104 University of California Publications in Zoology [Vol. 19 



twice, and four cysts once. The average number of cysts on this 

 ordinate was three cysts, and this was the number plotted in figure 

 18 on ordinate line 5 as representing the average number of cysts for 

 that day. In like manner the average number of cysts was determined 

 for the other days and plotted in figure 17. 



The modes 1, 2, and 4 are at once very evident and it will be found 

 that they lie on the same ordinate lines that were determined in 

 figure 16. The third mode is not so conspicuous, which I interpreted 

 to mean that the incidence of encystment was small and disturbed, 

 possibly by some environmental factor for all the rats whose modes 

 go to make up this common mode. But it will be seen, as was pointed 

 out in connection with figure 16, that 6 rats showed a distinct mode 

 at this area in their curve. Of the other six rats one was showing a 

 small number of cysts at this time (fig. 5), three were negative, and 

 no cysts were found in the following examinations (figs. 4, 6, 12), so 

 these curves could not be considered ; the other two rats were negative 

 during these days. Therefore, out of eight possible cases six of them 

 showed a distinct mode at point 19 (fig. 17), which made it justifiable 

 to pick out this line as representing a common mode for most of the 

 curves. It was the day when a maximum number of cysts was found. 



The mode at 26 (fig. 17) was chosen rather than the mode at 28 

 because more individual curves showed a distinct mode at this day, 

 and so represented more truly the day when the maximal number of 

 cysts was found for the majority of all the rats. 



The average interval between the modes 1, 2, 3, and 4 (fig. 17) 

 is found to be 6% days, which corresponds to the interval between 

 the ordinate lines determined in figure 16. 



Discussion 



Prom the data just presented the conclusion is derived that there 

 is a cycle of encystment in Giardia in the rat, that this cycle is regu- 

 larly periodic, and that the interval between successive maximum 

 numbers of cysts is about seven days. 



It is fully realized that the possibility of error is fairly large in 

 the study that has been made in determining this cycle. In the first 

 place because of the time required for examination only one examina- 

 tion was made to determine whether or not a rat was negative for any 

 given day. To eliminate this error at least three ordinary examina- 

 tions should be made to determine whether a rat is negative. Had it 



