vol. 45, p. 2. 



t Brown and Escombe, "On the Depletion of Hordevm vidgare during Germination," 

 'Proc. Eoy. Soc.,' 1898, vol. 63. 



