全文获取类型
收费全文 | 26277篇 |
免费 | 91篇 |
国内免费 | 145篇 |
专业分类
系统科学 | 124篇 |
丛书文集 | 139篇 |
教育与普及 | 63篇 |
理论与方法论 | 72篇 |
现状及发展 | 11743篇 |
研究方法 | 1177篇 |
综合类 | 12879篇 |
自然研究 | 316篇 |
出版年
2013年 | 178篇 |
2012年 | 408篇 |
2011年 | 678篇 |
2008年 | 480篇 |
2007年 | 550篇 |
2006年 | 563篇 |
2005年 | 528篇 |
2004年 | 470篇 |
2003年 | 490篇 |
2002年 | 487篇 |
2001年 | 927篇 |
2000年 | 896篇 |
1999年 | 569篇 |
1994年 | 350篇 |
1992年 | 539篇 |
1991年 | 409篇 |
1990年 | 476篇 |
1989年 | 417篇 |
1988年 | 401篇 |
1987年 | 455篇 |
1986年 | 452篇 |
1985年 | 558篇 |
1984年 | 449篇 |
1983年 | 385篇 |
1982年 | 370篇 |
1981年 | 358篇 |
1980年 | 355篇 |
1979年 | 897篇 |
1978年 | 686篇 |
1977年 | 593篇 |
1976年 | 578篇 |
1975年 | 570篇 |
1974年 | 649篇 |
1973年 | 569篇 |
1972年 | 559篇 |
1971年 | 676篇 |
1970年 | 868篇 |
1969年 | 647篇 |
1968年 | 701篇 |
1967年 | 667篇 |
1966年 | 604篇 |
1965年 | 420篇 |
1964年 | 175篇 |
1959年 | 226篇 |
1958年 | 373篇 |
1957年 | 247篇 |
1956年 | 229篇 |
1955年 | 213篇 |
1954年 | 202篇 |
1948年 | 173篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
A deficiency of the homeotic complex of the beetle Tribolium 总被引:10,自引:0,他引:10
In Drosophila, the establishment of regional commitments along most of the anterior/posterior axis of the developing embryo depends on two clusters of homeotic genes: the Antennapedia complex (ANT-C) and the bithorax complex (BX-C). The red flour beetle has a single complex (HOM-C) representing the homologues of the ANT-C and BX-C in juxtaposition. Beetles trans-heterozygous for two particular HOM-C mutations spontaneously generate a large deficiency, presumably by an exchange within the common region of two overlapping inversions. Genetic and molecular results indicate that this deficiency spans at least the interval between the Deformed and abdominal-A homologues. In deficiency homozygous embryos, all gnathal, thoracic and abdominal segments develop antennal appendages, suggesting that a gene(s) has been deleted that acts to distinguish trunk from head. There is no evidence that beetles have a homologue of the segmentation gene fushi tarazu of similar genomic location and function. On the basis of the genetic tractability, convenient genome size and organization of Tribolium, and its relatively long phylogenetic divergence from Drosophila (>300 million years), we have integrated developmental genetic and molecular analyses of the HOM-C. We isolated about 70 mutations in the complex representing at least six complementation groups. The homeotic phenotypes of adults and lethal embryos lead us to believe that these beetle genes are homologous with the Drosophila genes indicated in Fig. 1 (see text). 相似文献
112.
Effects of a change in the level of inbreeding on the genetic load 总被引:10,自引:0,他引:10
"The effects of inbreeding may not be as noticeable in the first generation as the invigoration immediately apparent after crossing". This statement, published in 1919, has received little attention, and has apparently never been tested empirically, although the reduction of the genetic load of populations by inbreeding is well known in theoretical terms. Because inbreeding increases homozygosity, and hence the effectiveness of selection against recessive or partially recessive detrimental alleles, changes in levels of inbreeding can lead to a reduction in the frequencies of such mutant alleles. This results in equilibration at higher population mean fitness and is referred to as 'purging' populations of their genetic load. Severe inbreeding can also reduce genetic load due to overdominant alleles, provided selection coefficients are not symmetrical at all loci, because alleles giving lower fitness will be reduced in frequency at equilibrium. With either fitness model, however, reduction in genetic load takes time, and the initial effect of an increase in inbreeding is reduced fitness due to homozygosity. There are few data relating to the extent to which fitness is reduced during inbreeding in a set of lines and to how long the reduction lasts before increasing again to the initial level, or higher. Inbreeding experiments involving sib mating in mice and Drosophila subobscura, and successive bottlenecks in house flies have yielded some evidence consistent with the purging hypothesis. Here, we report results of an experiment demonstrating a prolonged time-course of recovery of mean fitness under self-fertilization of a naturally outcrossing plant, and also compare our results with expectations derived by computer calculations. Our results show that the genetic load present in an outcrossing population can be explained only with a high mutation rate to partially recessive deleterious alleles, and that inbreeding purges the population of mutant alleles. 相似文献
113.
114.
Summary The in vitro absorption of calcium from the duodenum was significantly less in a group of rats fed on a wheat diet than in a group fed a wheat and Bengal gram (7030) diet. 相似文献
115.
Hypothalamic histamine modulates adaptive behavior of rats at high environmental temperature 总被引:1,自引:0,他引:1
K. Fujimoto T. Sakata K. Ookuma M. Kurokawa A. Yamatodani H. Wada 《Cellular and molecular life sciences : CMLS》1990,46(3):283-285
Summary Histamine content in the rat hypothalamus was lower at 4°C and higher at 31°C compared to that at 21°C. Pretreatment with -fluoromethylhistidine, a suicide inhibitor of histidine decarboxylase, attenuated both the increased level of hypothalamic histamine and rat adaptive behavior at 31°C. Increase of histamine content in the hypothalamus appears to be an important factor contributing to rat adaptive behavior to high environmental temperature. 相似文献
116.
A. Rakovska K. Milenov P. Henklein 《Cellular and molecular life sciences : CMLS》1990,46(10):1037-1041
Summary Suc-Tyr-(SE)-Met-Gly-Trp-Met-Asp--phenethylamide (GE 410) competitively antagonized the contractions of smooth muscle strips from guinea pig ileum (pA2=7.6, n=0.95) induced by cholecystokinin-octapeptide (CCK8). GE 410 inhibited the electrically-induced cholinergically mediated contractile responses and the [3H]ACh release in the ileum, as well as the CCK-stimulated electrical contractile responses and the [3H]ACh release in the cholinergic nerve terminals. The results suggest the existence of CCK-receptors not only in the smooth muscles but also on the neurons. 相似文献
117.
A. Alkofahi J. K. Rupprecht Y. -M. Liu C. -J. Chang D. L. Smith J. L. McLaughlin 《Cellular and molecular life sciences : CMLS》1990,46(5):539-541
Summary Gigantecin (I), a novel tetrahydroxy-di-tetrahydrofuran fatty acid -lactone (acetogenin), was isolated from an ethanolic extract of the stem bark ofGoniothalamus giganteus Hook. f., Thomas (Annonaceae), by means of activity-directed fractionation (brine shrimp lethality test). This new compound is extremely cytotoxic to human tumor cells, inhibits crown gall tumors on potato discs, and is active in an assay designed to detect antimitotic agents (9 ASK). 相似文献
118.
119.
Evidence from carbon isotope measurements for diverse origins of sedimentary hydrocarbons 总被引:12,自引:0,他引:12
The organic matter found in sedimentary rocks must derive from many sources; not only from ancient primary producers but also from consumers and secondary producers. In all of these organisms, isotope effects can affect the abundance and distribution of 13C in metabolites. Here, by using an improved form of a previously described technique in which the effluent of a gas chromatograph is continuously analysed isotopically, we report evidence of the diverse origins of sedimentary organic matter. The record of 13C abundances in sedimentary carbonate and total organic carbon can be interpreted in terms of variations in the global carbon cycle. Our results demonstrate, however, that isotope variations within sedimentary organic mixtures substantially exceed those observed between samples of total organic carbon. Resolution of isotope variations at the molecular level offers a new and convenient means of refining views both of localized palaeoenvironments and of control mechanisms within the global carbon cycle. 相似文献
120.