STUDIES OF NON-PARASITIC PROTOZOA 15 



liminary step before dividing, or in order to undergo a period of 

 rest, or, what is more common, in order to shield themselves from 

 desiccation or other injurious influences. In the two former 

 instances the cysts are temporary, but when the adverse conditions 

 last a long time the cysts are of slightly different character, and the 

 term 'duration-cysts' (dauercysten) is applied to them. Within these 

 duration-cysts the animalcules, reduced to a minute ball of proto- 

 plasm, condensed by loss of water, and now apparently homogeneous 

 in structure, may survive for years. All aerial dust contains such 

 encysted ciliates, which, when they fall into water that is sufficiently 

 pure, escape from their cysts and resume an active life. Thus arose 

 the erroneous idea of spontaneous generation. Bearing in mind that 

 binary division is the sole method of reproduction that has been 

 firmly established for the ciliata, and that the article to be referred 

 to appeared before the work of Maupas was published, it will be none 

 the less interesting, especially as a study of cyst-formation, to peruse 

 a piece of work by a most careful observer and veteran biologist, von 

 Rhumbler, who describes the different forms of cyst that occur in a 

 ciliate (Colpoda cucullus) that appears very often in infusions. Those 

 who wish to study the matter practically should read the original 

 article, where ingenious but simple methods for studying the life- 

 processes in this protozoon are given. 



The different forms of encystment described by von Rhumbler 

 are well known, but the various appearances described as sporulation 

 of individuals that have formed duration-cysts and development of 

 the mobile individual from such spores are thought by some biologists 

 to be involution-forms. This brings us to an important topic in biology, 

 and especially in parasitology. The reader will be already familiar 

 with the idea of involution in bacteriology, and he will find that in 

 protozoology the range of possible involution-forms becomes much 

 greater. Whether all forms described as involuted are really 

 degenerated beyond recall i.e., are in a phase of death or whether 

 some now regarded as such are really phases of parasitic adaptation, 

 still remains to be seen. With the reservation that this part of the 

 subject merits reconsideration, we may review the description of the 



