The ultrastructural changes occurring during encystment of acanthamoeba palestinensis have been investigated. The cyst wall consists of endocyst and exocyst, both having the same fine structure. At irregular intervals in the cyst wall ostioles occupied by opercula are present. The nuclear membrane forms bulb-shaped projections and releases vesicles bounded by double membranes into the cytoplasm. Dense nucleolus-like bodies of different sizes and variable numbers are found in the nucleus of every cyst. The importance of the cyst structure as a taxonomic criterion is discussed.

The fine structure of the trophozoite, encysting cells, and the cyst of Acanthamoeba astronyxis has been examined. In the trophic form a microtubule organizing center was associated with a well developed Golgi complex. During encystment the organelles of the amoeba changed considerably. The profiles of rough endoplasmic reticulum elongated and were often arranged in circles of multilayered concentric systems, enclosing mitochondria, the nucleus, or other inclusions. The mitochondria showed a tendency toward elongation and constriction. One or two nucleolus-like bodies appeared in the nucleus. Lipid droplets increased considerably in amount and were distributed individually or as aggregates. The mature cyst was star-shaped and surrounded by an almost circular exocyst and an endocyst that was closely apposed to the cell membrane. Both walls differed in their thickness and granulation. The exocyst was continuous over the entire cyst, while the endocyst was interrupted by gaps, ostioles, in the region of the rays. Within the ostioles was a bell-shaped structure, the operculum. The latter was composed of a granular material comparable in electron density to that of the endocyst.

SYNOPSIS. Acanthamoeba (Mayorella) palestinensis produces high basal levels of α‐ and β‐glucosidases, the latter being much more active than the former. Glucose, an essential growth substance, has a dual effect on the glucosidase activity. Growth concentrations (1%) of glucose inhibit, while low levels elevate the activity of both enzymes. Natural α‐glucosides support growth in the same manner as glucose and raise the activity of both enzymes to the same extent. β‐glucosides, on the other hand, are weak growth substrates, but stronger inducers, especially for β‐glucosidase activity. The role of the glucosidases in the over‐all metabolism of the ameba is discussed.

Acid phosphatzse activity has been studied in the ameba Mayorella palestinensis. Optimum activity of the enzyme was found to be at a pH of 3.2. The enzyme is inhibited by fluoride ion, but is not sensitive to Mg++.The activity was found to be correlated with age of culture. Two maxima have been obtained, one from cultures in the logarithmic phase, and the other during the period of maximal cell encystation. These results suggest that acid phosphatase play an important role in cell metabolism during growth and differentiation processes of this ameba.

SYNOPSIS. The effect of osmotic pressure, different electrolytes and organic compounds on cyst formation in Acanthamoeba palestinensis has been tested. The optimal osmolarity for encystment was similar to that of the growth medium. Iso‐osmotic solutions of NaCl, KCl, MgCl2, CaCl2, glycine and sucrose led to maximum cyst formation. The involvement of various agents in the induction of encystment is discussed.