全文获取类型
收费全文 | 8222篇 |
免费 | 25篇 |
国内免费 | 42篇 |
专业分类
系统科学 | 104篇 |
丛书文集 | 80篇 |
教育与普及 | 10篇 |
理论与方法论 | 24篇 |
现状及发展 | 4033篇 |
研究方法 | 375篇 |
综合类 | 3557篇 |
自然研究 | 106篇 |
出版年
2012年 | 112篇 |
2011年 | 214篇 |
2010年 | 47篇 |
2008年 | 161篇 |
2007年 | 186篇 |
2006年 | 160篇 |
2005年 | 152篇 |
2004年 | 187篇 |
2003年 | 143篇 |
2002年 | 159篇 |
2001年 | 246篇 |
2000年 | 259篇 |
1999年 | 165篇 |
1992年 | 167篇 |
1991年 | 111篇 |
1990年 | 111篇 |
1989年 | 99篇 |
1988年 | 74篇 |
1987年 | 105篇 |
1986年 | 114篇 |
1985年 | 172篇 |
1984年 | 129篇 |
1983年 | 96篇 |
1982年 | 91篇 |
1981年 | 87篇 |
1980年 | 113篇 |
1979年 | 223篇 |
1978年 | 183篇 |
1977年 | 208篇 |
1976年 | 173篇 |
1975年 | 181篇 |
1974年 | 239篇 |
1973年 | 183篇 |
1972年 | 170篇 |
1971年 | 229篇 |
1970年 | 328篇 |
1969年 | 243篇 |
1968年 | 237篇 |
1967年 | 252篇 |
1966年 | 220篇 |
1965年 | 152篇 |
1964年 | 76篇 |
1962年 | 50篇 |
1959年 | 94篇 |
1958年 | 130篇 |
1957年 | 119篇 |
1956年 | 95篇 |
1955年 | 78篇 |
1954年 | 76篇 |
1948年 | 71篇 |
排序方式: 共有8289条查询结果,搜索用时 15 毫秒
951.
Summary Cells of human ascitic fluid, cultivated in vitro, showed an activity of cytoplasmic DNA synthesis apparently non-concomitant with a nuclear DNA synthesis. 相似文献
952.
Lin JinFirst Academy Ministry of Aero -Space Industry P.R China P.O.Box -- Beijing China 《系统工程与电子技术(英文版)》1992,(3)
Fundamental definitions of distance and velocity in radar measurement principle are examined and revised from strict theoretical point of view. Synchronization scheme - for clocks in uniform, translatory relative motion is introduced as theoretical foundation for GPS and GLONASS type navigation and positioning technology. Traditional definitions of two-way radar measurement, based on arithmetic mean vlaue concept, turn out to be special cases of revised definitions for one-way radar measurement, based on geometric mean concept, derived from synchronization of moving clocks in accordance with the principle of relativity. The essential physical meaning of Lorentz transformation is interpreted in terms of radar measured parameters. Invariance or absoluteness of four dimensional interval turns out to be invariance or absoluteness of geometric mean time interval. The Lorentz factor turns out to be ratio of geometric mean and arithmetic mean time intervals in terms of radar measured parameters. Theoretical re 相似文献
953.
The segregation of voltage-dependent sodium channels to specialized regions of the neuron is crucial for propagation of an action potential. Studies of their lateral mobility indicate that sodium channels are freely mobile on the neuronal cell body but are immobile at the axon hillock, presynaptic terminal and at focal points along the axon. To elucidate the mechanisms that regulate sodium channel topography and mobility, we searched for specific proteins from the brain that associate with sodium channels. Here we show that sodium channels labelled with 3H-saxitoxin (STX) are precipitated in the presence of exogenous brain ankyrin by anti-ankyrin antibodies and that 125I-labelled ankyrin binds with high affinity to sodium channels reconstituted into lipid vesicles. The cytoplasmic domain of the erythrocyte anion transporter competes for the latter interaction. Neither the neuronal GABA (gamma-aminobutyric acid) receptor channel complex nor the dihydropyridine (DHP) receptor bind brain ankyrin. The results indicate that brain ankyrin links the voltage-dependent sodium channel to the underlying cytoskeleton and may help to maintain axolemmal membrane heterogeneity and control sodium channel mobility. 相似文献
954.
O Kah P Dubourg P Chambolle 《Comptes rendus des séances de l'Académie des sciences. Série D, Sciences naturelles》1979,289(13):951-953
The nucleus preopticus (NPO) and the pars lateralis of the nucleus lateralis tuberis (NLT) have been examined at the ultrastructural level. In the NPO two types of large neurons occur, both containing 140-150 nm secretory granules. The pars lateralis of the NLT contains only one cell type with similar granules. Both of these nuclei may be the source of type A fibres innervating the gonadotropic cells of Gambusia. 相似文献
955.
956.
Recently discovered representatives of the Class Bdelloidea from Tertiary amber from the Dominican Republic represent the oldest known fossils of the Phylum Rotifera. Assuming that the fossil bdelloids had a similar mode of reproduction as present day members of the Class (apomictic thelytoky), then contrary to current thought, some lines of parthenogenetic organisms are not doomed to an early extinction and have evolved built-in mechanisms for genetic diversity. 相似文献
957.
The thymus is the major, if not the sole site of maturation of T lymphocytes from their haematopoietic precursors. During embryonic life (at a few well-defined intervals, at least in birds) the thymus receives thymus-homing haematopoietic precursors that give rise to antigen-specific functional T lymphocytes. Although the number and thymic location of distinct T-cell lineages destined to form the peripheral T-cell pool are not yet well defined, at least two independent pathways have been proposed. First, thymic subcapsular lymphoblasts divide and differentiate to give rise to small deep cortical thymic lymphocytes, medullary lymphocytes and thymus emigrants (I.W., unpublished data) and second, the medulla contains an independent self-renewing population that contains the precursors of the peripheral T-cell pool. Following irradiation the thymus may be repopulated by injected haematopoietic cells presumably related to the thymus-homing haematopoietic cells of the embryo. Here we have reconstituted irradiated mice with limiting numbers of bone marrow cells from Thy-1 congeneic donors and have found distinct clones of cells within the thymus. The pattern of reconstitution by the precursor cells indicates that two independent thymus lineages exist: cortex plus medulla, and medulla alone. 相似文献
958.
Differential expression of calmodulin-binding proteins in B, T lymphocytes and thymocytes 总被引:2,自引:0,他引:2
Changes in intracellular free Ca2+ are involved in the transmembrane signalling of different cells, including lymphocytes. Since calmodulin (CaM) is a primary receptor for Ca2+ (ref. 4), it may mediate the activation of crucial enzymes after antigen-induced increases in cytosolic Ca2+. Using a biotinylated-CaM (Bio-CaM) detection procedure to identify such proteins, we found that a peptide of relative molecular mass 59,000 (59K) was the predominant soluble CaM-binding protein (CaM-BP) in T cells and B lymphocytes from murine spleen; immunoblotting experiments identified it as a subunit of the CaM-dependent phosphatase, 'calcineurin' (CN). Smaller amounts of larger CaM-BPs, thought to be cytoskeletal-binding proteins, were also detected. CaM-BPs were expressed differentially, with B lymphocytes having four times more of the CN-like protein than T lymphocytes, while in thymocytes, a 65K polypeptide was the major CaM-BP. However, limited proteolysis analysis suggested that this thymus-specific peptide may be a precursor of CN. These data suggest that Ca2+-stimulated protein dephosphorylation may be an important and highly regulated function in lymphoid cells. 相似文献
959.
Summary Incubation of calf rib cartilage slices with antiproteoglycan serum elevates CO2-production, but inhibits biosynthesis of proteochondroitinsulfate and of total protein. Absorbing antiserum with proteoglycan abolishes these effects.Acknowledgment. This work was supported by grant K1 193/9 and 10 from the Deutsche Forschungsgemeinschaft. 相似文献
960.