122 University of C alif or ivia Publications in Zoology [Vol.20 



flagellate, where it usually is appressed against the blepharoplasts 

 (pi. 15, fig. 5). In other cases it is slightly withdrawn, spinning out 

 the rhizoplast between the centrosome on its anterior face and the 

 primary blepharoplast. It lies to the left of the cytostome, asj-m- 

 rtietrically and excentrically in the cytoplasm as a whole, both in the 

 free and encysted stages, thus adding an important factor in the 

 fundamental asymmetry of the organism. 



As encystment approaches, and even before the cyst wall is formed, 

 the nucleus withdraws from the anterior end posteriori}^ to a position 

 slightly in advance of the middle of the cyst. 



The close physical connection between the blepharoplasts at the 

 base of the flagella and the nucleus, the metabolic center of the cell, 

 throughout the period of flagellar activity, is indicative of the intimate 

 relation which the nucleus bears to these energj'-expending structures. 

 It can not be merely the physical tug of the moving flagella which 

 pulls the nucleus to its anterior position, for the blepharoplasts retain 

 this anterior location when the nucleus moves posteriorly, the con- 

 nection between them being retained merely by the slender nuclear 

 rhizoplast. The rounding-up process in the cytoplasm of the encysting 

 individual doubtless exerts some pressure leading to spatial readjust- 

 ments, but the translation of the nucleus appears to be out of propor- 

 tion to this single factor. In the active phase of the organism the 

 nucleus is nearest to the center of metabolic activity, and in the passive 

 phase of encystment it tends to assume a place as near as possible to 

 the center of the cytoplasmic mass, the c3-tostomal pouch appearing to 

 hold it off from the fully central location. 



The presence of the centrosome on its anterior border gives evi- 

 dence of a permanent polarization of the nucleus. The distribution 

 of the chromatin within the nucleus lends support to this interpreta- 

 tion. There is a tendency for the chromatin, in the encysted stage, 

 to accumulate at or near the anterior and the posterior poles of the 

 nucleus, adacent to and opposite to the centrosome (pi. 16, figs. 8, 9), 

 in plaques flattened against the membrane. In numbers of instances 

 a radial fiber can be traced from the centrosome (fig. B) to the central 

 karyosome, as in Giardia, where the intranuclear rhizoplast occupies 

 a similar position. Several radial fibers within the nucleus are often 

 found in the cysts. In other instances (pi. 16, figs. 11, 14) the chro- 

 matin is broken into several smaller blobs or plaques, which are 

 distributed on the membrane irregularly and are sometimes connected 

 by linin threads to the central karyosome. The intranuclear rhizoplast 



