全文获取类型
收费全文 | 39831篇 |
免费 | 4135篇 |
国内免费 | 11篇 |
出版年
2022年 | 237篇 |
2021年 | 670篇 |
2020年 | 486篇 |
2019年 | 566篇 |
2018年 | 701篇 |
2017年 | 631篇 |
2016年 | 1043篇 |
2015年 | 1727篇 |
2014年 | 1881篇 |
2013年 | 2329篇 |
2012年 | 2865篇 |
2011年 | 2783篇 |
2010年 | 1849篇 |
2009年 | 1609篇 |
2008年 | 2246篇 |
2007年 | 2234篇 |
2006年 | 2125篇 |
2005年 | 1894篇 |
2004年 | 1887篇 |
2003年 | 1682篇 |
2002年 | 1651篇 |
2001年 | 652篇 |
2000年 | 607篇 |
1999年 | 621篇 |
1998年 | 432篇 |
1997年 | 352篇 |
1996年 | 339篇 |
1995年 | 350篇 |
1994年 | 308篇 |
1993年 | 306篇 |
1992年 | 413篇 |
1991年 | 354篇 |
1990年 | 349篇 |
1989年 | 363篇 |
1988年 | 364篇 |
1987年 | 334篇 |
1986年 | 268篇 |
1985年 | 298篇 |
1984年 | 305篇 |
1983年 | 273篇 |
1982年 | 259篇 |
1981年 | 231篇 |
1980年 | 209篇 |
1979年 | 215篇 |
1978年 | 181篇 |
1977年 | 183篇 |
1976年 | 184篇 |
1975年 | 198篇 |
1974年 | 165篇 |
1973年 | 165篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Dr. Martin P. Schreibman Ellen J. Berkowitz Rob van den Hurk 《Cell and tissue research》1982,224(1):81-87
Summary The gonads of 3-day- to 7-month-old male and female platyfish (Xiphophorus maculatus) were examined for the presence of 5-3-hydroxysteroid dehydrogenase (3-HSD) and glucose-6-phosphate dehydrogenase (G6PD) by histochemical means. In 3-day-old males a positive response for both enzymes is localized in the Leydig cells. With subsequent testicular development, these cells increase in number and display greater activity at the periphery of the testis and around the efferent ducts. In 3-day-old females 3-HSD and G6PD are localized in the stromal cells of the ovary. These cells increase in number and activity as the animals become sexually mature. Sertoli cells, efferent duct epithelium, and ovarian granulosa cells are negative at all stages of development examined. Our findings suggest that the gonads of neonatal fish possess the potential for steroidogenesis. The role played by sexsteroid hormones in the maturation of the brain-pituitary-gonad axis is discussed. 相似文献
992.
993.
994.
Cranial morphology and adaptations in Eocene Adapidae. II. The Cambridge skull of Adapis parisiensis
One of the most complete skulls of the early primate Adapis parisiensis is in the collection of the Department of Zoology, Cambridge University. This exceptionally well-preserved male skull, from Quercy in southern France, is important in showing relatively small orbits that are highly convergent, a distinct ethmoid component in the medial orbital wall, very small infraorbital foramina, a well-preserved auditory region with the stapedial canal about twice the diameter of the canal for the promontory artery, and a well-preserved braincase 8.8 cm3 in endocranial volume. The frontal lobe of the brain in the Cambridge skull described here is less expanded than that reported previously in a British Museum skull. The average body weight of Adapis parisiensis is estimated to have been about 2.0 kg, and that of Adapis magnus is estimated to have been about 8.4 to 9.0 kg. The encephalization quotient (EQ) of Adapis parisiensis is estimated to have been 0.45, which is well below the range found in modern prosimians. There is some indication that the size of the foramen magnum has increased with increasing brain size during primate evolution. Adapis parisiensis appears to have been a medium-sized, visually oriented, diurnal, sexually dimorphic arboreal folivore. 相似文献
995.
Conversion of Cellulose to Methane and Carbon Dioxide by Triculture of Acetivibrio cellulolyticus, Desulfovibrio sp., and Methanosarcina barkeri 总被引:6,自引:6,他引:0 下载免费PDF全文
The fermentation of cellulose by monocultures of Acetivibrio cellulolyticus and cocultures of A. cellulolyticus-Methanosarcina barkeri, A. cellulolyticus-Desulfovibrio sp., and A. cellulolyticus-M. barkeri-Desulfovibrio sp. was studied. The monoculture produced ethanol, acetate, H2, and CO2. More acetate and less ethanol was formed by the cocultures than by the monoculture. Acetate was utilized by M. barkeri in coculture with A. cellulolyticus after a lag period, whereas ethanol was metabolized by the sulfate reducer only under conditions of low H2 partial pressure, i.e., when cocultured with A. celluloyticus-M. barkeri or when grown together with the methanogen. Only the three-component culture carried out the rapid conversion of cellulose to CO2 and methane. Furthermore, this culture hydrolyzed the most cellulose—85% of that initially present. This amount was increased to 90% by increasing the population of M. barkeri in the triculture. Methane production was also increased, and a quicker fermentation rate was achieved. 相似文献
996.
997.
Enzymatic cleavage prior to antibody incubation as a method for neuropeptide immunocytochemistry 总被引:4,自引:0,他引:4
Summary Five anti-gastrin releasing peptide (GRP) sera have been characterized against GRP, bombesin and related polypeptides spotted on cellulose acetate discs. Antibodies reacting with the C-terminal G-14 sequence of bombesin and the 19–27 sequence of GRP, were detected in all sera. Antibodies directed exclsively against the bombesin unrelated 1–17 sequence of GRP were found only in one serum (R-6902). With parallel immunohistochemical tests only the C-terminal immunoreactivity was detected in endocrine-paracrine cells of the chicken proventriculus, while both immunoreactivities were present in nerve fibres and a few nerve cell bodies of the mammalian gut. The distribution of GRP- and bombesin-like immunoreactive nerves in the gastric mucosa of both pyloric and oxyntic type the submucosal and myenteric plexus along the whole gastrointestinal wall and at sphincter regions is detailed. 相似文献
998.
999.
R.D. Nolan G.J. Dusting J. Jakubowski T.J. Martin 《Prostaglandins & other lipid mediators》1982,24(6):887-902
Cyclo-oxygenase products of arachidonic acid metabolism formed by the pericardium and epicardial surface of dog heart were identified and quantitated by radioimmunoassay after separation by high-pressure liquid chromatography. Pieces of pariental pericardium, of dog, ox and rat, when incubated
produced mainly 6-keto-PGF1α, with lesser amounts of PGE2, PGF2α and thromboxane B2. Biosynthesis of all prostanoids increased during incubation of the pariental pericardium of each species with arachidonic acid, but 6-keto-PGF1α was still the major metabolite. When slices of dog heart were incubated with arachidonic acid (1 μg/ml) the rates of 6-keto-PGF1α formation by the pariental pericardium was much greater than that of the myocardium and endocardium. Epicardial slices appeared to be intermediate in 6-keto-PGF1α formation. The hearts of anesthetized dogs were also irrigated
with Krebs' solution, and during the first 5 min of epicardial irrigation the pericardial fluid leaving the heart again contained high levels of 6-keto-PGF1α, with lesser amounts of the other prostanoids. Addition of arachidonic acid (3 μg/ml) to the irrigating fluid caused an increase in all measured prostanoid levels, although 6-keto-PGF1α remained the predominant metabolite. In contrast, intravenous infusion of isoproterenol selectively increased the release of 6-keto-PGF1α from the irrigated heart. It is concluded that the pericardium and epicardium continuously release prostacyclin into the pericardial fluid, and that the increased release of this substance observed when cardiac workload increases derives mainly from these membranous sources. This raises the interesting possibility that pericardial prostacyclin might influence coronary vascular tone and chemoreflexes which arise from the epicardium during myocardial ischemia. 相似文献
1000.
Martin L. Ogletree John A. Oates Kenneth L. Brigham Walter C. Hubbard 《Prostaglandins & other lipid mediators》1982,23(4):459-468
Leukocyte trapping in the pulmonary circulation may be an important component of the lung vascular injury response to endotoxin, but mediators of the pulmonary leukostasis and increased lung vascular permeability are unknown. The leukocyte 5-lipoxygenation pathway of arachidonic acid metabolism yields highly biologically active products including leukotrienes C4 and D4 (formerly slow reacting substance of anaphylaxis) and the potent chemotaxin, leukotriene B4. A major product of 5-lipoxygenation is 5-hydroxy-6,8,11,14-eicosatetraenoic acid (5-HETE), for which a sensitive, stable isotope dilution assay employing combined gas chromatography-mass spectrometry is available. This assay was used to test the hypothesis that 5-lipoxygenation products might participate in pulmonary vascular responses to endotoxin. We measured 5-HETE concentrations in lung lymph at three intervals during endotoxemia in unanesthetized sheep. Concentrations of 5-HETE in lung lymph exceeded those in aortic blood plasma. Lymph 5-HETE concentrations increased from 1.7±0.3 (mean ± SEM, N = 7) ng/ml during baseline to peak values of 6.1±1.8 ng/ml (p < 0.05) during the
hours after endotoxemia and preceeding the steady state increased lung vascular permeability response. During the increased permeability steady state from 240 to 270 minutes after endotoxin, lymph 5-HETE concentrations (1.4±0.3 ng/ml) and lymph 5-HETE flow (i.e., 5-HETE concentration x lung lynph flow rate) returned to baseline values. Although these observations are consistent with the hypothesis that 5-lipoxygenation products participate in the pulmonary vascular injury response to endotoxin, lymph 5-HETE concentrations did not correlate with any of the other experimental measurements. It may be only coincidence that the increase in lymph 5-HETE concentrations appeared contemporaneous with the onset of lung vascular injury. 相似文献