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University of California Publications in Zoology [Vol. 19 



It is significant that a plot made of analogous data in my own 

 study reveals only one mode (fig. 15) and sets the period at seven days 

 as the interval between maximum numbers of cysts in the faeces. 

 The curve is a typical frequency curve with a single mode. 



In plotting the interval between the modes in the curves sub- 

 mitted by Porter (1916) there are two distinct modes present (fig. 18). 

 It is a bimodal frequency curve. The solid line in figure 18 repre- 

 sents the interval plotted by single-day units on the abscissa, just as 

 was done in figure 15 of my own study. Two distinct modes are seen 

 at points 8 and 15 and a small mode at point 24. There was only one 

 instance of an interval of 24 days (see chart 1, Porter, 1916). When 

 these same data were plotted by two-day units on the abscissa, the 



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 



Fig. 19 



A plot of the average number of cysts for each day for rats 1, 2, 4, 5, 6, 8, 9, 

 10, 11, 12, 13, 14, leaving out of consideratoin the number of cysts which occurred 

 on the last examination in each rat. 



Modes 1, 2, and 4 are very definite; mode 3, although not so pronounced as 

 the other modes, was selected since in six rats this point represented the days of a 

 maximal number of cysts in the faeces. Mode 4 at point 27 on the abscissa was 

 selected instead of the mode at point 29 because it was more representative of 

 all the curves; five curves had a mode at or near point 27, while only two curves 

 had a mode at point 29. 



The average day-intervals between modes 1, 2, 3, 4, is 6% days are almost 

 identical with the intervals found in figure 15. 



broken curve results, and again we find that there are two modes 

 present. This curve thus plotted represents two, if not three (point 

 24), distinct periods at which time maximum numbers of cysts were 

 found in the faeces for Giardia intestinalis. 



From this study of Porter's charts and the plottings of the modes 

 of her graphs it is evident that the cycle of encystment in G. intes- 

 tinalis has an interval of about seven to eight days between the maxi- 

 mum number of cysts in the faeces. The mode at point 24 (fig. 18) is 

 the locus of a third occurrence in one instance only of a maximum num- 

 ber of cysts, which suggests that there was a tendency at least for the 

 cycle to continue at the second interval of fourteen days on the interval 



