全文获取类型
收费全文 | 12564篇 |
免费 | 1107篇 |
国内免费 | 4篇 |
专业分类
13675篇 |
出版年
2023年 | 44篇 |
2022年 | 98篇 |
2021年 | 209篇 |
2020年 | 100篇 |
2019年 | 160篇 |
2018年 | 178篇 |
2017年 | 164篇 |
2016年 | 343篇 |
2015年 | 540篇 |
2014年 | 692篇 |
2013年 | 786篇 |
2012年 | 1006篇 |
2011年 | 985篇 |
2010年 | 654篇 |
2009年 | 566篇 |
2008年 | 825篇 |
2007年 | 809篇 |
2006年 | 780篇 |
2005年 | 708篇 |
2004年 | 674篇 |
2003年 | 690篇 |
2002年 | 633篇 |
2001年 | 95篇 |
2000年 | 80篇 |
1999年 | 125篇 |
1998年 | 204篇 |
1997年 | 123篇 |
1996年 | 112篇 |
1995年 | 129篇 |
1994年 | 97篇 |
1993年 | 117篇 |
1992年 | 103篇 |
1991年 | 70篇 |
1990年 | 68篇 |
1989年 | 56篇 |
1988年 | 54篇 |
1987年 | 39篇 |
1986年 | 50篇 |
1985年 | 49篇 |
1984年 | 63篇 |
1983年 | 40篇 |
1982年 | 52篇 |
1981年 | 42篇 |
1980年 | 33篇 |
1979年 | 27篇 |
1978年 | 23篇 |
1977年 | 35篇 |
1976年 | 23篇 |
1974年 | 25篇 |
1973年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
The frontal sinuses of 143 Eskimo skulls from two sites in the Hudson Bay region of Canada were examined radiographically. No between-site or sex differences were noted in the size of the sinuses. On average, the sinuses are small and often bilaterally absent. The Canadian samples have smaller sinuses than reported for Alaskan Eskimos or American Indian groups. 相似文献
142.
Structural Analysis of the dur Loci in S. CEREVISIAE: Two Domains of a Single Multifunctional Gene 总被引:13,自引:1,他引:12 下载免费PDF全文
In Saccharomyces cerevisiae, the degradation of urea to carbon dioxide and ammonia is catalyzed by urea carboxylase and allophanate hydrolase. The loci coding for these enzymes (dur1 and dur2) are very tightly linked on the right arm of chromosome II between pet11 and met8. Pleiotropic mutations that fail to complement mutations in either of the dur loci were found to be predominantly located in or near the dur2 locus. We interpret these data as suggesting that the two dur loci might in reality be domains of a single gene that codes for a multifunctional polypeptide. In view of this conclusion, we have renamed the dur loci as the dur1,2 locus. 相似文献
143.
Summary Four cultivars ofTrifolium subterraneum were nodulated by five strains ofRhizobium leguminosarum; all combinations except one gave 100% nodulation. Rates of nodule formation and total nodule numbers were similar to those with an effectiveR. trifolii strain. The nodules were more commonly associated with lateral roots and were ineffective in nitrogen fixation. 相似文献
144.
Christine Dreyer Helen Singer Peter Hausen Ursula Müller Elisabeth Siegel 《Development genes and evolution》1981,190(4):197-207
Summary A library of hybridoma cell lines has been established which produce monoclonal antibodies against antigens from the germinal vesicle ofXenopus laevis oocytes. Many of the antigens are also found in the nuclei ofXenopus embryonic cells in culture. The fate of two of these antigens during embryogenesis was traced by immunofluorescence on embryo and tadpole sections. Early in development these antigens appear to be evenly distributed in the nuclei of all cells. In later stages they gradually disappear from most embryonic structures but are strongly accumulated in the nuclei of some specific cell types and organs. 相似文献
145.
146.
Ute Gröschel-Stewart Susi Ceurremans Irmtraut Lehr Christine Mahlmeister Eltriede Paar 《Histochemistry and cell biology》1977,50(4):271-279
Summary The injection of rabbits with insoluble or soluble G-actin from chicken smooth or striated muscle will produce antibodies that are equally reactive, and species and tissue non-specific in immunoprecipitation, immunofluorescence and actin-activated Mg2+-ATPase** inhibition test. These antibodies have been used for the identification of actin-containing fibrils in a variety of tissues. When G-actins from chicken smooth or striated muscle are immobilized by chemical linkage to Affi-Gel 702 microbeads, their immunogenicity is increased, but the antibodies obtained against them are species-specific and will only react with actin and actin-containing structures from chicken and are therefore limited in use. It is concluded from this work that insoluble G-actin is the preferable immunogen to obtain precipitating antibodies for wide use.Abbreviations ATPase
Adenosinetriphosphatase
- FITC
Fluoresceinisothiocyanate
- SDS
Sodiumdodecylsulfate
This paper is dedicated to Dr. Dorothy M. Needham, University of Cambridge, England, in honour of her eightieth birthday 相似文献
147.
Christine A. Williams 《Phytochemistry》1978,17(4):729-734
In a leaf survey of 61 species of the Bromeliaceae, an unexpectedly wide spectrum of flavonoid constituents was encountered. The family is unique amongst the monocotyledons in the frequency and variety of flavonoids with extra hydroxylation or methoxylation at the 6-position. More common flavonols (in 43% of species) and flavones (in 13%) are distributed throughout the family whereas the rarer flavonoid classes are restricted to one or two of the three subfamilies. Thus 6-hydroxyflavones were found in both the Pitcairnioideae (in 50%) and the Tillandsioideae (in 14%) but patuletin (in 19%), gossypetin (in 1 species) and methylated 6-hydroxymyricetin derivatives (in 24%) were detected only in the Tillandsioideae. A new flavonol, 6,3′,5′-trimethoxy-3,5,7-4′-tetrahydroxyflavone, was identified as the 3-glucoside in Tillandsia usneoides and a new glycoside, patuletin 3-rhamnoside, in Vriesea regina. Myricetin glycosides were found only in the Bromelioideae and their presence here and the concomitant absence of 6-hydroxyflavonoids could indicate the primitive condition of this subfamily. The flavonoid results, in toto, confirm the view based on morphology, that the Bromeliaceae occupies an isolated position in relation to other monocot families. 相似文献
148.
We have studied LTA4 and LTB4 synthesis in a cell-free system from RBL-1 cells. All the enzymes leading to the formation of LTB4 from arachidonic acid are localized in the soluble fraction (100, 000 x g supernatant) of these cells. The formation of LTA4 and LTB4 is complete by 10 min. When we varied the arachidonic acid concentration from 1 to 300 μM, the synthesis of LTB4 leveled off at 30 μM and of LTA4 at 100 μM while 5-HETE had not reached a plateau at 300 μM. This enzyme system has the capacity to generate relatively large amounts of 5-HETE and LTA4 and only a relatively small amount of LTB4. Therefore, the rate limiting step is not the 5-lipoxygenase, the first step in the pathway, but the conversion of LTA4 to LTB4. This is in contrast to cyclooxygenase pathway where the first step is rate limiting. A second addition of arachidonic acid at submaximal concentration for LTA4 synthesis did not produce any additional LTA4 or LTB4. Further study of this phenomenon showed that the 5-lipoxygenase and LTA-synthase were inactivated with time by preincubation with arachidonic acid and that peroxy fatty acids seem to be the inactivating species. 相似文献
149.
Ralph L. Holloway Marie Christine De La Costelareymondie 《American journal of physical anthropology》1982,58(1):101-110
Observations on petalial asymmetry for 190 hominoid endocasts are reported, and their statistical differences assessed. While all taxa of hominoids show asymmetries to various degrees, the patterns or combinations of petalial asymmetries are very different, with fossil hominids and modern Homo sapiens showing an identical pattern of left-occipital, right-frontal petalias, which contrasts with those found normally in pongids. Of the pongids, Gorilla shows the greater degree of asymmetry in left-occipital petalias. Only modern Homo and hominids (Australopithecus, Homo erectus, Neandertals) show a distinct left-occipital, right-frontal petalial pattern. Analysis by x2 statistics shows the differences to be highly significant. Due to small sample size and incompleteness of endocasts, small-brained hominids, i.e., Australopithecus, are problematical. To the degree that gross petalial patterns are correlated with cognitive task specialization, we speculate that human cognitive patterns evolved early in hominid evolution and were related to selection pressures operating on both symbolic and spatiovisual integration, and that these faculties are corroborated in the archaeological record. 相似文献
150.
Pierre Abad Chantal Quiles Sophie Tares Christine Piotte Philippe Castagnose-Sereno Monique Abadon Antoine Dalmasso 《Journal of molecular evolution》1991,33(3):251-258
Summary To have a better understanding of the evolutionary history of mobile elements within the nematodes, we examined the distribution and the conservation of homologues to transposable elements fromCaenorhabditis elegans (Tc1, Tc2, Tc3, Tc4, Tc5, and FB1) in 19 nematode species belonging to the class Secernentea. Our results show that Tc1 elements display a distribution restricted to the family Rhabditidae with poor conservation. The Tc2 and FB1 homologous elements have the same patchy distribution within the Rhabditidae. They were only found inCaenorhabditis and inTeratorhabditis. The Tc3 element is widely distributed among nematode species. Tc3 homologous elements are present in the majority of the Rhabditidae but also in two genera within the family Panagrolaimidae, and inBursaphelenchus, which belongs to the order Aphelenchida. Tc4 and Tc5 homologues show the most limited distribution of all tested elements, being strictly limited toC. elegans. These data indicate that in some cases, the distribution of transposable elements in the nematode cannot be explained by strict vertical transmission. The distribution of Tc3, Tc4, and Tc5 suggests that horizontal transmission may have occurred between reproductively isolated species during their evolutionary history. 相似文献