148 RESEARCH IN PROTOZOOLOGY 



are a number of deeply staining fibrils. It is possible to identify 

 some of these as axostyles and intracytoplasmic portions of the 

 flagella that have persisted from the trophozoite stage, but other 

 fibrils have not been identified with certainty. A careful study of 

 encystation will be necessary in order to determine the origin of all 

 of these fibrils. 



c. Division of the trophozoite. The trophozoite of Giardia ap- 

 parently divides by longitudinal binary fission. Stages in this proc- 

 ess have been described by Wenyon and O'Connor (1917) and by 

 Kofoid and Swezy (1922). There are still, however, many details 

 that need elucidation. We owe descriptions of nuclear division par- 

 ticularly to Boeck (1917), who described mitosis in Giardia microti 

 including the formation of eight chromosomes, and in Giardia 

 lamhlia to Kofoid and Swezy (1922). This work needs confirma- 

 tion and more detailed studies of nuclear division in other species 

 are desirable. 



d. Excystation. Very little is known about excystation in 

 Giardia. Fragmentary accounts of this process in the case of 

 G. lamblia in the feces of a human host have been supplied by 

 Simon (1921) and the writer (Hegner, 1925). It is possible, how- 

 ever, to feed cysts of giardias to laboratory animals such as rats 

 and then kill the animals at intervals. The writer (Hegner, 1927) 

 found that under these conditions G. lamhlia did not excyst in the 

 stomach or duodenum but mostly in the small intestine from thirty 

 to sixty centimeters posterior to the stomach. The minimum time 

 required for excystation was about thirty minutes. The excysted 

 specimens appeared to remain in the portion of the intestine where 

 excystation occurred, whereas unexcysted cysts were carried down 

 the cecum. 



Giardias are stimulated to activity within the cysts by raising 

 the temperature ; if they are placed in water or saline solution 

 under a cover glass and kept in a warm chamber at about 40° C. 

 for approximately an hour movements begin within the cyst wall 

 but do not result in excystation. This method of stimulating activ- 

 ity within the cyst has one distinct advantage, however, in that it 

 can be used to determine whether or not cysts are alive. 



e. Cultivation. As noted above no one has succeeded in cultivat- 

 ing any species of Giardia outside of the body. Attempts have been 

 made in the writer's laboratory by a number of investigators but 

 without success. It should be remembered that giardias have been 



