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University of California Publications in Zoology [Vol. 19 



the numbers of the other rats. There were three distinct periods when 

 the cysts were found in the faeces. The first period lasted four days, 

 October 27 to 31. The mode of this period came on October 28, when 

 12 cysts were counted. A period of depression of eight days when 

 no cysts were found in the faeces preceded the next period when the 

 cysts recurred in the faeces. This period was short, lasting only two 

 days, with the mode on November 7. Two days followed during which 

 the cysts disappeared from the faeces. Then the third period when 

 cysts recurred again in the faeces began November 12. This period 

 extended to November 18, when a period of depression marked by a 

 fall in the number of cysts took place. The mode was reached on 



10 



t 



iii 



25 26 27 28 29 30 31 1 2 3 



10 11 12 13 14 15 16 17 18 19 20 21 



5 6 7 8 9 



Eat 15 

 Fig. 14 



November 16, when 10 cysts were recorded for twenty fields. After 

 the single day when the number of cysts decreased to five the number 

 immediately increased and when the last examination was made the 

 number had reached eighteen in twenty fields. This was the largest 

 number of cysts found in twenty fields for all the rats examined. The 

 intervals between the modes one and two and two and three, are 10 

 and 91/2 days, respectively. 



Rat 15. The infection in this rat was very light, for during the 

 twenty-eight days cysts occurred only three times and in small num- 

 bers, though the number of cysts was increasing when the last two 

 examinations were made. It will be seen that there are not two com- 

 plete periods when cysts were found in the faeces, and because of this 

 fact these data of rat 15 could not be used to determine the cycle of 

 encystment in Giardia of the rat. 



The series of graphs just presented may be divided into two groups. 

 In the first group may be placed those curves which show positive 



