全文获取类型
收费全文 | 1088篇 |
免费 | 37篇 |
出版年
2014年 | 10篇 |
2013年 | 22篇 |
2012年 | 28篇 |
2011年 | 46篇 |
2010年 | 58篇 |
2009年 | 53篇 |
2008年 | 45篇 |
2007年 | 58篇 |
2006年 | 43篇 |
2005年 | 31篇 |
2004年 | 18篇 |
2003年 | 17篇 |
2002年 | 10篇 |
2001年 | 11篇 |
1999年 | 15篇 |
1998年 | 23篇 |
1997年 | 24篇 |
1996年 | 15篇 |
1995年 | 24篇 |
1994年 | 20篇 |
1993年 | 21篇 |
1992年 | 14篇 |
1991年 | 10篇 |
1990年 | 17篇 |
1989年 | 14篇 |
1988年 | 15篇 |
1987年 | 12篇 |
1986年 | 16篇 |
1984年 | 9篇 |
1983年 | 12篇 |
1982年 | 8篇 |
1980年 | 17篇 |
1979年 | 11篇 |
1978年 | 14篇 |
1976年 | 12篇 |
1975年 | 8篇 |
1972年 | 14篇 |
1971年 | 18篇 |
1959年 | 23篇 |
1958年 | 30篇 |
1957年 | 27篇 |
1956年 | 12篇 |
1955年 | 16篇 |
1954年 | 20篇 |
1953年 | 22篇 |
1952年 | 16篇 |
1951年 | 19篇 |
1950年 | 10篇 |
1949年 | 7篇 |
1948年 | 11篇 |
排序方式: 共有1125条查询结果,搜索用时 101 毫秒
921.
THE possibility of an oncogenic virus in human breast cancer has been increased by recent findings. Virus-like particles resembling the mammary tumour virus (MTV) were found in electron micrographs of human breast cancer tissue1 and particles physically and morphologically similar to MTV particles have been seen in human milk2. These particles were found more frequently in the milk of American women with a history of breast cancer in their immediate families and in the milk of Parsi women in Bombay than in the milk of nonselected American women3. Parsi women are three times more likely to have breast cancer than other women in Bombay3. The detection of RNA-dependent DNA-polymer-ase activity in such particles isolated from human milk emphasized the possibility that these particles represent an oncogenic RNA virus4. In addition to the physical and morphological resemblance of human milk particles and MTV an immunological relationship between these two kinds of particles seems probable; sera from breast cancer patients neutralize the biological activity of MTV whereas normal human sera did not do so5. We report data supporting the hypothesis of an immunological cross relationship between MTV and human breast cancer. 相似文献
922.
We present primer sequences for 14 polymorphic microsatellite loci in the European beewolf, Philanthus triangulum. Microsatellites were isolated using an efficient enrichment procedure. The number of alleles ranged from two to 28, with observed levels of heterozygosity between 0.090 and 0.828. The microsatellites were designed to determine paternity in female offspring of P. triangulum and to study female choice in this species. We also report the applicability of these markers for a congeneric North American species, P. gibbosus. 相似文献
923.
Synopsis. Crude preparations of kinetoplast vesicles were used to investigate the respiratory chain components in Leishmania tropica promastigotes. In difference spectra from enzymically and chemically reduced preparations, cytochrome b was the predominant component. By utilizing special assays designed to minimize the influence of cytochrome b on difference spectra, cytochromes a, a3, and c333 were demonstrated. Difference spectra from chemically reduced preparations indicated that pyridine nucleotides (NADH) and flavoproteins were also part of the respiratory chain. The presence of these components as well as their response to respiratory inhibitors and ascorbate provide evidence for the presence of a typical trypanosomatid respiratory chain in L. tropica promastigotes. 相似文献
924.
Vertical lek placement of forest-dwelling manakin species (Aves, Pipridae) is associated with vertical gradients of ambient light 总被引:2,自引:0,他引:2
MARTIN HEINDL HANS WINKLER 《Biological journal of the Linnean Society. Linnean Society of London》2003,80(4):647-658
We studied the interaction of ambient light and plumage colour in four sympatric neotropical manakin species which differ in signal colours and vertical lek placement. We estimated bird conspicuousness by modelling chromatic and achromatic background contrast of signal colours in tetrachromatic colour space. Spectral composition of ambient light varies gradually from the understory to the canopy and may affect where manakins lek. Under the given spatial requirements for their horizontal display flights, manakins place their leks at that position along the vertical gradient where ambient light increases chromatic and/or achromatic contrast of their colour signals against the background and/or within their coloration patterns. This suggests that physical factors can be important for placement of display sites, since they may critically influence the effectiveness and efficiency of conspecific communication. © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 80 , 647–658. 相似文献
925.
926.
927.
928.
929.
ABSTRACT. A new heterotrophic flagellate has been discovered from sites in Maryland, Michigan and Wyoming. The flagellate resides within a lorica constructed of a meshwork of intertwined fibrils with the outer surface invested with nail-shaped spines. The organism "reclines" within the lorica with its ventral aspect directed upward, and has two heterodynamic flagella, neither of which bears mastigonemes. One flagellum is directed upward and the other is arched over the ventral aspect of the body. Ingestion of bacteria takes place at the left posterior half of the cell. The organism is anchored to the lorica on the right posterior side by a series of regularly spaced cytoplasmic bridges and at the left anterior of the cell by a cytoplasmic appendage similar to the "languette cytoplasmique" found in some bicosoecids. The right side of the cell is raised into a flattened lip with the outer margin reinforced by a ribbon of microtubules. The new flagellate has mitochondria with tubular cristae and lacks a Golgi. A new genus is created to accommodate both the new flagellate described herein and Histiona campanula Penard. A new family is proposed to include the new genus and Histiona. 相似文献
930.