全文获取类型
收费全文 | 6401篇 |
免费 | 757篇 |
国内免费 | 1篇 |
出版年
2021年 | 70篇 |
2020年 | 54篇 |
2018年 | 61篇 |
2016年 | 119篇 |
2015年 | 151篇 |
2014年 | 174篇 |
2013年 | 221篇 |
2012年 | 277篇 |
2011年 | 294篇 |
2010年 | 195篇 |
2009年 | 182篇 |
2008年 | 228篇 |
2007年 | 261篇 |
2006年 | 218篇 |
2005年 | 220篇 |
2004年 | 205篇 |
2003年 | 214篇 |
2002年 | 211篇 |
2001年 | 219篇 |
2000年 | 204篇 |
1999年 | 150篇 |
1998年 | 72篇 |
1997年 | 72篇 |
1996年 | 73篇 |
1995年 | 72篇 |
1994年 | 70篇 |
1993年 | 50篇 |
1992年 | 138篇 |
1991年 | 126篇 |
1990年 | 119篇 |
1989年 | 131篇 |
1988年 | 122篇 |
1987年 | 105篇 |
1986年 | 89篇 |
1985年 | 87篇 |
1984年 | 95篇 |
1983年 | 79篇 |
1982年 | 85篇 |
1981年 | 60篇 |
1980年 | 69篇 |
1979年 | 96篇 |
1978年 | 71篇 |
1977年 | 69篇 |
1976年 | 79篇 |
1975年 | 76篇 |
1974年 | 69篇 |
1973年 | 68篇 |
1972年 | 63篇 |
1971年 | 56篇 |
1968年 | 48篇 |
排序方式: 共有7159条查询结果,搜索用时 15 毫秒
111.
Aphids of Schizaphis graminum (Rondani) (biotype C) reared on its host-plant, Sorghum bicolor (L.) Moench, sequestered campesterol, stigmasterol and sitosterol. Aphids reared for 72 hr on holidic diets supplemented with [4-14C]-sitosterol contained both [14C]-sitosterol and [14C]-cholesterol, indicating that these aphids are capable of dealkylation at C-24. When aphids were reared on artificial diets containing [2-14C]-mevalonic acid, no detectable amounts of radioactively labelled desmethyl sterols, nor metabolic intermediates in sterol synthesis (i.e. squalene, 2,3-oxidosqualene, 4,4-dimethyl and 4-monomethyl sterols) were found to accumulate in their tissues. The relevance of these findings to previous research suggesting the ability of aphids, via their symbiotes, to synthesize sterols is discussed. 相似文献
112.
Growth of Chikungunya Virus in Baby Hamster Kidney Cell (BHK-21-Clone 13) Suspension Cultures 总被引:2,自引:2,他引:0 下载免费PDF全文
This report describes the methods used to obtain high titers of chikungunya virus with suspension cultures of BHK-21-clone 13 cells. The cells were grown at 37 C to a cell concentration of 10(6) to 2 x 10(6) per ml. After maximum cell growth, the cells were inoculated with chikungunya virus at a multiplicity of 1 to 2 50% suckling mouse intracerebral lethal doses (SMICLD(50)) per cell in the spent Eagle's minimum essential medium for suspension cultures (MEMS), or the cell cultures were centrifuged at 200 x g and resuspended in either fresh MEMS or medium 199 prior to inoculation. The medium used had no effect on virus titer. The inoculated cultures were incubated at 34 C until the cell viability dropped to 30%, which usually occurred 28 to 30 hr postinoculation. After these procedures, chikungunya virus titers of log(10) 10.3 to 11.8 SMICLD(50) per ml were obtained. 相似文献
113.
The aggregation of trypsinized BHK21 cells 总被引:2,自引:0,他引:2
114.
Dissociation by Chelating Agents and Substructure of the Thermophilic Bacteriophage TP84 总被引:2,自引:1,他引:1 下载免费PDF全文
The thermophilic bacteriophage TP84 is dissociated into its head, tail, and released deoxyribonucleic acid (DNA) by chelating agents such as ethylenediaminetetraacetic acid (EDTA) and phosphate. The phage is more sensitive to EDTA than to phosphate, and dialysis against either agent causes more effective dissociation than standing in their presence. The tail possesses a knobbed structure which is inserted into the head of the intact phage and to which the DNA appears to be attached. The method of dissociating TP84 described in this paper provides a source of undamaged structural components and intact strands of DNA for subsequent investigations. A possible mechanism of chelate inactivation is discussed. 相似文献
115.
The methodology and some of the requirements for the deoxyribonucleic acid-mediated transformation of an arginine auxotroph of Bacillus amyloliquefaciens to prototrophy are described. 相似文献
116.
Primitive erythropoiesis in early chick embryogenesis. II. Correlation between hemoglobin synthesis and the mitotic history 总被引:7,自引:3,他引:4 下载免费PDF全文
Primitive erythroblasts in the circulating blood of the chick embryo continue to divide while synthesizing hemoglobin (Hb). Hb measurements on successive generations of erythroblasts show that there is a progressive increase in the Hb content of both interphase and metaphase cells. Furthermore, for any given embryo the Hb content of metaphase cells is always significantly greater than that of interphase cells. The distribution of Hb values for metaphase cells suggests that there are six Hb classes corresponding to the number of cell cycles in the proliferative phase. The location of erythroblasts in the cell cycle was determined by combining Feulgen cytophotometry with thymidine radioautography on the same cells. Measurements of the Hb content for erythroblasts in different compartments of the cell cycle (G1, S, G2, and M) show a progressive increase through the cycle. Thus, the amount of Hb per cell is a function of the number of cell divisions since the initiation of Hb synthesis and, to a lesser degree, the stage of the cell cycle. Earlier generations of erythroblasts synthesize Hb at a faster rate than the terminal generation. Several models have been proposed to explain these findings. 相似文献
117.
An extensive study of adult and developing smooth muscle has revealed the widespread occurrence of a distinct filament with an average diameter of about 100 A (termed the 100 A filament). Unlike that of myofilaments, their appearance in longitudinal section is uniform, but in transverse section they have a round profile, occasionally exhibiting a less electron-opaque core. The 100 A filaments are almost invariably preserved under a variety of fixation procedures, whereas myofilaments, particularly the thicker filaments, are preserved inconsistently. The 100 A filaments appear to be randomly oriented throughout the cytoplasm, either singly or in small groups, although they are sometimes concentrated in the juxtanuclear region of the smooth muscle cells. The intimate association of 100 A filaments with dark bodies, in both developing and adult smooth muscle cells, may indicate that these filaments either play a role in dark body formation or, at least, constitute a part of the dark body. The 100 A filaments are conspicuous in developing smooth muscle cells and occasionally form networks or clusters; they appear to decrease in relative number as maturation proceeds, but considerable numbers are still present in adult tissue. 相似文献
118.
Arginine metabolism in insects. Role of arginase in proline formation during silkmoth development 总被引:4,自引:4,他引:0 下载免费PDF全文
1. Ornithine delta-transaminase (l-ornithine-2-oxo acid aminotransferase, EC 2.6.1.13) and Delta(1)-pyrroline-5-carboxylate reductase [l-proline-NAD(P) 5-oxidoreductase, EC 1.5.1.2] were demonstrated in fat-body and flight-muscle tissues of the silkmoth Hyalophora gloveri. Arginase (l-arginine ureohydrolase, EC 3.5.3.1) is also present in these tissues. 2. Arginase, ornithine transaminase and pyrroline-carboxylate reductase are generally considered to make up the catabolic pathway for the conversion of arginine into proline. The conversion of l-[U-(14)C]arginine into [(14)C]proline by intact fat-body tissue was used to show that the enzymes in insect fat body also function in this capacity. 3. Of the three enzymes of the catabolic pathway, only arginase increased during adult development and the increase coincided with the emergence of the winged adult moth. Since proline appears to be a major substrate utilized in insect flight metabolism, the increase in arginase activity at this stage suggests a major role for arginase in proline formation. 相似文献
119.
A. W. Asscher M. Sussman W. E. Waters Joy A. S. Evans H. Campbell K. T. Evans J. Edmund Williams 《BMJ (Clinical research ed.)》1969,1(5647):804-806
Short courses of nitrofurantoin and ampicillin produced an immediate cure in 80% of adult non-pregnant bacteriuric women. Of the subjects so treated, 55% remained cured at the end of one year. Over the same follow-up period 36% of untreated bacteriuric women developed a spontaneous remission of bacteriuria. Treatment failed to prevent the development of symptomatic infection, and the reinfections which followed successful treatment were more commonly associated with the development of symptoms than the persistent or relapsing infections in untreated or unsuccessfully treated subjects.It is concluded that a search for bacteriuria in non-pregnant women is unlikely to be of value as a preventive measure, since in many instances it fails to detect urinary tract infection at an early stage and since treatment by methods suitable for large-scale use is ineffective. 相似文献
120.
The incidence of impaired renal concentrating ability in pregnant women with asymptomatic significant bacteriuria is significantly less than previously reported when osmolality readings are performed on every urine specimen obtained during the 24 hours'' deprivation of fluids. The concentrating defect is more considerable as pregnancy progresses. The lower the maximum urinary osmolality the more difficult is the treatment of the patient, and the higher the incidence of acute pyelonephritis. 相似文献