222 University of California Publications in Zoology [Vol. 20 



Function op Encystment 



The encystment of Giardia has the same dual functions as in that 

 of the intestinal amoebae. It provides, in the first place, a resistant 

 stage discharged in the faeces of the infected host, which survives under 

 favorable conditions of temperature and moisture and may serve to 

 infect a new host by the oral route. It serves a second function in 

 providing for the period of multiple fission of the encysted Giardia 

 as Wenyon (1915) proposed. It results in the formation in the common 

 cytoplasm of the organs of 2, 4, and 8 individuals which presumably 

 separate at encystment. We find no evidence whatever to support the 

 opinion of Schaudinn (1903, p. 550) and later defended by Penfold, 

 "Woodcock, and Drew (1916), that there are copulation cysts in Giardia, 

 or that of Hartmann (1910, p. 47) that autogamy occurs in the cysts, 

 and that there is an Octomitus stage. It is to be noted that Hartmann 

 and Schilling (1917. p. 169) admit the uncertainty of sexual processes 

 in the cysts. We find neither maturation, fertilization, sexual behavior, 

 nor zygotes. We regard the so-called copulation cysts as cysts in 

 which binary fission and plasmotomy are completed and the two indi- 

 viduals are sister zooids, separated and awaiting escape, not gametes 

 to fuse into a zygote. We have not found such binary cysts in G. 

 ent erica from man, though we did find them (Kofoid and Christiansen, 

 1916) among the cysts of G. microti and G. muris. Furthermore, the 

 opinion hazarded by Penfold et al., that only a single individual will 

 be liberated from the cyst of Giardia finds no support in our analysis 

 of multiple fission in the cyst. 



Specific Differences between 6. entebica and Giardia from 

 Rodents 



Giardia enterica of man differs from G. muris (Grassi) of the house 

 and albino mouse and Peromyscus maniculatiis, from G. microti 

 (Kofoid and Christiansen), from Microtus calif ornicus, and from G. 

 cunicidi (Bensen) of the rabbit, in a number of important and readily 

 recognized morphological characters. The Giardia of the rat requires 

 further elucidation before conclusions can be drawn regarding it. The 

 following table summarizes the known differences. 



