SYNOPSIS. A study of regeneration of trichocysts in Paramecium caudatum was made in the electron microscope. Organisms, from which trichocysts were removed en masse with electric current, were allowed opportunity to regenerate their lost organelles before they were prepared for microscopical examination of developing stages. Thin sections of organisms from various regeneration periods demonstrate clearly the endoplasmic origin and development of trichocysts. The first stage seen is a membrane‐limited, endoplasmic vesicle enclosing an homogeneous mass, the primordium, within which appears a minute, point condensation of greater electron density. This is subsequently elaborated as a closely packed system of linearly oriented fibrous elements (30–50 Å)—the first definitive sign of the presumptive trichocyst. By progressive utilization of the intra‐vesicular primordial matter in association with apparently continuous provision of the building material by extra‐vesicular biosynthetic activity, the form and structure of the juvenile trichocysts recognizable as a system of closely packed, multi‐layered, fibrous sheets become increasingly evident. The basic structural patterns extend also into the now developing tip. Further growth and differentiation involving the transition of its gross shape from oval to oblong effect a second notable morphological transformation: the formation of a dense U‐shaped cortex enclosing a lightly stained center. Full maturation of the trichocyst includes the formation of the cap, disappearance of the limiting membrane, reduction and ultimate loss of the dense cortex with simultaneous assumption of light electron stain homogeneity of the body, and elaboration of the characteristic periodic striation of the tip. These neo‐formative events, which also occur—although with reduced frequency—in normal vegetative forms, in conjugating pairs, and in dividing organisms, are concluded prior to the final positioning of the organelle in the bipolar ciliary meridian. The implications of these findings are discussed in light of current hypotheses concerning the role of the nucleus and ciliary basal bodies in the genesis of cortical organelles.

SYNOPSIS. An electron microscope study of regenerating trichocysts in Frontonia vesirztlosa showed that these organelles oripnate from primordial, membrane‐limited vesicles which appear randomly in the endoplasm. The characteristic pattern of development leading to the first definitive organization of the trichocyst involves the elaboration of the amorphous intravesicular substance into a series of closely packed fibrous sheets. These densely staining elements, laid down in two dimensional arrays, constitute the fundamental plan upon which the various structural subdivisions of the organelle are subsequently formed. Maturation involves, however, the re‐transformation of the distinctly fibrillar pattern of the body into a homogeneous, structureless matrix. The mature trichocyst, consisting of the tip, cap and body, is a pyriform structure bounded by a thin external membrane which is continuous with the pellicle. Extrusion of the organelle results in the transformation of the homogeneous tip and body into a slender shaft with transverse striations having a familiar periodicity of about 560 Å. During discharge the entire organelle is ejected, but the cap does not participate in the formation of the shaft. Elementary units of 120Å and 250 Å periodicities are also described in intracellularly discharged trichocysts. These observations are discussed in relation to current theories on the nature of the extrusion process. The endoplasmic origin of trichocysts in Frontonia does not support the classical theory of the morphogenetic pluripatency of kinetosomes insofar as i t relates to the direct derivation of these organelles from pre‐existing ciliary basal bodies.

SUMMARY. A comparative cytological study of the buccal organelles in seven species of Paramecium and a critical analysis of their morphogenesis during binary fission are presented. The. results show several heretofore overlooked features of the buccal system which may possess considerable systematic significance. On the basis of the morphology of the buccal apparatus and its mode of stomatogenesis, the genus Paramecium, traditionally placed among the trichostomes, is considered to be a true member of the order Hymenostomatida. The taxonomic utility of the buccal organelles and their phylogenetic implications are discussed in the light of the fundamental principles underlying the “new systematics” of ciliated protozoa.

No abstract

SYNOPSIS. A description of atypical mitochondria found in specimens of Paramecium caudatum, which had been treated with shocks of electric current, is reported. The morphological alterations of these organelles, noted only in certain percentages of specimens from several experimental groups, usually involved a three‐ to fivefold increase in diameter accompanied by distortions of their regular form into various configurations. The structural integrity of the internal microtubules, however, was for the most part preserved. These abnormal forms are interpreted in the light of the experiment as examples of cellular reactions to injury.