Effect of light adaptation on electrical responses of the retinas of four species of bats.

The levels of light adaptation at which the retinas of 4 species of microchiropteran bats became unable to generate electroretinograms were progressively ordered. The order correlated well with light preferences based on activity patterns of the 4 species. These results suggest that the ability of the retina to function in ambient light may govern some natural behaviors of these bats.     

Splenic cellular responses in vitro after tumour-immunization

Résumé L'incorporation de³Htdr, en l'absence de PHA, a obtenu un niveau élevé chez les animaux immunisés. Les cellules témoins montrèrent une radioactivité faible, à moins qu'elles n'aient été stimulées par la PHA. Les cellules spléniques des souris immunisées ne présentèrent aucune stimulation significative par l'utilisation d'une dose optimum de PHA.     

Histone hydrolase in tadpole liver

Résumé Le cytoplasme du foie de têtard contient une enzyme qui hydrolyse spécifiquement les protéines basiques telle que la polylysine, la protamine et les histones. L'addition d'autres protéines telles que l'albumine, la globuline et l'acide polyglutamique inhibe l'activité de cette enzyme. Les résultats obtenus montrent que cette hydrolysase extraite du foie de têtard diffère des cathépsines ou de l'histone-hydrolase du rein de rat.

---

---

This work was supported by research grants from the National Institute of Arthritis and Metabolic Diseases No. AM 09602, National Science Foundation No. GB-22663 and from the National Cancer Institute No. CA 07174.     

A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way

Many galaxies are thought to have supermassive black holes at their centres-more than a million times the mass of the Sun. Measurements of stellar velocities and the discovery of variable X-ray emission have provided strong evidence in favour of such a black hole at the centre of the Milky Way, but have hitherto been unable to rule out conclusively the presence of alternative concentrations of mass. Here we report ten years of high-resolution astrometric imaging that allows us to trace two-thirds of the orbit of the star currently closest to the compact radio source (and massive black-hole candidate) Sagittarius A*. The observations, which include both pericentre and apocentre passages, show that the star is on a bound, highly elliptical keplerian orbit around Sgr A*, with an orbital period of 15.2 years and a pericentre distance of only 17 light hours. The orbit with the best fit to the observations requires a central point mass of (3.7 +/- 1.5) x 10(6) solar masses (M(*)). The data no longer allow for a central mass composed of a dense cluster of dark stellar objects or a ball of massive, degenerate fermions.     

Progenitor cell maintenance requires numb and numblike during mouse neurogenesis

Neurons in most regions of the mammalian nervous system are generated over an extended period of time during development. Maintaining sufficient numbers of progenitors over the course of neurogenesis is essential to ensure that neural cells are produced in correct numbers and diverse types. The underlying molecular mechanisms, like those governing stem-cell self-renewal in general, remain poorly understood. We report here that mouse numb and numblike (Nbl), two highly conserved homologues of Drosophila numb, play redundant but critical roles in maintaining neural progenitor cells during embryogenesis, by allowing their progenies to choose progenitor over neuronal fates. In Nbl mutant embryos also conditionally mutant for mouse numb in the nervous system, early neurons emerge in the expected spatial and temporal pattern, but at the expense of progenitor cells, leading to a nearly complete depletion of dividing cells shortly after the onset of neurogenesis. Our findings show that a shared molecular mechanism, with mouse Numb and Nbl as key components, governs the self-renewal of all neural progenitor cells, regardless of their lineage or regional identities.     

Chemical investigation of hassium (element 108)

The periodic table provides a classification of the chemical properties of the elements. But for the heaviest elements, the transactinides, this role of the periodic table reaches its limits because increasingly strong relativistic effects on the valence electron shells can induce deviations from known trends in chemical properties. In the case of the first two transactinides, elements 104 and 105, relativistic effects do indeed influence their chemical properties, whereas elements 106 and 107 both behave as expected from their position within the periodic table. Here we report the chemical separation and characterization of only seven detected atoms of element 108 (hassium, Hs), which were generated as isotopes (269)Hs (refs 8, 9) and (270)Hs (ref. 10) in the fusion reaction between (26)Mg and (248)Cm. The hassium atoms are immediately oxidized to a highly volatile oxide, presumably HsO(4), for which we determine an enthalpy of adsorption on our detector surface that is comparable to the adsorption enthalpy determined under identical conditions for the osmium oxide OsO(4). These results provide evidence that the chemical properties of hassium and its lighter homologue osmium are similar, thus confirming that hassium exhibits properties as expected from its position in group 8 of the periodic table.