108 University of California Publications in Zoology [Vol.19 



DEVELOPMENT OF GIARDIA MICROTI WITHIN THE CYSTS 

 Introduction 



The author (1917) has pointed out the presence of three distinct 

 types of cysts for Giardia microti. These types were placed in three 

 categories because their morphology warranted such a classification. 

 The first of these types is the "single individual" cyst (pi. 1, fig. 1). 

 This cyst harbors only one individual which has the usual two nuclei. 

 The second type is the "binary" cyst (pi. 1, figs. 5-8), containing four 

 nuclei. The cytoplasmic body may not have undergone plasmotomy, 

 (pi. 1, figs. 4, 5) or plasmotomy may be completed with two daughter 

 flagellates formed (pi. 1, figs. 7, 8). The third type is the "multinu- 

 cleate" cysts, so called because it always contains more than four 

 nuclei and never more than sixteen nuclei (pi. 1, figs. 12-16). 



A study of these cysts and their location in the intestine of the 

 mouse or rat at once suggest definite progressive development of the 

 flagellate within the cyst. To give an account of this development 

 with a view to supplying more evidence toward completing all the 

 stages in the life history of this parasite, the following data are 

 submitted. 



Materials and Data 



The first part of these data were gathered from a study of the 

 material from five different meadow mice, Microtus calif ornicus cali- 

 f ornicus (Peale). Other preparations were made but only five series 

 revealed the presence of cysts. The following table shows the types 

 of cysts found in the different regions of the digestive tract of the 

 meadow mice. 



Cysts were not always found in all the regions of the digestive tract 

 (series 28, A4) and this was explained by virtue of a cycle of encyst- 

 ment. In series 28 the process of encystment had been going on for 

 some time and the cysts had already progressed as far as the colon and 

 rectum, while in series A4, the process of encystment had just begun 

 and so the cysts were found only in the small intestine. In series 29 

 and 33 preparations made from all the regions of the intestine showed 

 the presence of cysts. Encystment was in the midst of its cycle. 

 Series 29 and 33 present two parallel lines of data, identical in all 

 respects except that in series 29 only single and binary cysts were 



