全文获取类型
收费全文 | 2104篇 |
免费 | 252篇 |
专业分类
2356篇 |
出版年
2022年 | 20篇 |
2021年 | 20篇 |
2019年 | 20篇 |
2018年 | 33篇 |
2017年 | 19篇 |
2016年 | 32篇 |
2015年 | 71篇 |
2014年 | 72篇 |
2013年 | 93篇 |
2012年 | 97篇 |
2011年 | 126篇 |
2010年 | 66篇 |
2009年 | 50篇 |
2008年 | 87篇 |
2007年 | 77篇 |
2006年 | 83篇 |
2005年 | 61篇 |
2004年 | 73篇 |
2003年 | 72篇 |
2002年 | 64篇 |
2001年 | 54篇 |
2000年 | 62篇 |
1999年 | 53篇 |
1998年 | 26篇 |
1997年 | 22篇 |
1996年 | 36篇 |
1995年 | 20篇 |
1994年 | 23篇 |
1993年 | 20篇 |
1992年 | 46篇 |
1991年 | 44篇 |
1990年 | 44篇 |
1989年 | 34篇 |
1988年 | 46篇 |
1987年 | 49篇 |
1986年 | 28篇 |
1985年 | 35篇 |
1984年 | 25篇 |
1983年 | 29篇 |
1982年 | 21篇 |
1981年 | 17篇 |
1980年 | 19篇 |
1979年 | 31篇 |
1978年 | 25篇 |
1977年 | 21篇 |
1975年 | 17篇 |
1974年 | 29篇 |
1972年 | 19篇 |
1968年 | 16篇 |
1967年 | 17篇 |
排序方式: 共有2356条查询结果,搜索用时 15 毫秒
61.
62.
63.
64.
Knowledge of the reproductive life history of giraffe in the wild is sparse. Giraffe have two fairly unusual reproductive patterns among large mammals: they can become pregnant while lactating, and calf mortality is extremely high. Longitudinal records are largely absent, so tracking reproductive parameters tends to combine information from captive and field studies. In this study, we examine longitudinal data obtained over a 33‐year period in one population of Thornicroft’s giraffe in order to chart their reproductive careers. We found that age at first parturition was 6.4 years, or slightly later than in captivity. Giraffe bred throughout the year, with cows producing offspring on average every 677.7 days. About half of the calves died before one year of age, but death of a calf did not reduce interbirth interval. We conclude that the lifetime reproductive success of giraffe is more dependent on longevity and calf survivorship than on reproductive rate. 相似文献
65.
For studies on cathodoluminescence, we equipped a scanning electron microscope with a prism spectrometer and sensitive photomultiplier. The apparatus is described and our initial results are presented on the analyse of cathodoluminescence. The material observed promarily involved studies of immunofluorescent specimens. Humal lymphocytes were labelled with a fluorescent antibody and cryosections of rat kidney with Masugi nephritis were labelled with a fluorescent specific antibody. Our apparatus permitted monochromatic imaging of cathodoluminescence emissions and resulted in much improved micrographs. Some possible improvements of the technique are discussed. 相似文献
66.
M Berry R Flinn 《Proceedings of the Royal Society of London. Series B, Containing papers of a Biological character. Royal Society (Great Britain)》1984,221(1224):321-348
All networks are made up of vertices (points interconnected by segments), which include terminals interconnected by terminal segments, nodes interconnected by link segments and the root point connected to the tree by the root segment. All nodes may be classified into unique types according to the number of terminal and link segments they drain. For example, there are three distinct dichotomous nodes, a 'primary' node draining two terminal segments, a 'secondary' node draining one terminal segment and a link segment, and a 'tertiary' node draining two link segments. The numbers of primary and tertiary nodes approximate to equality in large networks and thus the ratio of primary to secondary nodes defines topology. All higher order nodes ( trichotomous and beyond) may be resolved into dichotomous forms and incorporated into the analysis. Different forms of growth may thus be analysed by comparing the frequency distributions of nodes with those generated by computer simulated growth models. Moreover, all vertices can be ordered so that metrical parameters are easily incorporated and the hierarchical arrangements of vertices of different order discerned. The dendritic trees of 48 Purkinje cells, taken from folia along the primary fissure, were analysed using vertex analysis. The mean number of segments in Purkinje cell trees was 881 +/- 23 (s.e.) and mean total dendritic length 7959 +/- 233 (s.e.) micrometers. Segment lengths were longest over proximal segments but over most of the tree segment lengths were constant at 10 +/- 0.2 (s.e.) micrometers. Vertex, segment and terminal frequency distributions of equivalent orders were all normal with a slight positive skew. Peak frequencies were recorded at the 12th equivalent order. The mean primary/secondary nodal vertex ratio was 0.93 and the proportion of trichotomous branch points in the tree was 5%. Comparison of the frequency distribution of all vertices with computer generated models showed that growth of the Purkinje cell was most closely simulated by a random terminal growth model, incorporating 5% trichotomy , in which the branching of high order terminals was more likely than low order terminals. It was concluded that growth of the Purkinje cell tree could proceed by random terminal branching with growth occurring preferentially over a front composed of terminals that are ascending through a corridor in the molecular layer whose margins are defined by neighbouring trees. 相似文献
67.
68.
Chloe I. Bloom Christine M. Graham Matthew P. R. Berry Fotini Rozakeas Paul S. Redford Yuanyuan Wang Zhaohui Xu Katalin A. Wilkinson Robert J. Wilkinson Yvonne Kendrick Gilles Devouassoux Tristan Ferry Makoto Miyara Diane Bouvry Valeyre Dominique Guy Gorochov Derek Blankenship Mitra Saadatian Phillip Vanhems Huw Beynon Rama Vancheeswaran Melissa Wickremasinghe Damien Chaussabel Jacques Banchereau Virginia Pascual Ling-pei Ho Marc Lipman Anne O’Garra 《PloS one》2013,8(8)
69.
Fouad El Baidouri Laure Diancourt Vincent Berry Fran?ois Chevenet Francine Pratlong Pierre Marty Christophe Ravel 《PLoS neglected tropical diseases》2013,7(6)
Leishmaniasis is a complex parasitic disease from a taxonomic, clinical and epidemiological point of view. The role of genetic exchanges has been questioned for over twenty years and their recent experimental demonstration along with the identification of interspecific hybrids in natura has revived this debate. After arguing that genetic exchanges were exceptional and did not contribute to Leishmania evolution, it is currently proposed that interspecific exchanges could be a major driving force for rapid adaptation to new reservoirs and vectors, expansion into new parasitic cycles and adaptation to new life conditions.To assess the existence of gene flows between species during evolution we used MLSA-based (MultiLocus Sequence Analysis) approach to analyze 222 Leishmania strains from Africa and Eurasia to accurately represent the genetic diversity of this genus. We observed a remarkable congruence of the phylogenetic signal and identified seven genetic clusters that include mainly independent lineages which are accumulating divergences without any sign of recent interspecific recombination. From a taxonomic point of view, the strong genetic structuration of the different species does not question the current classification, except for species that cause visceral forms of leishmaniasis (L. donovani, L. infantum and L. archibaldi). Although these taxa cause specific clinical forms of the disease and are maintained through different parasitic cycles, they are not clearly distinct and form a continuum, in line with the concept of species complex already suggested for this group thirty years ago. These results should have practical consequences concerning the molecular identification of parasites and the subsequent therapeutic management of the disease. 相似文献
70.
Anvesha Srivastava Helle Goldberger Alexander Dimtchev Malathi Ramalinga Juliet Chijioke Catalin Marian Eric K. Oermann Sunghae Uhm Joy S. Kim Leonard N. Chen Xin Li Deborah L. Berry Bhaskar V. S. Kallakury Subhash C. Chauhan Sean P. Collins Simeng Suy Deepak Kumar 《PloS one》2013,8(10)
Prostate cancer (PCa) is the most common type of cancer in men in the United States, which disproportionately affects African American descents. While metastasis is the most common cause of death among PCa patients, no specific markers have been assigned to severity and ethnic biasness of the disease. MicroRNAs represent a promising new class of biomarkers owing to their inherent stability and resilience. In the present study, we investigated potential miRNAs that can be used as biomarkers and/or therapeutic targets and can provide insight into the severity and ethnic biasness of PCa. PCR array was performed in FFPE PCa tissues (5 Caucasian American and 5 African American) and selected differentially expressed miRNAs were validated by qRT-PCR, in 40 (15 CA and 25 AA) paired PCa and adjacent normal tissues. Significantly deregulated miRNAs were also analyzed in urine samples to explore their potential as non-invasive biomarker for PCa. Out of 8 miRNAs selected for validation from PCR array data, miR-205 (p<0.0001), mir-214 (p<0.0001), miR-221(p<0.001) and miR-99b (p<0.0001) were significantly downregulated in PCa tissues. ROC curve shows that all four miRNAs successfully discriminated between PCa and adjacent normal tissues. MiR-99b showed significant down regulation (p<0.01) in AA PCa tissues as compared to CA PCa tissues and might be related to the aggressiveness associated with AA population. In urine, miR-205 (p<0.05) and miR-214 (p<0.05) were significantly downregulated in PCa patients and can discriminate PCa patients from healthy individuals with 89% sensitivity and 80% specificity. In conclusion, present study showed that miR-205 and miR-214 are downregulated in PCa and may serve as potential non-invasive molecular biomarker for PCa. 相似文献