462 
DORIS L. MACKINNON. 
The course of encystment seems to be as follows. The 
organism loses its flagellum (PI. 31, flg. 8), but the rhizostyle 
usually persists. The body becomes rounded off, and there 
appear in the cytoplasm, hitherto flnely alveolar, two or three 
large vacuoles (PI. 31, flgs. 9 and 10). These vacuoles run 
together, and form one very large vacuole, which lies to one 
side of the now spherical cyst. By this time a deflnite cyst- 
wall, double contour,’’ has formed. The diameter of the 
finished cyst is 5 to 6 ju. The nucleus lies between the cyst- 
wall and the vacuole (PI. 31, fig. 11). At this stage it still 
retains the characteristic features of the adult flagellate 
nucleus, with a large central karyosome, and peripherally 
arranged chromatin granules, from which linin threads extend 
towards the centre, like the spokes of a wheel, fifiie rhizostyle 
is still clearly visible as a dark-staining line, often equatorial 
in position. About this time it would appear that chromatin 
escapes into the cytoplasm from the nucleus ; the cyst contents 
stain darkly, especially just within the cyst- wall. This suggests 
that possibly the nuclear material plays some part in the 
formation of this envelope.^ 
The nucleus seems to swell, and its outlines become vague. 
The karyosome breaks up into a number of small round 
masses (PI. 31, figs. 12 and 13). A spindle now makes 
its appearance, on which the karyosome granules occupy the 
position of the equatorial plate (PI. 31, figs. 14 and 15). 
It is not clear what becomes of the peripheral chromatin. 
Possibly it unites with the central mass, but as the cytoplasm 
stains even more intensely and contains more dark granules 
than before, I think it possible that a good deal of the peri- 
pheral chromatin is absorbed. 
Though the spindle is perfectly well defined, I have found 
no indication of centrioles at its poles. In one cyst (PL 31, 
^ Cf. Alexeieff's observations (1913a) on the part played by chromidia 
in the cyst-formation of Chilomastix and other protozoa. Hartmann 
and Whitmore (1910) think that the extrusion of chromatin from the 
nucleus of Entamoeba coli has something to do with the formation of 
the large central vacuole in the cyst. 
