Ultrastructure of the Giant Amoeba Pelomyxa palustris*

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Abstract
SYNOPSIS. The ultrastructure of the herbivorous amoeba Pelomyxapalustris was studied. Nuclear division is not understood in this amoeba, and evidence for the method of nuclear division was sought. This species typically has many spheroidal nuclei which are similar within a given cell. However, some amoebae from our collections differed from this common type in both the number and structure of their nuclei. This suggested stages associated with nuclear division. One current hypothesis of nuclear division in this organism is that of nuclear budding. Our evidence is more in accord with this method than with mitosis.The cytoplasm contained no mitochondria, Golgi bodies, contractile vacuoles or crystals. Most amoebae had 2 types of bacteria (bacteroids or endosymbionts) in their cytoplasm; a separate vesicle enclosed each of these. Characteristically, only 1 type of bacterium (Bn) surrounded the nucleus. Another type (B) was found elsewhere in the cytoplasm. Also in the cytoplasm were the following: food vacuoles enclosing various algae, relatively clear vacuoles and vesicles, glycogen, various electron‐opaque particles, and occasional microtubules. The plasmalemma was smooth, lacking the external fringe which characterizes other large fresh‐water amoebae.

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