STUDIES OF THE MITOTIC FIGURE

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Abstract
In earlier studies of Echinarachnius eggs the writer reached the conclusion that their central bodies are the coagulated focal area of converging rays and have no existence as individualized structures in the living cell. The present investigation is the first of a group analyzing central body behavior in a variety of cell types in diverse organisms, for the purpose of ascertaining in each case whether the central body behaves like that of Echinarachnius eggs or like a typical centriole, such as that found in many spermatocytes. The present study confines itself to first-cleavage metaphase asters in Chaetopterus eggs, because, in preliminary experiments, the writer failed to find the typical centrioles of that egg previously described by Mead, whose observations were later confirmed by Wilson. The effects upon central body structure produced by the following modifications of technique were observed: the formula, pH, and temperature of the reagent, the duration of fixation, the depth of stain, and other points. Ray structure is readily modified in coarseness; the shape is also affected, the rays being either rippled (serpentine) or undulating (almost straight). Central body structure is closely related to ray structure: if rays are rippled, centers are disrupted; if rays are undulating but delicate or medium coarse, centers are large homogeneous centrosomes, undemarked from the ray area; only if rays are both undulating and very coarse is there a slightly delnarked centrosome, which, in 58 per cent of the cases, contains single or double centrioles, which vary in size and shape. Centrioles can be demonstrated only under a very limited set of chemical conditions. Among the many modifications of the formula employed, the only ones coagulating centrioles and very coarse undulating rays are those in which the reagent was diluted with about 80 parts of water. In these the percentage by weight of the picric acid is between about 0.01 per cent and 0.2 per cent, and of the acetic acid, between about 0.05 per cent and 0.4 per cent. Any deviations from these conditions produce other types of both centers and rays. If the central body in Chaetopterus eggs is a true centriole, it is atypical in several respects: the great instability of the central body area, the close relation between its morphology and the exact configuration of the fixed rays, and the narrowly limited conditions of fixation under which centrioles can be shown. The suggestion is made that, as in Echinarachnius eggs, the various kinds of central bodies occurring may be various types of coagulation products of the focal area of converging rays, the morphology of the center depending upon the way in which the rays are fixed; only by chance do certain of them simulate individualized structures. Further consideration of this point awaits additional data. Various classes of minute bodies occurring at the centers of fixed asters are discussed, and the suggestion is made that heretofore we have applied the single term centriole to several very diverse classes of structures, which often look alike but actually behave so differently as to warrant a careful consideration of whether or not they belong to several different categories.