L. E. Roth

Dividing nuclei from the giant ameba Pelomyxa carolinensis were fixed in osmium tetroxide solutions buffered with veronal acetate to pH 8.0. If divalent cations (0.002 M calcium, magnesium, or strontium as chlorides) were added to the fixation solution, fibrils that are 14 mmicro in diameter and have a dense cortex are observed in the spindle. If the divalent ions were omitted, oriented particles of smaller size are present and fibrils are not obvious. The stages of mitosis were observed and spindle components compared. Fibrils fixed in the presence of calcium ions are not so well defined in early metaphase as later, but otherwise have the same diameter in the late metaphase, anaphase, and early telophase. Fibrils are surrounded by clouds of fine material except in early telophase, when they are formed into tight bundles lying in the cytoplasm unattached to nuclei. Metaphase and anaphase fibrils fixed without calcium ions are less well defined and are not observably different from each other. The observations are consistent with the concept that spindle fibrils are composed of polymerized, oriented protein molecules that are in equilibrium with and bathed in non-oriented molecules of the same protein. Partially formed spindle fibrils and ribosome-like particles were observed in the mixoplasm when the nuclear envelope had only small discontinuities. Remnants of the envelope are visible throughout division and are probably incorporated into the new envelope in the telophase. Ribosome-like particles are numerous in the metaphase and anaphase spindle but are not seen in the telophase nucleus, once the envelope is reestablished, or in the interphase nucleus.

The release process of bacteria into the cytoplasm of soybean nodule cells has been studied, and three functional zones of the infection thread are delineated. Zone 1 is found over the greatest length of very long infection threads. Zone 2 is a short region where membrane mobilization by exocytosis of endoplasmic reticulum (ER) into the infection-thread membrane takes place; the result is that much new membrane and wall degradation enzymes can be provided. In addition, de novo membrane formation takes place inside the infection thread in apposition to the bacterial outer membrane. Zone 3 is the endocytic region where both bacteria and infection-thread wall degradation vesicles are released into the host cytoplasm and constitute a second product of endocytosis at the infection thread tip. Evidence is presented indicating that the symbiosome membrane, even at its time of origin, is composed of membrane from three sources: the host infection-thread membrane, ER, and de novo synthesis; the membrane formation that is so large for these purposes is probably carried out both from the ER directly and also through the Golgi-apparatus synthesis. Evidence is also given that the bacteria have lost their exopolysaccharide coatings before release into symbiosomes.

The large oligotrich rumen protozoa Diplodinium ecaudatum and Ophryoscolex caudatus have been studied by electron microscopy during interphase and division. The structure of mature cilia is contrasted with that seen during their formation particularly in a tuft where development lags and is arrested. Here the shaft is only a few micra long and is composed of filaments that have circular cross-sections not in the typical circular arrangement. In their diameter and appearance the filaments are similar to filaments associated with the nuclei during division. The macronucleus has within it randomly directed filaments, while the micronucleus contains well aligned filaments and other arrangements typical of an intranuclear mitotic process. An extranuclear filament system is also present and is elaborated during division. The infraciliary filament system is particularly elaborate in these organisms. Filaments ranging from 14 to 22 mmicro have been observed with some tendency for a bimodal distribution in diameters of 15 and 21 mmicro. Formation of such filaments has been observed and consists of an initial orientation of very fine elements followed by filament formation. The observations are discussed in relation to filament involvements in cell movements. The concepts are discussed that filaments are metastable structures and that the transitions from one state to another are functionally significant.