Structure and Variation in the Sarcotubular System of the Extraocular Muscle Fibres in the Shark Scyliorhinus canicula L.

The microscopic anatomy of the extraocular eye muscles of the shark Scyliorhinus canicula is described. In contrast to swimming muscles, the fibres are not differentiated into distinct fibre type groups. The sarcoplasmic reticulum has narrow terminal cisternae in the Z band region, longitudinal tubules and a fenestrated cisterna in the H band region. Triads or dyads are mostly located at the Z discs. A number of variations from this general pattern is described, with main emphasis on triad location, orientation and structure. Since triads in vertebrates usually are located either at the Z disc or at the A-I junction, and with a fixed structure, these variations in shark muscles are of phylogenetic interest, as they may provide basis for an explanation of development to the two different triad locations.     

Purification and serology of the W strain of cucumber mosaic virus

During studies on the purification of cucumber mosaic virus (strain W) it was found that preparations were most infective and stable when made from tobacco leaves (10–12 days after inoculation) homogenized in phosphate buffer containing EDTA and thioglycollic acid and clarified with diethyl ether. The preparations were further purified by centrifugation in sucrose density gradients containing EDTA at pH 9.0 and were then stable at 2 °C for > 100 days. When mounted in neutralized ammonium molybdate they were shown to consist of predominantly intact particles. In tube and ring precipitin tests and in agar gel-diffusion tests, specific precipitation with homologous antiserum occurred only in media containing alkaline adjusted solutions (ammonium molybdate and dipotassium hydrogen phosphate).     

<Emphasis Type="Italic">De novo</Emphasis> Synthesis of Transfer and 55 RNA<Superscript>1f</Superscript> in Cleaving Sea Urchin Embryos

SIGNIFICANT changes in RNA metabolism have been described during early sea urchin development. Until recently the only detectable class of RNA synthesized during cleavage stages was that with a low G + C base composition and heterogeneous sedimentation properties (DNA-Jike RNA)¹. The genes for nucleolar ribosomal RNA (26S and 18S) were believed to become active only following gastrulation^2–4 and the products of nuclear transfer RNA (4S) genes were first detected at the mesenchyme blastula stage⁵. Any label in the 4S region of sucrose gradients of RNA from the cleavage stages of embryogenesis was interpreted as reflecting the turnover of the pCpCpA region of pre-existing transfer RNA (tRNA) molecules.