1915] Kof Old and Christiansen: On Giardia muris (Grassi) 41 



There is some evidence, resting on several preparations (pi. 5, figs. 

 6-8) that the granule on the nuclear membrane at the entrance of 

 the rhizoplast is the centrosome which divides to form the polar centro- 

 somes with extranuelear parad&smose between (fig. 8). 



A considerable elongation of the nucleus follows this stage and in 

 some of such nuclei (pi. 6, fig. 20; pi. 8, fig. 59) a centrosome is seen 

 at either pole. In other eases it is revealed at the upper end, but the 

 lower is obscured by the chromatic posterior peristome. In some 

 (pi. 5, fig. 11) it is not visible at either pole. Spindle fibers are not 

 visible till the metaphase and then but faintly. No external astral 

 rays and no centrosphere were found and the paradesmose does not 

 persist as in Trichomonas (Kofoid and Swezy, 1915 a, b). 



The organization of the chromosomes, however, follows somewhat 

 the same history as in Tricliomonas. The stage of encrusted nuclear 

 membrane (pi. 5, fig. 3) if followed by that of the intranuclear 

 chromidial cloud with halo about the karyosome (pi. 5, fig. 4), the 

 enlargement of the karyosome (pi. 8, fig. 55), its breaking up into 

 scattered granules (pi. 5, figs. 8-11), their subsequent arrangement in 

 a split skein (pi. 5, figs. 13, 12; pi. 8, fig. 58) from which there later 

 emerge four larger chromosomes (pi. 5, figs. 14, 15; pi. 6, figs. 16, 17). 

 This stage is certainly subsequent to that of more (eight) scattered 

 granules (figs. 8-11) as proved by its later phase of division in the 

 axostyle. We interpret this earlier phase (figs. 8-13) as one of pre- 

 cocious splitting of the chromosomes or chromatic thread, followed by 

 a subsequent fusion as in TricJiomonas (Kofoid and Swezy, 1915 a,h), 

 prior to the appearance of the definitive number in the equatorial 

 plate, and their final division in the metaphase. Traces of their 

 bivalent character appear in some nuclei (pi. 6, fig. 17). 



The formation of the "amphiaster" stage (pi. 6, figs. 18-22) with 

 the chromosomes in the equatorial plate follows the refusion. There 

 are two rather different types of spindles present, one (figs. 18-20, 22) 

 in which the spindle is shrunken away from the membrane, possibly 

 an artefact or pathological. In this the chromatin in the plate is small 

 in amount, the chromosomes indistinct, and the centrosomes somewhat 

 enlarged. In the other (pi. 6, fig. 21) the spindle is phunp, fills the 

 nucleus, the chromatin is larger in amount, the chromosomes more 

 distinct, and general appearance more normal. 



In the metaphase (pi. 6, figs. 23, 24) the chromatin masses elongate, 

 constrict in the middle, and part. We are unable to see any connec- 



