首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1752篇
  免费   154篇
  2021年   28篇
  2020年   9篇
  2019年   11篇
  2018年   12篇
  2017年   24篇
  2016年   23篇
  2015年   57篇
  2014年   90篇
  2013年   90篇
  2012年   118篇
  2011年   101篇
  2010年   67篇
  2009年   54篇
  2008年   82篇
  2007年   84篇
  2006年   82篇
  2005年   54篇
  2004年   73篇
  2003年   61篇
  2002年   62篇
  2001年   37篇
  2000年   41篇
  1999年   38篇
  1998年   38篇
  1997年   37篇
  1996年   26篇
  1995年   24篇
  1994年   20篇
  1993年   25篇
  1992年   37篇
  1991年   50篇
  1990年   24篇
  1989年   26篇
  1988年   25篇
  1987年   28篇
  1986年   29篇
  1985年   19篇
  1984年   14篇
  1983年   13篇
  1982年   10篇
  1981年   10篇
  1980年   16篇
  1979年   18篇
  1978年   11篇
  1977年   13篇
  1976年   15篇
  1975年   7篇
  1973年   9篇
  1972年   7篇
  1970年   7篇
排序方式: 共有1906条查询结果,搜索用时 15 毫秒
21.
A nasal, so called ethmoidal, tumor from a fallow deer is described. It appears to be the first reported case of that species. The etiology is discussed.  相似文献   
22.
Rat immunoglobulin E heavy chain locus   总被引:5,自引:0,他引:5  
A 2100 base-pair long sequence has been established which covers all four constant domains of the rat epsilon-chain. An analysis of messenger RNA from an immunoglobulin E producing rat immunocytoma revealed two separate epsilon-chain mRNA species, 2.3 X 10(3) and 2.8 X 10(3) base-pairs long. The latter mRNA encodes the membrane binding form of the epsilon-chain. The membrane exons which are located approximately 2 X 10(3) base-pairs away from the 3'-side of the CH4 exon were also sequenced. A comparison between the rat and mouse epsilon-chains at the protein sequence level revealed an overall homology of 80% which, as expected, is considerably higher than the homology found between rat and human epsilon-chains. The fourth constant domain together with the two membrane exons exhibited the highest degree of homology, 81 to 89%. Only two differences were found when the epsilon-chains from LOU and Sprague Dawley rats were compared. The most striking difference at the nucleotide sequence level between the rat, mouse, and human epsilon genes was found within the first intron. The mouse genome contains a unique 366 base-pair long sequence in this region. The inserted sequence is repetitive and present in approximately 100 copies in the mouse genome. It is flanked by 22 base-pair long direct repeats and contains also 14 base-pair long inverted repeats, thus having properties in common with transposable elements.  相似文献   
23.
Summary Nest humidity ( ) under an incubating bantam hen was measured at ambient ranging from 1.3 to 25.0 mmHg. Weight loss of eggs was recorded in clutches of varying size. Nest and ambient were also measured in wild incubating willow ptarmigan nests in dry and wet habitats.Nest increased linearly with ambient in a way predictable on the assumption that the water vapour conductance ( ) of brood patch skin, plumage and eggs were constant and independent of ambient . Nest was also dependent of clutch size. Egg dehydration was quantitatively predicted from measured values and the laws of diffusion.Our findings confirm earlier conclusions that the adult bird does not actively regulate nest at varying ambient . Birds can presumably achieve appropriate egg dehydration by a strategy combining nest site, nest construction, egg shell conductance and incubation behaviour which meets the requirements of their breeding climate.Abbreviations water vapur pressure - water vapur conductance - water flux  相似文献   
24.
K Skarstad  E Boye    H B Steen 《The EMBO journal》1986,5(7):1711-1717
The synchrony of initiation of chromosome replication at multiple origins within individual Escherichia coli cells was studied by a novel method. Initiation of replication was inhibited with rifampicin or chloramphenicol and after completion of ongoing rounds of replication the numbers of fully replicated chromosomes in individual cells were measured by flow cytometry. In rapidly growing cultures, with parallel replication of several chromosomes, cells will end up with 2n (n = 1, 2, 3) chromosomes if initiation occurs simultaneously at all origins. A culture with asynchronous initiation may in addition contain cells with irregular numbers (not equal to 2n) of chromosomes. The frequency of cells with irregular numbers of chromosomes is a measure of the degree of asynchrony of initiation. After inhibition of initiation and run-out of replication in rapidly growing B/r A and K-12 cultures, a small fraction of the cells (2-7%) contained 3, 5, 6 or 7 chromosomes. From these measurements it was calculated that initiation at four origins in a single cell occurred within a small fraction, 0.1, of the doubling time (tau). A dnaA(Ts) mutant strain grown at permissive temperature exhibited a very large fraction of cells with irregular numbers of chromosomes after drug treatment demonstrating virtually random timing of initiation. A similar pattern of chromosome number per cell was found after treatment of a recA strain.  相似文献   
25.
Summary EDA (ectodermal dysplasia, anhidrotic) is an X-linked recessive disorder characterized by hypohidrosis, hypoor anodontia, and hypotrichosis. A possible linkage between the gene for EDA and a number of restriction fragment length polymorphisms (RFLPs) spread over the X chromosome was investigated in two Danish families segregating EDA. No recombination between the gene for EDA and our probe pTAK8, which detects a two allele polymorphism in the region Xp11-q12, was found in nine informative meiotic events (seven of which are phase known), giving a maximal lod score of 2.41 at a recombination fraction of 0.00. This juxtacentromeric location of the gene for EDA agrees well with the linkage data obtained with the other markers used in this study.  相似文献   
26.
Summary Ducks (Anas platyrhynchos) were fed hypertonic saline for eight days, resulting in an activation and hypertrophy of the salt gland. The Na+–K+-dependent adenosine triphosphatase, an enzyme generally assumed to be part of the active Na transport system, increased its specific activity by about 200% during this activation. Sulfatides, the major glycolipids of the salt gland, increased their concentration to the same extent. Cholesterol, cerebrosides, and six phospholipid classes showed an increase of 20–80%.A preliminary report on this work was given at the Second International Meeting of the International Society for Neurochemistry, Milan, September 1–5, 1969, and at the XIIIth International Conference on the Biochemistry of Lipids, Athens, September 7–12, 1969.  相似文献   
27.
28.
29.
30.
Pregnancy-associated murine protein-1 (PAMP-1) could not be detected in peripheral blood of female dwarf mice (genotype dw/dw of the DW strain). By contrast the normal size females of the DW strain (genotypes +/+ and +/dw) had PAMP-1 serum levels of 18.9 AU +/- 15.7 AU/ml. Following administration of biosynthetic human growth hormone (hGH) every 2 h for 52 h PAMP-1 was detected in all dwarf females at concentrations of 16.0 AU +/- 3.3 AU/ml. The albumin levels in the circulation of DW females of normal size were significantly higher (P less than 0.05) than those of DW dwarfs, and the hGH administration did not change the serum albumin levels. The present experiment adds weight to the suggestion that the PAMP-1 serum level is regulated by GH.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号