全文获取类型
收费全文 | 14756篇 |
免费 | 1172篇 |
国内免费 | 2篇 |
专业分类
15930篇 |
出版年
2023年 | 71篇 |
2022年 | 161篇 |
2021年 | 286篇 |
2020年 | 170篇 |
2019年 | 204篇 |
2018年 | 274篇 |
2017年 | 271篇 |
2016年 | 437篇 |
2015年 | 705篇 |
2014年 | 804篇 |
2013年 | 979篇 |
2012年 | 1328篇 |
2011年 | 1224篇 |
2010年 | 741篇 |
2009年 | 673篇 |
2008年 | 957篇 |
2007年 | 993篇 |
2006年 | 827篇 |
2005年 | 811篇 |
2004年 | 735篇 |
2003年 | 702篇 |
2002年 | 685篇 |
2001年 | 163篇 |
2000年 | 140篇 |
1999年 | 146篇 |
1998年 | 172篇 |
1997年 | 104篇 |
1996年 | 101篇 |
1995年 | 82篇 |
1994年 | 85篇 |
1993年 | 97篇 |
1992年 | 71篇 |
1991年 | 62篇 |
1990年 | 66篇 |
1989年 | 46篇 |
1988年 | 49篇 |
1987年 | 41篇 |
1986年 | 37篇 |
1985年 | 35篇 |
1984年 | 36篇 |
1983年 | 34篇 |
1982年 | 22篇 |
1981年 | 24篇 |
1980年 | 25篇 |
1979年 | 22篇 |
1978年 | 25篇 |
1975年 | 19篇 |
1974年 | 23篇 |
1972年 | 20篇 |
1971年 | 23篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
L. Karlsson 《Journal of fish biology》1983,23(5):511-524
A light and electron microscopic study of the gills of the zebrafish, Brachydanio rerio , were made to serve as a morphological basis for future investigations. It was found that for fixation of B. rerio gills, a mixture of 1·5% gluturaldehyde and 1·5% paraformaldehyde gives a mucus-free surface. Morphometric measurements of structural components of the gill secondary lamellae were made. Observations at SEM were correlated with those made at TEM. The different cell types in the branchial epithelium were characterized. Chloride cells were mainly located in the interlamellar regions and on the afferent side of the primary lamellae. Two morphologically different chloride cells were seen. The first type communicates with the external environment through a reservoir-like lumen, which is normally absent in freshwater fishes. The second type of chloride cell has more direct contact with the ambient water, resembling chloride cells from other freshwater fishes. Another cell type with features similar to those of the rodlet cell was frequently observed. This cell is interposed between other types of cells in the epithelium, and sometimes junctional complexes were present between the rodlet cell and surrounding cells. 相似文献
22.
23.
Effects of Population Size and Selection Intensity on Responses to Disruptive Selection in DROSOPHILA MELANOGASTER 下载免费PDF全文
Disruptive selection for sternopleural bristle number with opportunity for random mating was done in the four treatment combinations of two population sizes (40 pairs and 8 pairs of selected parents) and two selection intensities (1 in 40 and 1 in 2). In each generation, matings among selected parents were observed in a mating chamber, and progeny collected separately from each female parent. In the high number, high selection intensity treatment, divergence between the high and low parts ceased about generation 11. The isolation index increased rapidly to generation 3, but then fluctuated to termination of the population at generation 17. The overall isolation index was significant, indicating a real tendency to assortative mating. The failure of the isolation index to increase after generation 3 was attributed to lower average mating fitness of high males (due to inbreeding) and reduced receptivity of low females (due to a homozygous lethal gene with a large effect on sternopleural bristle number in heterozygotes). In the two low number treatments, isolation indices fluctuated from generation to generation with no obvious trends, and none of the overall isolation indices were significantly different from zero. The high number, low selection intensity treatment showed very little divergence, and one of the replicates showed, in contrast with expectation and the high number, high selection intensity treatment, a significant tendency to disassortative mating. Intense disruptive selection may lead to assortative mating. 相似文献
24.
R Jonsson A L Karlsson R Holmdahl 《The journal of histochemistry and cytochemistry》1989,37(2):265-268
Administration of biotinylated monoclonal antibodies provides the basis of a simple technique for identifying immunoreactive sites in vivo. Biotinylated anti-type II collagen antibodies were injected intraperitoneally into normal DBA/1 mice. The mice were sacrificed after 96 hr and the front paws removed and decalcified to allow tissue sectioning before snap-freezing. Binding of antibodies in vivo was visualized with affinity cytochemical staining using avidin-biotin-peroxidase complexes. Specific binding of antibodies to cartilaginous structures was seen after injection of 20-500 micrograms biotinylated monoclonal or polyclonal anti-type II collagen antibodies, but not after injection of a biotinylated control antibody. This technique should further the detection and localization studies of tissue components involved in the dynamics of physiological and pathological processes. 相似文献
25.
Creatine Transport in Cultured Cells of Rat and Mouse Brain 总被引:7,自引:3,他引:4
Astroglia-rich cultures derived from brains of newborn rats or mice use a transport system for the uptake of creatine. The uptake system is saturable, Na+-dependent, and highly specific for creatine and Na+. Kinetic studies on rat cells revealed a Km value for creatine of 45 microM, a Vmax of 17 nmol x h-1 x (mg of protein)-1, and a Km value of 55 mM for Na+. The carrier is competitively inhibited by guanidinopropionate (Ki = 15 microM). No such transport system was found in neuron-rich primary cultures from embryonic rat brain. It is hypothesized that creatine transport is an astroglial rather than a neuronal function. 相似文献
26.
27.
David K. Chapman ers Johnsson Christina Karlsson Allan Brown David Heathcote 《Physiologia plantarum》1994,90(1):157-162
In a spaceflight experiment, autotropism by oat ( Avena sativa L.) coleoptiles following gravitropic responses was prominent in weightlessness: counter-reactions led to the straightening of the curved coleoptiles. This was not the case during clinorotation on earth. The autotropic reactions appeared to be related to the stimulus received during the stimulus period, i.e. the greater the response the greater the autotropic counter-reaction. Previous models of the gravitropic system which predicted that coleoptiles would not straighten in weightlessness are disproved. A modification to one of the models is proposed which includes the autotropic response observed in spaceflight. The nature of the counter-reactions in the absence of gravitropic stimulation is discussed. 相似文献
28.
29.
High-temperature gas chromatography and gas chromatography-inass spectrometry for the analyses of oligosaccharides derived
from glycoproteins or glycosphingolipids has been developed. Pcrmethylatcd oligosaccharides with up to about 12 sugar residues
and masses up to 2500 Daltons can be analyzed. This approach is discussed and exemplified. 相似文献
30.
Molecular evaluation of an Alu repeat including a polymorphic variable poly(dA) (AluVpA) in the vitamin D binding protein (DBP) gene 总被引:1,自引:0,他引:1
We investigated an Alu element at the end of intron 8 of the human vitamin D-binding protein (hDBP, group-specific component, GC) gene that shows a polymorphic poly(A) tail due to a variable number of tandem repeats (AluVpA) forming the 3 end of this member of the most abundant class of short interspersed repeated DNA element (SINES). The Alu element sequence in intron 8 of the GC gene was identical in all three common GC alleles (GC*1F, GC*1S, and GC*2) and could be classified as an Alu-Sa or Alu class-II sequence. The polymerase chain reaction was used to amplify selectively a fragment of about 200 bp containing the identified (TAAA)n repeat from genomic DNA of 188 unrelated human subjects. The size of the amplified products was determined by polyacrylamide gel electrophoresis. Four alleles (named GC-18*6, GC-I8*8, GC-I8*10, and GC-18*11) were found that differed in size by multiples of four nucleotides. The allele frequencies ranged from 0.0053 to 0.8511 and the observed heterozygosity was 26%. The stable inheritance of this polymorphic patterned poly(A) sequence was confirmed by a segregation study of a highly informative family with 19 members. Statistically significant linkage disequilibrium between the AluVpA and the GC isoelectric focusing (IEF) phenotypes was found in a sample of 188 unrelated individuals and delta values were calculated from the observed haplotype distribution. 相似文献