1939 1 Boeck: Studies on Giardia Microti 113 



off when the cysts reach the small intestine. Those cysts which eon- 

 tain two individuals, products of binary fission, would discharge two 

 flagellates into the intestine when the cyst wall was digested away. 

 It is also possible that cysts might be ingested which had not completed 

 all the stages of binary fission, because such cysts were found in the 

 rectum, and if this happened and the cyst wall was digested away 

 then a somatella would be liberated which could continue its develop- 

 ment in a free stage. 



A word may be said here regarding these somatella stages de- 

 scribed many times by Kofoid and Christiansen (1915) for G. muris 

 and by the same investigators and myself for G. microti. Most of 

 these somatella stages represent plasmodial bodies resultant from 

 previous mitotic activity of individuals which had not encysted. 

 Obviously the large number of individuals present in the intestine 

 can be explainable on the ground of binary and multiple fission in 

 the free stage of the flagellate, but another interpretation may be 

 proposed for the somatella stages. 



These stages may be interpreted to be somatellas liberated from 

 cysts while the flagellate body within was in the course of binary 

 fission. Especially does this seem plausible when we compare the 

 free somatella described by Kofoid and Christiansen (pi. 8, fig. 55; 

 1915) and the encysted somatella described in this paper (pi. 1, fig. 6). 

 Furthermore, these authors figure cysts with part of the cyst wall 

 apparently digested away and the somatella in a process of binary 

 fission (pi. 7, fig. 38; Kofoid and Christiansen, 1915). The interpre- 

 tation that these somatellas may have originated from encysted indi- 

 viduals which have escaped from the cyst seems justifiable in view 

 of the facts just presented. But most of these somatella stages no 

 doubt originate from an individual undergoing binary fission in the 

 free state. 



Multiple Fission Within the Cysts 

 During the process of multiple fission within the cysts the two 

 nuclei of the encysted flagellate go through three successive divisions 

 to form a total of sixteen daughter nuclei. The method of division 

 is that of mitosis, for anaphase spindles were found in many cases 

 (pi. 1, fig. 10), but the other phases of mitosis were much obscured 

 and were therefore detected with gerat difficulty. The chromatin of 

 each nucleus was never seen to be divided into chromosomes. A soma- 

 tella containing sixteen nuclei is found in every multinucleate cyst 

 which has completed all its nuclear divisions (pi. 1, figs. 10, 11). 



