191!) ] Boeck: Studies on Giardia Microti 117 



division of the nuclei to form four daughter nuclei follows soon after 

 the encystment of the flagellate. The cysts are defaecated as binary 

 cysts, for over two hundred preparations from the faeces were exam- 

 ined and showed the cysts at a stage of development in binary fission 

 the same as that found in the small intestine. 



In the rat the binary cysts occurred throughout the large intestine 

 and were found in the faeces in a stage of development which was not 

 in advance of the stage that was found in the jejunum and ileum 

 (pi. , figs. 14, 16). A few of the single-individual cysts were also 

 found in the large intestine, but the number was almost insignificant 

 compared with the number of binary cysts. The single-individual 

 cysts of Giardia found iu the rat resembled very closely those of 

 G. microti which were found in the meadow mouse (cf. pi. 1, 

 figs. 1, 12). 



Mitosis, with its phases somewhat modified, is the method of nuclear 

 division. At no time in the cysts examined could chromosomes be 

 detected during the process of mitosis. Anaphase spindles (pi. 1, 

 fig. 12) were very common. When the new nuclei are formed two 

 remain in the anterior region of the cyst ; but the other pair 

 appear to migrate to a more posterior position, and they lie upon a 

 different optical plane from that occupied by the other pair (pi. 1, 

 figs. 13-16). This difference in optical planes of the two pairs of 

 nuclei is due to the direction of the major axis of the spindle during 

 the division of the parent nuclei. The major axis of the spindle is 

 directed dorsoventrally, but in many cases the spindle may be tilted 

 obliquely in an anteroposterior direction (pi. 1, fig. 12). 



The fact that cysts found in the faeces are in the same stage of 

 development as those found in the small intestine makes it appear 

 that further development of these cysts in the intestine is arrested. 

 The two-zooid somatella present in tin* binary cysts was never found 

 to have undergone plasmotomy while in the cyst, as was the case in 

 G. microti found in the meadow mouse. It is very probable that 

 further development is dependent on the cysts being ingested by 

 another rat. The action of the enzymes of the stomach it is believed 

 serves to prepare the cyst wall for digestion when the cyst reaches 

 the small intestine ; otherwise, we should expect those cysts found in 

 the small intestine before the defaecation to lose their walls, without 

 making ejection of the cysts from the rat and subsequent ingestion 

 necessary. The reason why the development of this species of Giardia 

 found in the rat should be arrested during encystment is a matter of 



