全文获取类型
收费全文 | 173981篇 |
免费 | 5242篇 |
国内免费 | 822篇 |
出版年
2021年 | 1076篇 |
2020年 | 939篇 |
2019年 | 1008篇 |
2018年 | 13432篇 |
2017年 | 12052篇 |
2016年 | 9820篇 |
2015年 | 4167篇 |
2014年 | 4012篇 |
2013年 | 5235篇 |
2012年 | 9711篇 |
2011年 | 17299篇 |
2010年 | 14737篇 |
2009年 | 10370篇 |
2008年 | 13350篇 |
2007年 | 14741篇 |
2006年 | 4051篇 |
2005年 | 3710篇 |
2004年 | 4150篇 |
2003年 | 3853篇 |
2002年 | 3383篇 |
2001年 | 2344篇 |
2000年 | 2123篇 |
1999年 | 1547篇 |
1998年 | 721篇 |
1997年 | 551篇 |
1996年 | 577篇 |
1995年 | 495篇 |
1992年 | 973篇 |
1991年 | 894篇 |
1990年 | 865篇 |
1989年 | 920篇 |
1988年 | 772篇 |
1987年 | 794篇 |
1986年 | 711篇 |
1985年 | 771篇 |
1984年 | 654篇 |
1983年 | 580篇 |
1982年 | 479篇 |
1979年 | 681篇 |
1978年 | 497篇 |
1977年 | 496篇 |
1975年 | 597篇 |
1974年 | 627篇 |
1973年 | 599篇 |
1972年 | 805篇 |
1971年 | 772篇 |
1970年 | 546篇 |
1969年 | 622篇 |
1968年 | 576篇 |
1967年 | 483篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Local administration (microiontophoretically) of naloxone was tested in 57 parafascicularis thalamic (PF) neurons of morphine-naive and morphine-dependent rats. In morphine-naive rats microiontophoretic applications of naloxone induced changes in 52% of the PF neurons. Reduction in neuronal activity was observed in the majority of them; this reduction phenomena exhibited dose response characteristics, i.e., each incremental naloxone dose caused further decrease of the neuronal discharges. In morphine-dependent animals, 64% of the PF neurons were affected. The changes seen after naloxone were mainly increases of electrical discharges (i.e. the opposite effects obtained in morphine-naive animals). 相似文献
992.
Calli derived from leaves and radicles of B. ternifolia were grown on Murashige and Skoog (MS) basal medium, and the effects of different nitrogen sources on the rate of callus growth and on the enzymes related to nitrogen assimilation were studied. Ammonium alone did not support callus growth unless a Krebs-cycle intermediate was added to the medium. The activities of glutamine synthetase (EC 6.3.1.2), glutamate synthase (EC 1.4.7.1), and glutamate dehydrogenase (EC 1.4.1.2) were measured in homogenates of callus grown on media supplied with different nitrogen sources. The results indicate that leaf and root calli have similar levels of these enzymes when grown on MS medium (Murashige and Skoog 1962. Physiol. Plant. 15, 473–497). However, when the calli were supplied with glutamine as the sole nitrogen source, the activity of glutamate synthase increased in leaf callus but was almost completely inhibited in root callus. The results indicate that calli originated from different B. ternifolia tissues do not have the same biochemical dedifferentiated state. 相似文献
993.
Two ammonites belonging to the genusPictonia Bayle, 1878 (subgenusPachypictonia Schneid, 1940) are described from the Lower Kimmeridgian of the Subbetic of Western Andalusia, this being the first evidence of the presence of this genus in the Mediterranean (Tethydian) Upper Jurassic. 相似文献
994.
Nagy F. Lázár G. Menczel L. Maliga P. 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1983,66(3-4):203-207
Theoretical and Applied Genetics - Mitochondrial DNA (mtDNA) restriction patterns were studied in mutant, cybrid and somatic hybrid plants regenerated from Nicotiana protoplasts. It has been shown... 相似文献
995.
996.
J Janosek M Simkovicová 《Journal of hygiene, epidemiology, microbiology, and immunology》1983,27(2):155-162
A total of 107 donor strains of Staphylococcus aureus isolated from clinical material, with a high incidence of multiresistant strains belonging predominantly to phage group III, were tested for transmission of determinants of resistance to 6 antibiotics using mixed cultures of donor strains and the recipient Staphylococcus aureus strain 5849-fur-r, rif-r. The capability of strains to transfer resistance markers to the recipient was found to depend neither on phage group nor phage type to which the donor strain belonged, but strains possessing multiple resistance to antibiotics effectuated transfers at comparatively higher frequencies. 相似文献
997.
V Svoboda Z Kotasková 《Journal of hygiene, epidemiology, microbiology, and immunology》1983,27(3):329-335
Survival, radiosensitivity and capability to produce differentiated progeny were followed in CFU-S from lumbar vertebrae of mice injected with 198.6 kBq 239Pu/kg or 208.6 kBq 241Am/kg. The CFU-S assay and 59Fe uptake into spleen colonies were used. The number of CFU-S from treated mice was significantly lower than in controls. Higher radiosensitivity of CFU-S from 239Pu- or 241Am-treated mice was demonstrated using additional exposure to 0.5 Gy X-rays 1, 24, 48, 72 hrs after cell transplantation and expressed more precisely by survival curves obtained 1 hr after the marrow cell injection. The effect of 239Pu on CFU-S was characterized by Do 0.58 Gy (n = 0.91) and that of 241Am by Do 0.64 Gy (n = 0.91); corresponding control values were Do 0.89 Gy, n = 1.11. Lower iron utilization due not only to the decreased CFU-S numbers, but also to the defective production of erythroid cells per one CFU-S was found. Complexity of radiation effect on hemopoietic stem cells was demonstrated by the present study. 相似文献
998.
The injection of as little as 0.5 microgram phosphorylcholine-(PC) conjugated mouse immunoglobulin into BALB/c neonates within 48 hr of birth results in complete unresponsiveness to PC for 3 to 4 wk. Thereafter, anti-PC responses can be detected in tolerized animals, but these responses differ significantly from those of normal BALB/c mice. First, the magnitude of responsiveness does not approach normal levels even 9 mo after birth. Second, although the initial responses as tolerance is broken can be T15+, idiotypic dominance is not established; instead, a heterogeneous T15- population eventually emerges, which includes clones with higher and with lower avidity than T15. Unirradiated unresponsive mice will help transplanted normal B cells to produce T15+ responses to thymus-dependent PC antigens. The responses of animals recovered from tolerance are stable upon adoptive transfer. We have, moreover, found no evidence of either loss of idiotype-specific T cell help or generation of suppression. Therefore, neonatal exposure to PC tolerogen can effect profound, permanent changes in the antigen-specific B cell compartment independent of any influence on conventional T cell regulatory mechanisms. 相似文献
999.
Karel Mikulík Anna Jiráňová Jaroslav Weiser Ivan Janda Jana Šťastná Nguyen Quyen 《Biochimica et Biophysica Acta (BBA) - Gene Structure and Expression》1983,740(1):99-107
Slowly cooled cells of Streptomyces aureofaciens contained mainly tight-couple ribosomes. Maximum rate of polyphenylalanine synthesis on ribosomes of S. aureofaciens was observed at 40°C, while cultures grew optimally at 28°C. Ribosomes of S. aureofaciens differed from those of E. coli in the amount of poly(U) required for maximum synthetic activity. The polyphenylalanine-synthesizing activity of E. coli ribosomes was about 3-times higher than that of S. aureofaciens ribosomes. The addition of protein S1 of E. coli or the homologous protein from S. aureofaciens had no stimulatory effect on the translation of poly(U). In order to localize alteration(s) of S. aureofaciens ribosomes in the elongation step of polypeptide synthesis we developed an in vitro system derived from purified elongation factors and ribosomal subunits. The enzymatic binding of Phe-tRNA to ribosomes of S. aureofaciens was significantly lower than the binding to ribosomes of E. coli. This alteration was mainly connected with the function of S. aureofaciens 50 S subunits. These subunits were not deficient in their ability to associate with 30 S subunits or with protein SL5 which is homologous to L7/L12 of E. coli. 相似文献
1000.
In pregnant rats significant interstitial fluid pressure changes could be detected by means of capsules chronically implanted into the subcutaneous tissue. The capsular pressure increased significantly from a control value of -4.3 +/- 0.5 mmHg to -0.7 +/- 0.5 mmHg during the first period of pregnancy. Immediately before parturition the capsular pressure returned to the control level. During lactation the pressure rose as high as + 0.5 +/- 0.9 mmHg. After lactation the pressure returned again to the control value. By determining the extracellular fluid and plasma volume, as well as protein concentration in plasma and capsular fluid, the hydrostatic and colloid osmotic forces operating in the extracellular space could be analysed. It has been concluded that the observed capsular pressure changes during pregnancy are not solely of volumetric or colloid osmotic origin. 相似文献