全文获取类型
收费全文 | 2298篇 |
免费 | 97篇 |
国内免费 | 233篇 |
出版年
2024年 | 1篇 |
2023年 | 6篇 |
2022年 | 17篇 |
2021年 | 19篇 |
2020年 | 6篇 |
2019年 | 2篇 |
2018年 | 11篇 |
2017年 | 11篇 |
2016年 | 32篇 |
2015年 | 94篇 |
2014年 | 74篇 |
2013年 | 67篇 |
2012年 | 115篇 |
2011年 | 432篇 |
2010年 | 297篇 |
2009年 | 85篇 |
2008年 | 56篇 |
2007年 | 49篇 |
2006年 | 34篇 |
2005年 | 171篇 |
2004年 | 406篇 |
2003年 | 119篇 |
2002年 | 93篇 |
2001年 | 110篇 |
2000年 | 44篇 |
1999年 | 9篇 |
1998年 | 22篇 |
1997年 | 21篇 |
1996年 | 31篇 |
1995年 | 25篇 |
1994年 | 22篇 |
1993年 | 42篇 |
1992年 | 20篇 |
1991年 | 15篇 |
1990年 | 22篇 |
1989年 | 7篇 |
1988年 | 5篇 |
1987年 | 10篇 |
1986年 | 3篇 |
1985年 | 18篇 |
1974年 | 1篇 |
1963年 | 1篇 |
1950年 | 1篇 |
1943年 | 2篇 |
排序方式: 共有2628条查询结果,搜索用时 31 毫秒
71.
本研究以兔为实验材料,对细胞核移植过程中显微操作、电融合、电活化以及移核胚的培养等基本问题进行了研究。对兔进行PMSGhCG超数排卵,收集成熟卵母细胞和16细胞胚;后者经胰蛋白酶消化,去除胶膜和透明带,在不含Ca2+、Mg2+的分离液中分成单个卵裂球;然后,分别对两者做CB预处理;首次尝试采用Wiladsen法,去除卵母细胞核、并将单个卵裂球注入透明带,同时、与McGrathSolter法进行比较;通过电融合使供体核进入去核的卵母细胞内;将所得移核胚在体外或在中间受体内培养并观察。结果表明:一、Wiladsen法与McGrathSolter法比较,核移植操作的成功率及以后的电融合率均无明显差异(Tab.1)。相对于后者,Wiladsen法更简便、易于掌握并提高去核率。二、hCG超排注射后13~15h,观察卵母细胞发现:其中,678%保留有第一极体。此时的卵子若去除1/3胞质量,去核率可以达到583%。若推迟去核时间,笫一极体退化,失去去核标志。三、比较不同电脉冲条件,发现强度为063kv/cm,持续160μs的一次电脉冲可获较高移核胚的融合率(70.8%)(Tab.2);并可使611%的成熟 相似文献
72.
Patrik Mráz Myriam Gaudeul Delphine Rioux Ludovic Gielly Philippe Choler Pierre Taberlet the IntraBioDiv Consortium 《Journal of Biogeography》2007,34(12):2100-2114
Aim The range of the subalpine species Hypochaeris uniflora covers the Alps, Carpathians and Sudetes Mountains. Whilst the genetic structure and post‐glacial history of many high‐mountain plant taxa of the Alps is relatively well documented, the Carpathian populations have often been neglected in phylogeographical studies. The aim of the present study is to compare the genetic variation of the species in two major European mountain systems – the Alps and the Carpathians. Location Alps and Carpathians. Methods The genetic variation of 77 populations, each consisting of three plants, was studied using amplified fragment length polymorphism (AFLP). Results Neighbour joining and principal coordinate analyses revealed three well‐supported phylogeographical groups of populations corresponding to three disjunct geographical regions – the Alps and the western and south‐eastern Carpathians. Moreover, two further clusters could be distinguished within the latter mountain range, one consisting of populations from the eastern Carpathians and the second consisting of populations from the southern Carpathians. Populations from the Apuseni Mountains had an intermediate position between the eastern and southern Carpathians. The genetic clustering of populations into four groups was also supported by an analysis of molecular variance, which showed that most genetic variation (almost 46%) was found among these four groups. By far the highest within‐population variation was found in the eastern Carpathians, followed by populations from the southern and western Carpathians. Generally, the populations from the Alps were considerably less variable and displayed substantially fewer region‐diagnostic markers than those from the south‐eastern Carpathians. Although no clear geographical structure was found within the Alps, based on neighbour joining or principal coordinate analyses, some trends were obvious: populations from the easternmost part were genetically more variable and, together with those from the south‐western part, exhibited a higher proportion of rare AFLP fragments than populations in other areas. Moreover, the total number of AFLP fragments per population, the percentage of polymorphic loci and the proportion of rare AFLP fragments significantly decreased from east to west. Main conclusions Deep infraspecific phylogeographical gaps between the populations from the Alps and the western and south‐eastern Carpathians suggest the survival of H. uniflora in three separate refugia during the last glaciation. Our AFLP data provide molecular evidence for a long‐term geographical disjunction between the eastern and western Carpathians, previously suggested from the floristic composition at the end of 19th century. It is likely that Alpine populations survived the Last Glacial in the eastern part of the Alps, from where they rapidly colonized the rest of the Alps after the ice sheet retreated. Multiple founder effects may explain a gradual loss of genetic variation during westward colonization of the Alps. 相似文献
73.
Jun Lv Wei Chen Dianjianyi Sun Shengxu Li Iona Y. Millwood Margaret Smith Yu Guo Zheng Bian Canqing Yu Huiyan Zhou Yunlong Tan Junshi Chen Zhengming Chen Liming Li China Kadoorie Biobank collaborative group 《PloS one》2015,10(4)
Objectives
Lifestyle factors are well-known important modifiable risk factors for obesity; the association between tobacco smoking and central obesity, however, is largely unknown in the Chinese population. This study examined the relationship between smoking and central obesity in 0.5 million Chinese adults, a population with a low prevalence of general obesity, but a high prevalence of central obesity.Subjects
A total of 487,527 adults (200,564 males and 286,963 females), aged 30-79 years, were enrolled in the baseline survey of the China Kadoorie Biobank (CKB) Study conducted during 2004-2008. Waist circumference (WC) and WC/height ratio (WHtR) were used as measures of central obesity.Results
The prevalence of regular smokers was significantly higher among males (60.6%) than among females (2.2%). The prevalence of central obesity increased with age and BMI levels, with a significant gender difference (females>males). Of note, almost all obese adults (99.4%) were centrally obese regardless of gender. In multivariable regression analyses, adjusting for age, education, physical activity, alcohol use and survey site, regular smoking was inversely associated with BMI in males (standardized regression coefficients, β= -0.093, p<0.001) and females (β= -0.025, p<0.001). Of interest, in the BMI stratification analyses in 18 groups, all βs of regular smoking for WHtR were positive in both genders; the βs showed a significantly greater increasing trend with increasing BMI in males than in females. In the analyses with model adjustment for BMI, the positive associations between regular smoking and WHtR were stronger in males (β= 0.021, p<0.001) than in females (β= 0.008, p<0.001) (p<0.001 for gender difference). WC showed considerably consistent results.Conclusions
The data indicate that tobacco smoking is an important risk factor for central obesity, but the association is gender-specific and depends on the adjustment for general obesity. 相似文献74.
Gwinyai Masukume Ali S. Khashan Louise C. Kenny Philip N. Baker Gill Nelson SCOPE Consortium 《PloS one》2015,10(4)
Background
Anaemia in pregnancy is a major public health and economic problem worldwide, that contributes to both maternal and fetal morbidity and mortality.Objective
The aim of the study was to calculate the prevalence of anaemia in early pregnancy in a cohort of ‘low risk’ women participating in a large international multicentre prospective study (n = 5 609), to identify the modifiable risk factors for anaemia in pregnancy in this cohort, and to compare the birth outcomes between pregnancies with and without anaemia in early gestation.Methods
The study is an analysis of data that were collected prospectively during the Screening for Pregnancy Endpoints study. Anaemia was defined according to the World Health Organization’s definition of anaemia in pregnancy (haemoglobin < 11g/dL). Binary logistic regression with adjustment for potential confounders (country, maternal age, having a marital partner, ethnic origin, years of schooling, and having paid work) was the main method of analysis.Results
The hallmark findings were the low prevalence of anaemia (2.2%), that having no marital partner was an independent risk factor for having anaemia (OR 1.34, 95% CI 1.01-1.78), and that there was no statistically significant effect of anaemia on adverse pregnancy outcomes (small for gestational age, pre-tem birth, mode of delivery, low birth weight, APGAR score < 7 at one and five minutes). Adverse pregnancy outcomes were however more common in those with anaemia than in those without.Conclusion
In this low risk healthy pregnant population we found a low anaemia rate. The absence of a marital partner was a non-modifiable factor, albeit one which may reflect a variety of confounding factors, that should be considered for addition to anaemia’s conceptual framework of determinants. Although not statistically significant, clinically, a trend towards a higher risk of adverse pregnancy outcomes was observed in women that were anaemic in early pregnancy. 相似文献75.
76.
Genomic Resources Development Consortium Mariella Baratti Federica Cattonaro Tiziana Di Lorenzo Diana Maria Paola Galassi Valentina Iannilli Alessio Iannucci Just Jensen Peter Foged Larsen Rasmus O. Nielsen Cino Pertoldi Dragos Postolache Jose Martin Pujolar Ettore Randi Aritz Ruiz‐Gonzalez Janne Pia Thirstrup Giovanni Giuseppe Vendramin Andrzej Zalewski 《Molecular ecology resources》2015,15(2):458-459
77.
Genomic Resources Development Consortium Wolfgang Arthofer Laura Bertini Carla Caruso Francesco Cicconardi Lynda F. Delph Peter D. Fields Minoru Ikeda Yuki Minegishi Silvia Proietti Heike Ritthammer Birgit C. Schlick‐Steiner Florian M. Steiner Gregor A. Wachter Herbert C. Wagner Laura A. Weingartner 《Molecular ecology resources》2015,15(4):1014-1015
78.
Genomic Resources Development Consortium P. Álvarez Wolfgang Arthofer Maria M. Coelho D. Conklin A. Estonba Ana R. Grosso S. J. Helyar J. Langa Miguel P. Machado I. Montes Joana Pinho Alexander Rief Manfred Schartl Birgit C. Schlick‐Steiner Julia Seeber Florian M. Steiner C. Vilas 《Molecular ecology resources》2015,15(6):1510-1512
79.
Francesco Infarinato Anisur Rahman Claudio Del Percio Yves Lamberty Regis Bordet Jill C. Richardson Gianluigi Forloni Wilhelmus Drinkenburg Susanna Lopez Fabienne Aujard Claudio Babiloni Fabien Pifferi IMI project "PharmaCog" Consortium 《PloS one》2015,10(11)
The gray mouse lemur (Microcebus murinus) is considered a useful primate model for translational research. In the framework of IMI PharmaCog project (Grant Agreement n°115009, www.pharmacog.org), we tested the hypothesis that spectral electroencephalographic (EEG) markers of motor and locomotor activity in gray mouse lemurs reflect typical movement-related desynchronization of alpha rhythms (about 8–12 Hz) in humans. To this aim, EEG (bipolar electrodes in frontal cortex) and electromyographic (EMG; bipolar electrodes sutured in neck muscles) data were recorded in 13 male adult (about 3 years) lemurs. Artifact-free EEG segments during active state (gross movements, exploratory movements or locomotor activity) and awake passive state (no sleep) were selected on the basis of instrumental measures of animal behavior, and were used as an input for EEG power density analysis. Results showed a clear peak of EEG power density at alpha range (7–9 Hz) during passive state. During active state, there was a reduction in alpha power density (8–12 Hz) and an increase of power density at slow frequencies (1–4 Hz). Relative EMG activity was related to EEG power density at 2–4 Hz (positive correlation) and at 8–12 Hz (negative correlation). These results suggest for the first time that the primate gray mouse lemurs and humans may share basic neurophysiologic mechanisms of synchronization of frontal alpha rhythms in awake passive state and their desynchronization during motor and locomotor activity. These EEG markers may be an ideal experimental model for translational basic (motor science) and applied (pharmacological and non-pharmacological interventions) research in Neurophysiology. 相似文献
80.
Claudia Geue Olivia Wu Yiqiao Xin Robert Heggie Sharon Hutchinson Natasha K. Martin Elisabeth Fenwick David Goldberg Consortium ECDC 《PloS one》2015,10(12)