86 University of California Publications in Zoology [Vol. 19 



THE CYCLE OF ENCYSTMENT IN GIARDIA 311 C ROT I 



Introduction 



Most of the Protozoa which have been studied intensively for a 

 long period of time have been shown to possess rhythms or cycles of 

 what might be termed general vitality. These rhythms may be seen 

 in the sensitiveness to environmental conditions of the individual 

 organism or races of organisms, as in the case of Paramoecium aurt tin 

 (Woodruff), but more often the rhythms or cycles are evident in 

 the reproductive activity of the protozoan. Cycles in the fission rate of 

 Paramoecium were found by Calkins (1904) and by Woodruff (1905). 

 Gregory (1909) showed that there were cycles of high and low vitality 

 in Stylonychia mytilus and Tillina magna. The cycles were found to 

 be fairly regular and the last work of Woodruff on Paramoecium 

 aurelia (1917) has shown that even changes in the culture media and 

 in temperature fail to modify the cycles of endomitis which are 

 characteristic for the species. There may be a slight initial influence 

 on the cycle of the Paramoecium when the'ciliates are suddenly placed 

 in a changed environment, but after a short period, during which a 

 readjustment of the organisms takes place, the endomitic interval 

 regains its normal length. 



Cycles in the life history of Haematozoa are also known, the most 

 classical examples of which are those of the various species of Plas- 

 modium. In these protozoans not only is the reproductive process 

 rhythmic in nature, but also that of sporulation. 



In the flagellates the presence of cycles of vitality expressed in 

 reproductive activities is also known. The haemoflagellates in their 

 life history present striking examples of cyclic development, and one 

 has only to watch the various free living flagellates in an ordinary 

 aquarium to notice that a rhythm or cycle must exist for them in view 

 of the fact that their numbers are seen to fluctuate from day to day 

 and week to week. 



For some time a cycle of encystment was suspected in Giardia 

 found in the intestine of rodents, but it was during an investigation 

 of mitosis in Giardia microti (Boeck, 1917) that evidence of such a 

 cycle was found. A table in the paper referred to above showed that 

 cysts were found upon examination in only five out of nineteen cases 

 of infection. This fact raised the suspicion of the presence of a cycle 

 of encystment, for if encystment occurs continuously in the life history 



