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   6512篇
  免费   684篇
  国内免费   2篇
  7198篇
  2022年   54篇
  2021年   85篇
  2020年   55篇
  2019年   65篇
  2018年   93篇
  2017年   72篇
  2016年   121篇
  2015年   240篇
  2014年   225篇
  2013年   338篇
  2012年   393篇
  2011年   378篇
  2010年   230篇
  2009年   206篇
  2008年   320篇
  2007年   364篇
  2006年   308篇
  2005年   315篇
  2004年   305篇
  2003年   288篇
  2002年   253篇
  2001年   75篇
  2000年   71篇
  1999年   92篇
  1998年   93篇
  1997年   74篇
  1996年   74篇
  1995年   91篇
  1994年   44篇
  1993年   64篇
  1992年   75篇
  1991年   76篇
  1990年   66篇
  1989年   67篇
  1988年   53篇
  1987年   61篇
  1986年   50篇
  1985年   60篇
  1984年   61篇
  1983年   60篇
  1982年   72篇
  1981年   77篇
  1980年   66篇
  1979年   41篇
  1978年   62篇
  1977年   42篇
  1976年   46篇
  1974年   43篇
  1972年   37篇
  1971年   32篇
排序方式: 共有7198条查询结果,搜索用时 15 毫秒
71.
We report a large two-generation pedigree with seven affected males segregating for an X-linked mixed conductive sensorineural deafness. The patients present with atypical Mondini-like dysplasia, dilated petrous facial canal, dilatation of the internal auditory meatus fully connected with enlarged cochlear canals, and, in one patient, a wide bulbous posterior labyrinth. Obligatory carrier females are mildly affected. Molecular characterization of this family revealed a deletion of locus DXS169, in Xq21.1. Loci DXS72 and DXS26, which, respectively, flank DXS169 proximally and distally, were intact. Since a gene responsible for X-linked progressive mixed deafness with perilymphatic gusher (DFN3) has previously been assigned by deletion mapping to a slightly more distal interval between DXS26 and DXS121, this study indicates either two different deafness genes or the involvement of a very large region in Xq21.  相似文献   
72.
The operational sex ratio (OSR), an important determinant of the intensity of sexual selection, can vary with the population sex ratio and the potential reproductive rates of males and females. In parental teleosts, different forms of brood cannibalism, filial and non-kin, could affect the potential reproductive rates of males and females, and thereby the OSR. The direction of the effect of brood cannibalism on the OSR would depend on the sex of the cannibal and the parent. No evidence has been found that brood cannibalism can affect the OSR by altering the sex ratio.  相似文献   
73.
Cytogenetic analysis of leukemic cells obtained at diagnosis from 122 patients with childhood acute lymphoblastic leukemia (ALL) disclosed chromosomal translocations in 36 cases. Two new nonrandom translocations were identified and found to be associated with specific immunophenotypes of the disease. The first, identified in 4 of 16 cases of T-cell ALL positive for sheep erythrocyte receptors (E+), involved the short arm (p) of chromosome 11 and the long arm (q) of chromosome 14 and was designated t(11;14) (p13;q13). The second, found in 7 of 23 cases with a pre-B-cell phenotype, involved the long arm of chromosome 1 and the short arm of chromosome 19; it was designated t(1;19) (q23;p13.3). A third abnormality involving a common breakpoint on chromosome 12 (band p 12) was also identified. These two new differentiation-specific translocations suggest a mechanism for aberrant expression of genes that influence lymphoid cell growth and development, as well as leukemogenesis.  相似文献   
74.
We examined multiple genetically regulated Immoral and cell-mediated immune (CMI) responses to poly(glu60ala30tyr10) (GAT) using a panel of mouse strains. We show that assignment of responder/nonresponder status depends upon the assay method. In addition, two distinct categories of nonresponder mice were found: (1) those which are unresponsive by all parameters tested (H-2 q and H-2 s haplotypes) and (2) those which are partially nonresponsive [H-2 bm12 mutant strain—a low/nonresponder by splenic plaque-forming cell (PFC) and delayed-type hypersensitivity (DTH) responses, but exhibits B6 parental levels of high GAT-specific T-cell proliferation (Tprlf) and interleukin-2 production]. The distinction between these two nonresponder types was confirmed by complementation tests in which significant GAT-specific PFC and DTH responses were seen in (H-2 q × H-2 bm12)F1 hybrids, but not in (H-2 q × H-2 s )F1 hybrids. Suppressor T cells (Ts) also play a selective role in nonresponsiveness to GAT. Cyclophosphamide treatment of nonresponders (to eliminate Ts activity) as well as immunization with GAT coupled to the immunogenic carrier MBSA result in the development of GAT-specific humoral, but not CMI responses. Our results indicate that the T cell is the cellular site of Ir gene expression and that Tprlf responses do not correlate with functional helper T-cell activity and suggest distinct, multi-step Th/Ts regulatory pathways in the development of humoral and CMI effector functions.  相似文献   
75.
Muralytic Activities of Ruminococcus albus 8   总被引:4,自引:3,他引:1       下载免费PDF全文
Ruminococcus albus 8 was cultured with isolated alfalfa cell walls as the carbon source. The culture broth was assayed for muralytic enzyme activities. The effect, with respect to the production of such muralytic enzymes, of growing the microorganism on different carbon sources was also investigated. Also, the rates of solubilization and utilization by R. albus of individual alfalfa cell wall sugars during a 96-h growth period were examined.  相似文献   
76.
The passage of orally fed proteins from mother to foetus in the rat   总被引:2,自引:0,他引:2  
Pregnant rats were orally fed with a mixture of bovine IgG, bovine albumin, ovalbumin and beta-lactoglobulin. Using immunoprecipitation methods, these proteins were detected in the maternal blood serum, urine and uterine fluids, and also in the foetal blood serum and the amniotic fluid. The results imply that a variety of dietary proteins, still immunoprecipitable, are able to cross the materno-foetal barriers to be detected in the foetal circulation, where they may influence the maturation of the immune system of the foetus.  相似文献   
77.
The dependence of growth on nickel supply was studied in Chlorella emersonii 211-8b. After transfer to Ni2+ deficient medium containing only 0.5±0.2 g/l of Ni2+, production of biomass or daughter cells dropped to 55±5% of the controls, and the cells became chlorotic. These symptoms of deficiency disappeared completely by supplying adequate amounts of nickel. They were, however, only partially reversible by cobalt. It is concluded that nickel is an essential micronutrient for C. emersonii, although this organism lacks the nickel enzyme, urease.Gratefully dedicated to Prof. Hans Adolf von Stosch on the occasion of his 70th birthday  相似文献   
78.
Summary Elevated levels of alpha-1-fetoprotein (AFP) were found in the amniotic fluid of a 36-year-old woman in the 15th week of gestation. Because of this and the results of repeated ultrasonography, abortion was induced. An anencephalic fetus with trisomy 18 was delivered. The possible correlation among neural-tube defects, chromosomal abnormalities, and level of AFP is discussed.  相似文献   
79.
Auchenorrhyncha of a new polder were sampled by different kinds of traps in four stations in new habitats, one station on the salt marsh and one just outside the polder. In the latter stations 20 species of Auchenorrhyncha were caught in numbers that declined during the four year study. The proportion of macropters of the common Streptanus sordidus was highest in the largest catches. In the polder proper 392 specimens of 13 species were caught, all had moved at least 1500 m, but some could be traced to origins at greater distances. The number of immigrants also declined during the study. The distribution of the leafhoppers over the trapping stations suggests that leafhoppers, although sometimes transported over great distances, as a rule quickly fall out once they are airborne. Probably only one species had founded a resident population in the new polder at the end of the study.  相似文献   
80.
Studies were conducted on Padre Island, Texas since 1968 to provide specifications for how to use vegetation to reconstruct destroyed portions of foredunes of Gulf Coast barrier islands. The area has a subtropical, semi-arid climate, moderated by maritime tropical air from the Gulf of Mexico. Annual precipitation averages 670 mm for the island. We established experimental foredunes in broad overwash areas, 1.5 m M.S.L. where foredunes were eroded by hurricanes or destroyed by blowouts and across hurricane overwash channels of lower elevation behind the berm and more susceptible to surges. Most successful establishment of a vegetated dune was by transplanting directly onto the level backbeach to reestablish the original duneline about 125 meters inland. Sea oats and bitter panicum proved best of all species tested. Bitter panicum was the more desirable because of higher transplant survival, a longer planting season, and ease of harvest. Transplant survival was influenced more by soil salinity, soil moisture, and plant vigor, than by planting season as successful plantings occurred during all seasons. December through February plantings were best for sea oats, but survival of bitter panicum transplants different little from November through May. Both species are capable of growth year round during periods of favorable climatic conditions, although little growth occurs from mid-December to mid-February. Transplants placed 0.6 m apart with total planting width of 15 m produced foredunes nearly 6 m high in 6 years. Such plantings trapped sand at the rate of 12.5 m3/m of beach. In hurricane overwash channels, we used 0.6 m high sand fence placed 3 m apart to produce a flat topped dune free of salt for a planting surface. Without this site preparation, no transplants survived due to high soil salinity. Dune width was effectively widened by planting a strip 15 m wide immediately beachward of a previously established 4-year old dune.Presented at the Seventh International Biometeorological Congress, 17–23 August 1975, College Park, Maryland, USA.  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