110 University of California Publications in Zoology [Vol. 19 



found, while in series 33 not only were the single and binary cysts 

 present but also multinucleate cysts. This latter type of cyst was 

 found chiefly in the small intestine. 



The probable reason why the cysts were found in so few of the 

 infected mice at autopsy was the fact that a negative period in the 

 cycle of encystment was then in progress. 



In Giardia microti there are two types of reproduction, binary and 

 multiple fission. In taking up the evidence for the development of 

 this flagellate within the cysts a review will be made first of the data 

 that are related to the reproductive method by binary fission and 

 then the data concerned with the method of multiple fission. 



Binary fission may occur in the free state of the flagellate, as has 

 been shown by the work of Kofoid and Christiansen (1915) and by 

 the author (1917), and also within the cyst; the latter fact has been 

 known for a long time. Previous to the work mentioned above, de- 

 scribing binary fission taking place in the free state, it had been held 

 that this method of fission took place only within the cyst. Schaudinn 

 (1903) had noticed two individuals within a single cyst wall and had 

 called such a cyst a "copulation cyst" because he thought the two 

 flagellates were in syngamous union. It had been previously shown, 

 however (Boeck, 1917), that these cysts were only binary cysts and 

 that there is no evidence of sex in Giardia as Schaudinn had inferred. 



Binary Fission Within the Cyst 



When encystment takes place it involves the formation of a wall 

 around a single flagellate. This process begins in the ileum and 

 caecum of the meadow mouse. The greatest number of single cysts 

 are consequently found in the ileum or caecum of the digestive tract. 

 From table 2 it will be seen that these single cysts may also be found 

 in the colon and rectum, but their number in these regions is very 

 small compared with the number found in the small intestine. The 

 ratio in series 29 in one preparation was fifty-five binary cysts to five 

 single individual cysts in the colon and rectum. 



The fact that there were great numbers of single cysts in the ileum 

 and only a few in the large intestine, great numbers of binary cysts 

 being present in their stead, coupled with the presence of nuclear 

 changes within the cysts, is conclusive evidence of progressive multi- 

 plication within the cysts. 



The single cyst (pi. 1, fig. 1) is strongly indicative of recent encyst- 

 ment because one can easily detect within it all the organelles of the 



