The Fine Structure of the Meiotic Spindle of the Crayfish

Although the existence of a fibrous orientation within the living spindle can no longer be doubted (3, 6, 7), spindle fibers have been demonstrated with the electron microscope only in a small number of vertebrate tissues (1, 4). Centrioles have previously been investigated in interphasic and dividing cells of vertebrates (1,4) and as basal granules in sperm middle pieces (2, 11). Centromeric regions, identified as the points of termination of spindle fibers at the chromosomes, have been demonstrated as granular condensations in a mi-crograph published by de Harven and Bernhard (4). Species of crayfish seemed suitable objects for a continuation of this work. The high chromosome number of about 100 bivalents offers the possibility of frequent encounter and identifications of centromeres and spindle fibers in the thin sections. Methods Thin sections from the testis of Cambarus virilis were used for this study. The testis was rapidly removed from the decapitated animal and fixed in 1 per cent osmium tetroxide which was buffered to a pH of 7.4 with veronal acetate. The tissue was embedded in n-butyl methacrylate and the plastic containing 1 per cent 2,4 dichlorobenzoylperoxide was polymerized by ultraviolet light. The thin sections were cut with a Porter-Blum microtome equipped with a glass knife. The electron microscope used was an RCA EMU-3b, operated at 50 kV. RESULTS AND DISCUSSION" Axially sectioned meiotic spindles show the centrioles as cylinders of about 0.15 to 0.2 ~ diameter and 0.3 ~ length. In favorably oriented longitudinal sections of spindles two such cylinders were seen side by side during the first maturation division. In such cases, one of these cylinders always appeared cut across while its sister structure is sectioned through the axis or parallel to it through the cylinder wall (Fig. 2). Both cylinders must therefore be oriented at right angles to each other and to the axis of the spindle. Micrographs from the second division (metaphase, or telophase, as in Fig. 3) showed only one centriolar cylinder. Mitotic metaphases are reported to have one cen-triole at each spindle pole (1, 4). The existence of a double structure at each pole of the first meiotic metaphase may well be peculiar to meiosis and suggests that all four prospective spermatid cen-trioles are formed during the first meiotic prophase. However, further observations on both meiotic and mitotic material are necessary before a definite conclusion about the number and orientation of the centrioles can be …