全文获取类型
收费全文 | 2645篇 |
免费 | 340篇 |
国内免费 | 2篇 |
出版年
2021年 | 26篇 |
2019年 | 28篇 |
2017年 | 27篇 |
2016年 | 35篇 |
2015年 | 45篇 |
2014年 | 77篇 |
2013年 | 93篇 |
2012年 | 110篇 |
2011年 | 93篇 |
2010年 | 84篇 |
2009年 | 77篇 |
2008年 | 108篇 |
2007年 | 116篇 |
2006年 | 99篇 |
2005年 | 106篇 |
2004年 | 111篇 |
2003年 | 72篇 |
2002年 | 88篇 |
2001年 | 79篇 |
2000年 | 79篇 |
1999年 | 63篇 |
1998年 | 56篇 |
1997年 | 42篇 |
1996年 | 33篇 |
1995年 | 30篇 |
1994年 | 30篇 |
1993年 | 26篇 |
1992年 | 65篇 |
1991年 | 64篇 |
1990年 | 56篇 |
1989年 | 64篇 |
1988年 | 52篇 |
1987年 | 47篇 |
1986年 | 40篇 |
1985年 | 49篇 |
1984年 | 44篇 |
1983年 | 36篇 |
1982年 | 33篇 |
1981年 | 27篇 |
1980年 | 27篇 |
1979年 | 31篇 |
1978年 | 27篇 |
1977年 | 25篇 |
1976年 | 35篇 |
1975年 | 33篇 |
1974年 | 26篇 |
1973年 | 24篇 |
1972年 | 39篇 |
1969年 | 25篇 |
1968年 | 24篇 |
排序方式: 共有2987条查询结果,搜索用时 31 毫秒
111.
112.
Liposomes containing 111In-labelled bleomycin were injected intravenously into normal and tumour-bearing rodents and the fate of radioactivity followed. 111In levels in tissues retained their maximum values for up to 48h after treatment thereby enabling accurate estimations of tissue participation which with a variety of tumours (Meth ‘A’, 6C3HED, Lewis lung carcinoma and Novikoff hepatoma) in mice and rats was secondary to that of the liver and spleen. Reductions in the size of liposomes decreased liver and spleen participation and increased tumour and kidney involvement. Uptake by lungs, skeletal muscle and brain was also augmented albeit to a lesser extent. Incorporation of anti-Meth ‘A’ cells IgG immunoglobulin into the liposomal carrier led to a modest increase in the uptake of co-entrapped 111In by the Meth ‘A’ tumour implanted subcutaneously. Although at the same time, liposomal IgG reduced uptake by the kidney, it effected a drastic increase in hepatic and splenic involvement. This could be prevented by the concurrent administration of excess “empty” liposomes which, however, did not interfere with uptake by tumour tissue. 相似文献
113.
The single envelope glycoprotein of vesicular stomatitis virus was used as a specific probe of glycosyltransferase activities in fibroblasts from two cystic fibrosis patients, an obligate heterozygous carrier and a normal individual. Gel filtration of pronasedigested glycopeptides from both purified virions and infected cell-associated VSV glycoprotein which had been labeled with [3H] glucosamine did not reveal any significant differences in the glycosylation patterns between the different cell cultures. All 4 cell lines were apparently able to synthesize the mannose- and glucosamine-containing core structure and branch chains terminating in sialic acid which are characteristic of asparagine-linked carbohydrate side chains in cellular glycoproteins. Analysis of tryptic glycopeptides by anion-exchange chromotography indicated that the same 2 major sites on the virus polypeptide were recognized and glycosylated in all 4 VSV-infected cell cultures. These studies suggest that the basic biochemical defect(s) in cystic fibrosis is not an absence or deficiency in enzymes responsible for the biosynthesis of complex carbohydrate side chains. 相似文献
114.
115.
The aunting behavior in a captive group of 22 squirrel monkeys containing three infants was done in terms of the age/sex classification
of those animals involved. The time course of the aunting phenomena and the type and intensity of the interactions between
the mothers and the aunts were recorded. Males as well as females were observed to ascertain if the babysitters were sex specific.
Observations were gathered before, during, and after a particular threat to any monkey who was carrying an infant. Three categories
of protective behavior (protect, retreat, and nothing) were tabulated. The results indicated that most aunting and protection
occurred between infant ages 2-1/2-5 weeks when the infants were growing rapidly but not as yet socially self-sufficient.
Mothers protected infants the most against juveniles, then subadult males, and least against other adult females. Subadult
males were occasionally observed to carry and protect older infants. Aunting behavior was discussed in terms of the selective
pressures by which it may have evolved. 相似文献
116.
Employing in situ SCUBA methods a seasonal carbon budget has been established for aLaminaria saccharina population in a Scottish sea-loch. Concurrent studies of photosynthesis, secretion rates, reserve fluctuations and frond growth were undertaken. Net annual production is in excess of 120 g C m–2 yr–1. Over 13% of gross carbon input is released as extracellular secretions (over 30% in autumn) and 40–50% is lost by distal decay, entering detrital food chains. The large concentrations of laminarin, synthesised in summer months, are nearly all lost in autumn-winter distal tissue loss and therefore not available for early spring growth. 相似文献
117.
A pregnancy defect in the osteopetrotic (op/op) mouse demonstrates the requirement for CSF-1 in female fertility. 总被引:20,自引:0,他引:20
Correlative evidence suggests that maternal production of the mononuclear phagocyte growth factor colony stimulating factor-1 (CSF-1) regulates placental development. In order to study the role of CSF-1 in pregnancy the fertility of CSF-1-less osteopetrotic (op/op) mutant mice was investigated. Homozygous mutant crosses (op/op x op/op) were consistently infertile. As expected, op/op males were almost completely fertile when crossed with heterozygous females. Surprisingly, op/op females when mated to heterozygote males were fertile, although at a rate that was 46% of the rate for +/op females x op/op males. These data suggest that CSF-1 is required for pregnancy. However, a maternal CSF-1 source is not absolutely necessary in that pregnancies involving +/op fathers were partially rescued, suggesting that +/op fetuses and/or +/op seminal fluid provides CSF-1 or CSF-1-induced factors which compensate for the absence of maternally produced CSF-1. Despite the complete absence of CSF-1 in the uterus and placenta of op/op mice placental weights were normal, suggesting that proliferation of decidual cells and trophoblasts, both of which express the CSF-1 receptor, may not be solely regulated by CSF-1. Histochemical staining for F4/80 antigen was used to identify macrophages in the uterus and placenta. Uterine macrophages could not be detected in virgin op/op mice although they were abundant in +/op uteri. Interestingly, macrophages could be detected in op/op uteri as uncharacteristically rounded cells in early gestation, however, they were not maintained and no macrophages were apparent beyond Day 14 of pregnancy in op/op mice. Further studies in the osteopetrotic mouse will be useful in delineating those functions required for pregnancy that are regulated by CSF-1. 相似文献
118.
J W Brown E S Haas B D James D A Hunt J S Liu N R Pace 《Journal of bacteriology》1991,173(12):3855-3863
The secondary structures of the eubacterial RNase P RNAs are being elucidated by a phylogenetic comparative approach. Sequences of genes encoding RNase P RNA from each of the recognized subgroups (alpha, beta, gamma, and delta) of the proteobacteria have now been determined. These sequences allow the refinement, to nearly the base pair level, of the phylogenetic model for RNase P RNA secondary structure. Evolutionary change among the RNase P RNAs was found to occur primarily in four discrete structural domains that are peripheral to a highly conserved core structure. The new sequences were used to examine critically the proposed similarity (C. Guerrier-Takada, N. Lumelsky, and S. Altman, Science 246:1578-1584, 1989) between a portion of RNase P RNA and the "exit site" of the 23S rRNA of Escherichia coli. Phylogenetic comparisons indicate that these sequences are not homologous and that any similarity in the structures is, at best, tenuous. 相似文献
119.
120.
M. Chartrain G. Hunt L. Horn A. Kirpekar D. Mathre A. Powell L. Wassel J. Nielsen B. Buckland R. Greasham 《Journal of industrial microbiology & biotechnology》1991,7(4):293-299
Summary An efrotomycin fermentation was characterized through physical, chemical and biochemical studies. Growth of the actinomycete,Nocardia lactamdurans occurred during the first 50 h of the fermentation cycle at the expense of glucose, protein, and triglycerides. The initiation of efrotomycin biosynthesis was observed when glucose dropped to a low concentration. Upon glucose depletion, cell growth ceased and a switch in the respiratory quotient occurred. Efrotomycin biosynthesis was supported by the utilization of soybean oil and starch. Analysis of triglyceride metabolism showed that no diglycerides or monoglycerides accumulated during the fermentation. The activity of extracellular enzymes (lipase, protease, and amylase) increase during the cell growth phase and decreased significantly after 150 h. The concentrations of DNA, tetrahydro-vitamin K2 (a membrane component), and free amino acids in the supernatant increased dramatically late in the fermentation cycle (225 h), indicating massive cell lysis. During this same time period, a reduction in cellular respiratory activity and efrotomycin biosynthesis were observed. 相似文献