全文获取类型
收费全文 | 11480篇 |
免费 | 1488篇 |
出版年
2021年 | 157篇 |
2019年 | 98篇 |
2018年 | 135篇 |
2017年 | 124篇 |
2016年 | 222篇 |
2015年 | 346篇 |
2014年 | 364篇 |
2013年 | 514篇 |
2012年 | 523篇 |
2011年 | 571篇 |
2010年 | 361篇 |
2009年 | 338篇 |
2008年 | 453篇 |
2007年 | 473篇 |
2006年 | 411篇 |
2005年 | 415篇 |
2004年 | 416篇 |
2003年 | 364篇 |
2002年 | 425篇 |
2001年 | 314篇 |
2000年 | 304篇 |
1999年 | 260篇 |
1998年 | 153篇 |
1997年 | 120篇 |
1996年 | 163篇 |
1995年 | 129篇 |
1994年 | 132篇 |
1993年 | 149篇 |
1992年 | 223篇 |
1991年 | 186篇 |
1990年 | 223篇 |
1989年 | 232篇 |
1988年 | 180篇 |
1987年 | 159篇 |
1986年 | 165篇 |
1985年 | 166篇 |
1984年 | 169篇 |
1983年 | 132篇 |
1982年 | 147篇 |
1981年 | 132篇 |
1980年 | 105篇 |
1979年 | 152篇 |
1978年 | 127篇 |
1977年 | 122篇 |
1976年 | 152篇 |
1975年 | 134篇 |
1974年 | 137篇 |
1973年 | 136篇 |
1972年 | 114篇 |
1971年 | 105篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
In a study of 2000 unselected pregnant women the development of a mature placental appearance (grade 3) on ultrasonography by 34-36 weeks'' gestation, observed in 15% of cases, was associated with maternal smoking, low parity, low maternal age, and being white. These women had an increased risk of problems during labour and their babies had an increased risk of low birth weight, poor condition at birth, and perinatal death. The women were randomly allocated to two groups: in one group the result of the placental grading was reported to the clinician responsible for care; in the second the result was noted but not reported. There was a significant decrease in the risk of perinatal death in the group where the grading was known. This reduction was responsible for a difference in the principal outcome index, a heterogeneous group of measures of mortality and morbidity, but this difference was not significant. This study alone does not justify routine late scanning, and further, larger trials are required. Nevertheless, the results do provide a basis for the reporting of placental grading when ultrasound examination is performed during the third trimester. 相似文献
2.
This paper is concerned with gene survival in a population which may increase without density dependence according to a generalization of the Moran model for haploid individuals. A selective advantage to one allele and the possibility of differential reproductive rates are allowed. Simple conditions are given for ultimate homozygosity to be certain and for the possibility of ultimate polymorphism. The results complement and extend those of Heyde (1981, 1982). 相似文献
3.
A putative GDP–GTP exchange factor is required for development of the excretory cell in Caenorhabditis elegans 下载免费PDF全文
Norio Suzuki Matthew Buechner Kiyoji Nishiwaki David H. Hall Hiroyuki Nakanishi Yoshimi Takai Naoki Hisamoto Kunihiro Matsumoto 《EMBO reports》2001,2(6):530-535
The Caenorhabditis elegans excretory cell extends tubular processes, called canals, along the basolateral surface of the epidermis. Mutations in the exc-5 gene cause tubulocystic defects in this canal. Ultrastructural analysis suggests that exc-5 is required for the proper placement of cytoskeletal elements at the apical epithelial surface. exc-5 encodes a protein homologous to guanine nucleotide exchange factors and contains motif architecture similar to that of FGD1, which is responsible for faciogenital dysplasia. exc-5 interacts genetically with mig-2, which encodes Rho GTPase. These results suggest that EXC-5 controls the structural organization of the excretory canal by regulating Rho family GTPase activities. 相似文献
4.
V. Grant 《Plant Systematics and Evolution》2001,230(1-2):89-96
This paper deals with the use of cladistic methods and cladograms in phylogeny reconstruction in plant groups containing
numerous taxa. How accurate are the cladograms as to details? Accuracy tests at the level of details require an independently
known phylogeny, which excludes most plant groups, but such tests can be carried out in domesticated and experimental plant
groups which have documented pedigrees. Four such tests are known and are presented here: a new case in Gilia and three previously published cases in Avena, Hordeum, and Helianthus. The four cases include domesticated and experimental plants, use of morphological and molecular evidence, and presence of
dichotomous as well as reticulate phylogenies. The cladograms of the four plant groups all differ in significant details from
the known pedigrees. These results are discussed in relation to problems of interpretation of cladograms.
Received March 21, 2000 Accepted August 16, 2001 相似文献
5.
Difficulties were encountered with the orcein method currently being used to demonstrate hepatitis B surface antigen and copper-associated protein in the liver when a new batch of dye was introduced. A survey of published material produced a plethora of methods with many contradictory recommendations. A number of methods and a variety of orceins were compared to determine which methods and orcein solutions would give the most consistent results. Two methods gave equally satisfactory results and can be recommended for routine use in screening paraffin sections of liver for hepatitis B surface antigen and copper-associated protein. 相似文献
6.
7.
8.
9.
10.
P Kendall-Taylor K Hall D G Johnston R W Prescott 《BMJ (Clinical research ed.)》1982,285(6340):465-467
The effect of pergolide mesylate was studied in two previously untreated men with large prolactinomas and exceptionally high prolactin concentrations. The study was designed to determine whether pergolide would be effective in alleviating symptoms, correcting hormonal abnormalities and shrinking the tumour. Starting with 50 micrograms daily the dose of pergolide was slowly increased over 10 weeks to 1 mg once daily, when repeat assessment was performed. Both patients reported complete relief of symptoms, with no side effects. Serum prolactin concentration was suppressed to normal in both subjects, and evidence to suggest tumour shrinkage was observed. Pergolide appears to be effective treatment for men with large prolactinomas. 相似文献