. Electron microscopy; proceedings of the Stockholm Conference, September, 1956 . •4iJ*.V Figs. 1-2. Electron micrograph (2) of a thin section of an osmium fixed nucleus in locust testis with an inset of an ultra-violet micrograph (1) of the same nucleus in an adja- cent thicker section. Two clusters of mitochondria (w) are indicated. They possess considerable ultra-violet absorption. solve this discrepancy, we have examined adjacent thick. (| /<) and thin (about 200 A) sections of osmium fixed testis in ultra-violet and electron microscopes respectively. Low magnification micrographs permi
. Electron microscopy; proceedings of the Stockholm Conference, September, 1956 . •4iJ*.V Figs. 1-2. Electron micrograph (2) of a thin section of an osmium fixed nucleus in locust testis with an inset of an ultra-violet micrograph (1) of the same nucleus in an adja- cent thicker section. Two clusters of mitochondria (w) are indicated. They possess considerable ultra-violet absorption. solve this discrepancy, we have examined adjacent thick. (| /<) and thin (about 200 A) sections of osmium fixed testis in ultra-violet and electron microscopes respectively. Low magnification micrographs permit the identification of identical nuclei in the two sec- tions. A comparison between two sections of the same nucleus is shown in figs. 1-2. Fig. I is an ultra- violet micrograph of the thick section and shows that the inhomogeneous distribution of chromatin in the live nucleus is at least partly preserved by osmium fixation. Fig. 2 is an electron micrograph c'f the ihin sectiiMi of the same nucleus, it shows the homogenecHis appearance typical of osmium fixed nuclei in this material and little trace of the inhomo- geneities of chromatin existing in the section is dis- cernible. This homogeneous appearance of nuclei in electron micrographs must, therefore, be ascribed not to the failure of osmium fixation to preserve chromatin distribution, but to its failure to produce appreciable contrast between chromatin and nuclear sap. We believe that this lack of contrast is evidence that osmium tetroxide does nt)l react with desoxyri- bonucleic acid (2). The general appearance of nuclei after osmium fixation is shown in fig. 2. The nucleus shows a rather homogeneous fine-grained structure with little trace of organisation. When examined at high magni- fication occasional traces of organised structure are seen appearing as groups. The general appearance of nuclei after formalde- hyde fixation is shown in fig. 3. The distribution of chromatin within the nucleus is easily visible because
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