全文获取类型
收费全文 | 185544篇 |
免费 | 8870篇 |
国内免费 | 923篇 |
专业分类
195337篇 |
出版年
2023年 | 360篇 |
2022年 | 808篇 |
2021年 | 1663篇 |
2020年 | 946篇 |
2019年 | 1247篇 |
2018年 | 13046篇 |
2017年 | 11746篇 |
2016年 | 9496篇 |
2015年 | 4452篇 |
2014年 | 4667篇 |
2013年 | 5540篇 |
2012年 | 11174篇 |
2011年 | 19288篇 |
2010年 | 15969篇 |
2009年 | 12104篇 |
2008年 | 15241篇 |
2007年 | 16780篇 |
2006年 | 5737篇 |
2005年 | 5808篇 |
2004年 | 6055篇 |
2003年 | 5923篇 |
2002年 | 5391篇 |
2001年 | 1211篇 |
2000年 | 881篇 |
1999年 | 1026篇 |
1998年 | 1315篇 |
1997年 | 910篇 |
1996年 | 828篇 |
1995年 | 770篇 |
1994年 | 663篇 |
1993年 | 767篇 |
1992年 | 686篇 |
1991年 | 601篇 |
1990年 | 573篇 |
1989年 | 556篇 |
1988年 | 529篇 |
1987年 | 489篇 |
1986年 | 442篇 |
1985年 | 617篇 |
1984年 | 692篇 |
1983年 | 641篇 |
1982年 | 731篇 |
1981年 | 659篇 |
1980年 | 664篇 |
1979年 | 393篇 |
1978年 | 465篇 |
1977年 | 401篇 |
1976年 | 400篇 |
1972年 | 458篇 |
1971年 | 463篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
Background
Substituted catechols are important precursors for large-scale synthesis of pharmaceuticals and other industrial products. Most of the reported chemical synthesis methods are expensive and insufficient at industrial level. However, biological processes for production of substituted catechols could be highly selective and suitable for industrial purposes. 相似文献22.
23.
Sex ratios under asymmetrical local mate competition in the parasitoid wasp Nasonia vitripennis 总被引:1,自引:0,他引:1
Sex ratio theory has proved remarkably useful in testing theadaptive nature of animal behavior. A particularly productivearea in this respect is Hamilton's theory of local mate competition(LMC), which has been extended in numerous directions to includegreater biological realism, allowing more detailed tests inspecific organisms. We have presented one such extension, termedasymmetrical LMC, which occurs when egg laying by females ona patch is asynchronous, and emerging males do not disperse,resulting in the extent of LMC on a patch varying over time.Our aim here is to test whether the parasitoid wasp Nasoniavitripennis responds to variation in the degree of asymmetricalLMC. Specifically, we show that females adjust their offspringsex ratios in response to (1) variation in the amount of asynchronyin emergence between broods on a patch and (2) the number andproportion of previously parasitized hosts on the patch. Ourresults provide qualitative support for the predictions of theory,suggesting new levels of complexity in the sex ratio behaviorof this much-studied organism. However, our results do not alwaysprovide quantitative support for theory, suggesting furthercomplexities that must be clarified. 相似文献
24.
25.
Dr. Patrick R. Cammarata Lawrence Oakford David Cantu-Crouch Robert Wordinger 《Cell and tissue research》1987,250(3):633-640
Summary The bovine lens capsule has previously been shown to provide an optimal surface for the examination of epithelial cell interaction with a basement membrane. This native substrate has been used to investigate some initial aspects of attachment of mouse blastocysts and trophoblastic cellular outgrowth. Mouse blastocysts were presented to the cell-free humoral side of the anterior lens capsule, incubated for 72 h, and examined by scanning and transmission electron microscopy. Blastocysts hatch and attach from their zonae pellucidae by 30 h. Trophoblastic cells proliferate rapidly in a coronal direction, display extensive surface microvilli, and advance by the extension of numerous filipodia, many of which terminate with bulbous projections. These projections were shown by transmission electron microscopy to contain numerous vacuoles and polysomes. To simulate further the initial blastocyst-uterine interaction, a suspension of lens epithelial cells was introduced to the capsule and permitted to form a monolayer prior to the addition of the blastocysts. At 72 h the monolayer of lens cells remained intact. We observed that: a) lens cells appear to recede from the advancing trophoblastic cells, and b) trophoblastic cells extend beneath the monolayer of lens cells and thereby dislodge the cells from the lens capsule substrate. No infiltration of the capsule by the advancing trophoblastic cells was observed. The lens capsule appears to offer a promising system for the study of trophoblast-epithelial cell interaction on a natural basement membrane. 相似文献
26.
27.
28.
David M. Anderson Richard H. Scheller James W. Posakony Linda B. McAllister Steven G. Trabert Clifford Beall Roy J. Britten Eric H. Davidson 《Journal of molecular biology》1981,145(1):5-28
Three repetitive sequence families from the sea urchin genome were studied, each defined by homology with a specific cloned probe one to a few hundred nucleotides long. Recombinant λ-sea urchin DNA libraries were screened with these probes, and individual recombinants were selected that include genomic members of these families. Restriction mapping, gel blot, and kinetic analyses were carried out to determine the organization of each repeat family. Sequence elements belonging to the first of the three repeat families were found to be embedded in longer repeat sequences. These repeat sequences frequently occur in small clusters. Members of the second repeat family are also found in a long repetitive sequence environment, but these repeats usually occur singly in any given region of the DNA. The sequences of the third repeat are only 200 to 300 nucleotides long, and are generally terminated by single copy DNA, though a few examples were found associated with other repeats. These three repeat sequence families constitute sets of homologous sequence elements that relate distant regions of the DNA. 相似文献
29.
Carbon and nitrogen stable isotope analyses have improved our understanding of food webs and movement patterns of aquatic
organisms. These techniques have recently been applied to diet studies of elasmobranch fishes, but isotope turnover rates
and isotope diet–tissue discrimination are still poorly understood for this group. We performed a diet switch experiment on
captive sandbar sharks (Carcharhinus plumbeus) as a model shark species to determine tissue turnover rates for liver, whole blood, and white muscle. In a second experiment,
we subjected captive coastal skates (Leucoraja spp.) to serial salinity reductions to measure possible impacts of tissue urea content on nitrogen stable isotope values.
We extracted urea from spiny dogfish (Squalus acanthias) white muscle to test for effects on nitrogen stable isotopes. Isotope turnover was slow for shark tissues and similar to
previously published estimates for stingrays and teleost fishes with low growth rates. Muscle isotope data would likely fail
to capture seasonal migrations or diet switches in sharks, while liver and whole blood would more closely reflect shorter
term movement or shifts in diet. Nitrogen stable isotope values of skate blood and skate and dogfish white muscle were not
affected by tissue urea content, suggesting that available diet–tissue discrimination estimates for teleost fishes with similar
physiologies would provide accurate estimates for elasmobranchs. 相似文献
30.