全文获取类型
收费全文 | 60784篇 |
免费 | 4989篇 |
国内免费 | 28篇 |
专业分类
65801篇 |
出版年
2022年 | 414篇 |
2021年 | 857篇 |
2020年 | 530篇 |
2019年 | 642篇 |
2018年 | 838篇 |
2017年 | 775篇 |
2016年 | 1362篇 |
2015年 | 2406篇 |
2014年 | 2526篇 |
2013年 | 3350篇 |
2012年 | 4253篇 |
2011年 | 4264篇 |
2010年 | 2769篇 |
2009年 | 2466篇 |
2008年 | 3557篇 |
2007年 | 3617篇 |
2006年 | 3417篇 |
2005年 | 3435篇 |
2004年 | 3373篇 |
2003年 | 3157篇 |
2002年 | 3133篇 |
2001年 | 721篇 |
2000年 | 553篇 |
1999年 | 748篇 |
1998年 | 870篇 |
1997年 | 601篇 |
1996年 | 610篇 |
1995年 | 596篇 |
1994年 | 578篇 |
1993年 | 601篇 |
1992年 | 543篇 |
1991年 | 479篇 |
1990年 | 392篇 |
1989年 | 413篇 |
1988年 | 414篇 |
1987年 | 343篇 |
1986年 | 348篇 |
1985年 | 400篇 |
1984年 | 449篇 |
1983年 | 384篇 |
1982年 | 465篇 |
1981年 | 412篇 |
1980年 | 363篇 |
1979年 | 245篇 |
1978年 | 295篇 |
1977年 | 288篇 |
1976年 | 235篇 |
1975年 | 217篇 |
1974年 | 241篇 |
1973年 | 220篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
The phylogeny of the fungus gnat family Mycetophilidae (Diptera) is reconstructed with a focus on the species‐rich and taxonomically difficult subfamilies Gnoristinae and Mycomyinae. The multigene phylogenetic analyses are based on five nuclear (18S, 28S, CAD, MCS, ITS2) and four mitochondrial (12S, 16S, COI, CytB) gene markers. The analyses strongly support the monophyly of Mycetophilidae and the subfamilies Manotinae, Sciophilinae, Leiinae, and Mycomyinae, although Gnoristinae is paraphyletic with respect to Mycetophilinae. All the genera and groups of genera included are supported as monophyletic, except for Acomoptera Vockeroth, Boletina Staeger, Dziedzickia Johannsen, Ectrepesthoneura Enderlein, and Neoempheria Osten Sacken. Ancestral character state reconstructions were applied to two morphological features present in Gnoristinae and Mycomyinae (i.e. presence of setae on wing membrane and wing vein R4) in order to assess their evolution. The wing vein R4 appears as an unstable character, spread throughout different clades. A dated phylogeny of the family Mycetophilidae showed that most of the subfamilies of Mycetophilidae originated and diversified during the Cretaceous. The youngest subfamilies, originated in the Paleogene, appear to be Mycomyinae and Mycetophilinae. 相似文献
2.
This paper is concerned with gene survival in a population which may increase without density dependence according to a generalization of the Moran model for haploid individuals. A selective advantage to one allele and the possibility of differential reproductive rates are allowed. Simple conditions are given for ultimate homozygosity to be certain and for the possibility of ultimate polymorphism. The results complement and extend those of Heyde (1981, 1982). 相似文献
3.
Plasma estradiol (E2), serum LH and FSH, and the gonadotropin response to two consecutive LHRH administrations (10 and 100 micrograms with an interval of 2 h) were determined in 19 patients with anorexia nervosa (AN) at the emaciation phase, before and after estradiol benzoate (E2B) injections (3 micrograms/kg/day for 7 days). The same investigations were repeated after weight restoration in 9 AN patients who remained amenorrheic. Both at the emaciation phase and after weight restoration, E2B enhanced the second LH response to LHRH and decreased serum FSH, suggesting that the functional capacities of the pituitary gonadotrophs are normal in AN. Unlike E2B injections, weight restoration increased all the hormone values, suggesting that the weight restoration effects on the abnormal gonadotropin secretory pattern of AN depend on another mechanism than the E2 lowering. That mechanism is probably a disorder of the hypothalamic LHRH secretion, the consequences of which could be reinforced by the low E2 levels. 相似文献
4.
5.
6.
7.
8.
9.
Peter Buchwald 《Theoretical biology & medical modelling》2009,6(1):5-13
Background
The function and viability of cultured, transplanted, or encapsulated pancreatic islets is often limited by hypoxia because these islets have lost their vasculature during the isolation process and have to rely on gradient-driven passive diffusion, which cannot provide adequate oxygen transport. Pancreatic islets (islets of Langerhans) are particularly susceptible due to their relatively large size, large metabolic demand, and increased sensitivity to hypoxia. Here, finite element method (FEM) based multiphysics models are explored to describe oxygen transport and cell viability in avascular islets both in static and in moving culture media. 相似文献10.