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   804710篇
  免费   94675篇
  国内免费   967篇
  900352篇
  2018年   7091篇
  2017年   6762篇
  2016年   9973篇
  2015年   14646篇
  2014年   16744篇
  2013年   23221篇
  2012年   27053篇
  2011年   27511篇
  2010年   18362篇
  2009年   16825篇
  2008年   24180篇
  2007年   25047篇
  2006年   23160篇
  2005年   22436篇
  2004年   22272篇
  2003年   20957篇
  2002年   20309篇
  2001年   34893篇
  2000年   35017篇
  1999年   28053篇
  1998年   10557篇
  1997年   10775篇
  1996年   10338篇
  1995年   9637篇
  1994年   9468篇
  1993年   9353篇
  1992年   23055篇
  1991年   22375篇
  1990年   21998篇
  1989年   21445篇
  1988年   19527篇
  1987年   18748篇
  1986年   17520篇
  1985年   17203篇
  1984年   14525篇
  1983年   12579篇
  1982年   9729篇
  1981年   8795篇
  1980年   8193篇
  1979年   13679篇
  1978年   10849篇
  1977年   9837篇
  1976年   9170篇
  1975年   10180篇
  1974年   11118篇
  1973年   10896篇
  1972年   9792篇
  1971年   9048篇
  1970年   7605篇
  1969年   7456篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
102.
The breeding cycles of two species of sandy beach whelk (Bulliadigitalis and B. pura) are presented and compared with a thirdspecies (B. rhodostoma). In all three species, egg maturationand copulation occur in spring while summer marks the depositionand spawning of egg capsules. B. digitalis and B. pura migrateoffshore to lay their eggs, the juveniles of both species restrictingthemselves to beyond the breaker zone. Newly hatched snailsof B. rhodostoma appear in the intertidal towards late summer.The discovery of a penis-like structure on females of B. rhodostomais investigated. The pseudopenis was also found on B. pura butnever on B. digitalis. The possibility of a sex-change was eliminatedon the histological investigation of the gonads of a range ofsnails and the examination of sex-ratios over a period of fivemonths. General trends in reproductive behaviour are also discussed. (Received 18 March 1984;  相似文献   
103.
In a multicentre, randomised, prospective trial 89 patients (67 children and 22 adults) with the minimal change nephrotic syndrome were treated with three intravenous pulses of methylprednisolone followed by low dose oral prednisone for six months (group given methylprednisolone) or with high dose oral prednisone for four weeks followed by low dose oral prednisone for five months (control group). Five patients in the group given methylprednisolone and one in the control group did not respond initially. The time to response was shorter in children treated with methylprednisolone. No significant differences between the two groups were observed in the number of patients who relapsed or number of relapses per patient per year. Patients given methylprednisolone tended to relapse earlier than patients in the control group. Side effects related to treatment were significantly fewer in the group given methylprednisolone than in the control group. These data suggest that a short course of methylprednisolone pulses followed by low dose oral prednisone is only marginally less effective than a regimen of high dose oral steroids but can improve the ratio of risk to benefit associated with treatment of the minimal change nephrotic syndrome.  相似文献   
104.
3H-labeled Acholeplasma DNA probes were generated in vitro by the nick-translation method and used to determine the nucleotide sequence homology among the type strains of the eight currently recognized species of Acholeplasma. Very little nucleotide sequence homology (less than or equal to 18%) was found among the eight species, with heteroduplexes showing at least 12% or more mismatching as determined by thermal elution midpoints. The small amount of nucleotide sequence homology among the eight species indicates that these species are quite distinct and are not closely related to each other genomically.  相似文献   
105.
Monosomy 21: a new case confirmed by in situ hybridization   总被引:4,自引:0,他引:4  
Summary A new case of total monosomy 21 in a newborn is described. The diagnosis was first made using the cytogenetic data; it was then confirmed by the dosage of copper-superoxide dismutase (SOD-1) which showed a 50% decrease. In situ hybridization with a probe previously assigned to chromosome 21 was used to rule out the possibility of a partial monosomy with an unbalanced reciprocal translocation.  相似文献   
106.
A series of measurements of blood pressure in normotensive and hypertensive subjects showed that measurements made with a sphygmomanometer with the arm dependent by the side were consistently higher than those made with the arm horizontal at heart level. The mean difference in a group of 90 hypertensive outpatients was 11/12 mm Hg. Failure to appreciate the importance of arm position may lead to erroneous measurements of blood pressure. This has important implications for clinical practice and research.  相似文献   
107.
108.
Four new species ofCayaponia are described and illustrated: three from Brazil (C. cogniauxiana, C. nitida andC. rugosa) and one from Brazil and Bolivia (C. ferruginea).  相似文献   
109.
Spinosyns A and D are the active ingredients in an insect control agent produced by fermentation of Saccharopolyspora spinosa. Spinosyns are macrolides with a 21-carbon, tetracyclic lactone backbone to which the deoxysugars forosamine and tri-O-methylrhamnose are attached. The spinosyn biosynthesis genes, except for the rhamnose genes, are located in a cluster that spans 74 kb of the S. spinosa genome. DNA sequence analysis, targeted gene disruptions and bioconversion studies identified five large genes encoding type I polyketide synthase subunits, and 14 genes involved in sugar biosynthesis, sugar attachment to the polyketide or cross-bridging of the polyketide. Four rhamnose biosynthetic genes, two of which are also necessary for forosamine biosynthesis, are located outside the spinosyn gene cluster. Duplication of the spinosyn genes linked to the polyketide synthase genes stimulated the final step in the biosynthesis — the conversion of the forosamine-less pseudoaglycones to endproducts. Duplication of genes involved in the early steps of deoxysugar biosynthesis increased spinosyn yield significantly. Journal of Industrial Microbiology & Biotechnology (2001) 27, 399–402. Received 31 May 2001/ Accepted in revised form 09 July 2001  相似文献   
110.
The heart rate and respiratory patterns in hypoxia are not well documented in unanaesthetized intact newborn animals. We studied heart rate and respiratory patterns during quiet sleep in 17% inspired O2 in 31 unanaesthetized newborns of five species: lamb, piglet, puppy, kitten, and rabbit. There was no significant change in mean heart rate and respiratory rate with hypoxia for any species. Brief apneas greater than 5 s were frequent (5-8/h), both in 21 and 17% O2 only in lambs and puppies. No sustained periodic breathing was induced by hypoxia. Thus, mild hypoxia has little steady-state effect on heart rate and respiratory rate and pattern in these unanaesthetized newborns. These findings are compatible with depressed chemoreceptor threshold, but indicate a remarkably mature respiratory pattern in full-term newborns of these species.  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