1919] ' Boeck: Studies on Giardia Microti 107 



of the first two maxima. The results are very striking and uniform 

 in this series of only seven cases, and taken together with the data 

 derived from my study of the cycle of encystment in Giardia of the rat 

 give ground for concluding that there is a regular periodicity in the 

 appearance of the cysts of Giardia in the faeces of about six to seven 

 days. 



It is quite possible that the species of Giardia found in some of 

 the rodents, especially in G. microti, is one and the same species as 

 that found in man, namely, G. intestinalis, because of the similarity 

 in structure. Another reason for believing that these species may be 

 identical is revealed by the study of periodicity made by myself on 

 G. microti and by Porter (1916) on G. intestinalis, and also by the 

 manipulation of her data in the plotting of the curve representing 

 the frequency of the interval between modes, or the days when the 

 maximum number of cysts was detected. This study shows that both 

 of these species have almost identical cycles of encystment. This 

 feature may be common for all the species of Giardia, therefore a 

 characteristic of the genus and not a peculiarity of a single species 

 only ; but in either case it is significant as evidence toward the solution 

 of the true etiology of giardiasis (lambliasis) of man. 



Summary 



1. There is a cycle of encystment in Giardia in the rat ; the recur- 

 ring interval between the maximum numbers of cysts in the faeces is 

 about seven days. 



2. The cycle of encystment in G. intestinalis is about seven or 

 eight days, as seen in the charts submitted by Porter (1916), and not 

 a fortnight, as she has concluded. This in reality, if not fully plotted, 

 represents two cycles of encystment. 



3. Because the cycles of encystment of Giardia in the rat and of 

 G. intestinalis are almost identical there is another reason to infer 

 that these two species may be one and the same. This fact may lend 

 some aid in the solution of the true etiology and prevention of 

 giardiasis (lambliasis) of man. 



