全文获取类型
收费全文 | 25887篇 |
免费 | 54篇 |
国内免费 | 106篇 |
专业分类
系统科学 | 90篇 |
丛书文集 | 365篇 |
教育与普及 | 99篇 |
理论与方法论 | 92篇 |
现状及发展 | 12098篇 |
研究方法 | 966篇 |
综合类 | 12055篇 |
自然研究 | 282篇 |
出版年
2013年 | 172篇 |
2012年 | 313篇 |
2011年 | 572篇 |
2008年 | 402篇 |
2007年 | 446篇 |
2006年 | 417篇 |
2005年 | 457篇 |
2004年 | 475篇 |
2003年 | 435篇 |
2002年 | 444篇 |
2001年 | 677篇 |
2000年 | 693篇 |
1999年 | 463篇 |
1994年 | 332篇 |
1992年 | 404篇 |
1991年 | 339篇 |
1990年 | 397篇 |
1989年 | 379篇 |
1988年 | 392篇 |
1987年 | 388篇 |
1986年 | 418篇 |
1985年 | 523篇 |
1984年 | 345篇 |
1983年 | 372篇 |
1982年 | 296篇 |
1981年 | 297篇 |
1980年 | 362篇 |
1979年 | 816篇 |
1978年 | 686篇 |
1977年 | 651篇 |
1976年 | 491篇 |
1975年 | 567篇 |
1974年 | 750篇 |
1973年 | 644篇 |
1972年 | 701篇 |
1971年 | 811篇 |
1970年 | 973篇 |
1969年 | 845篇 |
1968年 | 826篇 |
1967年 | 754篇 |
1966年 | 659篇 |
1965年 | 479篇 |
1964年 | 191篇 |
1959年 | 264篇 |
1958年 | 501篇 |
1957年 | 393篇 |
1956年 | 344篇 |
1955年 | 305篇 |
1954年 | 303篇 |
1948年 | 258篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
R. S. Verma S. Thomas M. Coleman R. T. Silver H. Dosik 《Cellular and molecular life sciences : CMLS》1986,42(4):440-441
Summary A random distribution of the Y-chromosome at somatic metaphase was found in 50 patients with Ph' positive chronic myelogenous leukemia (CML). Thus, it is concluded that the positive of the Y-chromosome at somatic metaphase does not appear to influence the loss from bone marrow cells. 相似文献
92.
Summary Glycosidases like sialidase,-galactosidase, -L-fucosidase, N-acetyl hexosaminidase and proteases were detected in toad testis. Neuraminic acid aldolase activity was also detected. The enzyme activities were found to vary as production of spermatozoa varied. All enzymes, except N-acetyl glucosaminidase, were shown to decrease after injection of toad pituitary extract and they were also found to be absent from testis containing no spermatozoa. The glycosidases were found to act on toad oviduct jelly and they may therefore be involved in the degradation of the jelly after fertilization, into smaller bits, which may be utilized as nutrients by the fertilized zygote.Acknowledgment. We thank Prof. T.R. Ramaiah, Head of the Department of Biochemistry, University of Mysore, for his help. We also acknowledge the financial assistance of University Grants Commission to one of us (MS) and CSIR through a grant No. 9 (165)83/EMR-II to HSS. Please address all correspondence to H.S. Seshadri. 相似文献
93.
On some significance tests in cluster analysis 总被引:1,自引:1,他引:0
Bock H.H. 《Journal of Classification》1985,2(1):77-108
We investigate the properties of several significance tests for distinguishing between the hypothesisH of a homogeneous population and an alternativeA involving clustering or heterogeneity, with emphasis on the case of multidimensional observationsx
1, ...,x
n
p
. Four types of test statistics are considered: the (s-th) largest gap between observations, their mean distance (or similarity), the minimum within-cluster sum of squares resulting from a k-means algorithm, and the resulting maximum F statistic. The asymptotic distributions underH are given forn and the asymptotic power of the tests is derived for neighboring alternatives. 相似文献
94.
S. S. Tobe R. P. Ruegg B. A. Stay F. C. Baker C. A. Miller D. A. Schooley 《Cellular and molecular life sciences : CMLS》1985,41(8):1028-1034
Summary Titres of juvenile hormone (JH) have been determined in both hemolymph and whole body extracts of femaleDiploptera punctata during the first gonotrophic cycle using a method employing gas chromatography/mass spectrometry for qualitative and quantitative analysis. JH III is the sole JH found in both adult and last instarD. punctata. Maximum values of 1500 ng/ml (6M) were observed at the middle of the gonotrophic cycle, when basal oocyte growth rate was greatest. Changes in rates of JH release in vitro by corpora allata paralleled closely the changes in JH titre, suggesting that biosynthesis is a major regulator of titre. JH levels per animal were calculated from observed JH titres, and at certain time points in the gonotrophic cycle JH levels obtained from analysis of whole bodies were significantly greater than those predicted from hemolymph titres. These results suggest the existence of a nonhemolymph JH pool inD. punctata. Decay in JH titre after allatectomy of 5 day females has also been studied. Following a rapid initial decline, the rate of decay slowed appreciably 4 h post-operation. Thus, use of a first-order rate constant to estimate half-life of JH significantly underestimated the longevity of the hormone. The apparent persistence of JH following allatectomy may be due to the existence of a nonhemolymph JH pool. 相似文献
95.
R. K. A. Giger H. R. Loosli M. D. Walkinshaw B. J. Clark J. M. Vigouret 《Cellular and molecular life sciences : CMLS》1987,43(10):1125-1130
Summary We report the synthesis, stereochemistry and preliminary pharmacological evaluation of DCN 203-922, a novel ergot alkaloid of the cyclol type, which contains in its peptide moiety the uncommon amino acid L-allo-isoleucine.Part of this paper was reported by this author at the Herbstversammlung der Schweizerischen Chemischen Gesellschaft, Bern, in October 1986. 相似文献
96.
Ross P Weinhouse H Aloni Y Michaeli D Weinberger-Ohana P Mayer R Braun S de Vroom E van der Marel GA van Boom JH Benziman M 《Nature》1987,325(6101):279-281
Cellulose is the most abundant renewable carbon resource on earth and is an indispensable raw material for the wood, paper, and textile industries. A model system to study the mechanism of cellulose biogenesis is the bacterium Acetobacter xylinum which produces pure cellulose as an extracellular product. It was from this organism that in vitro preparations which possessed high levels of cellulose synthase activity were first obtained in both membranous and soluble forms. We recently demonstrated that this activity is subject to a complex multi-component regulatory system, in which the synthase is directly affected by an unusual cyclic nucleotide activator enzymatically formed from GTP, and indirectly by a Ca (2+) -sensitive phosphodiesterase which degrades the activator. The cellulose synthase activator (CSA) has now been identified as bis-(3' 5')-cyclic diguanylic acid (5'G3'p5'G3'p) on the basis of mass spectroscopic data, nuclear magnetic resonance analysis and comparison with chemically synthesized material. We also report here on intermediary steps in the synthesis and degradation of this novel circular dinucleotide, which have been integrated into a model for the regulation of cellulose synthesis. 相似文献
97.
Chromosomal localization of human haemoglobin structural genes 总被引:15,自引:0,他引:15
98.
The significance of glycosylated proteins 总被引:15,自引:0,他引:15
99.
Pineal gland changes of rats exposed to heat 总被引:1,自引:0,他引:1
100.
Stereochemistry of cooperative effects in haemoglobin 总被引:136,自引:0,他引:136
M F Perutz 《Nature》1970,228(5273):726-739