全文获取类型
收费全文 | 38060篇 |
免费 | 2521篇 |
国内免费 | 3495篇 |
专业分类
44076篇 |
出版年
2024年 | 108篇 |
2023年 | 499篇 |
2022年 | 829篇 |
2021年 | 1061篇 |
2020年 | 958篇 |
2019年 | 1360篇 |
2018年 | 1341篇 |
2017年 | 923篇 |
2016年 | 1010篇 |
2015年 | 1215篇 |
2014年 | 2333篇 |
2013年 | 2703篇 |
2012年 | 1823篇 |
2011年 | 2314篇 |
2010年 | 1762篇 |
2009年 | 1931篇 |
2008年 | 2062篇 |
2007年 | 2248篇 |
2006年 | 1873篇 |
2005年 | 1758篇 |
2004年 | 1481篇 |
2003年 | 1364篇 |
2002年 | 1236篇 |
2001年 | 834篇 |
2000年 | 794篇 |
1999年 | 767篇 |
1998年 | 802篇 |
1997年 | 616篇 |
1996年 | 601篇 |
1995年 | 595篇 |
1994年 | 516篇 |
1993年 | 456篇 |
1992年 | 406篇 |
1991年 | 369篇 |
1990年 | 304篇 |
1989年 | 294篇 |
1988年 | 223篇 |
1987年 | 219篇 |
1986年 | 160篇 |
1985年 | 240篇 |
1984年 | 336篇 |
1983年 | 240篇 |
1982年 | 254篇 |
1981年 | 181篇 |
1980年 | 143篇 |
1979年 | 139篇 |
1978年 | 108篇 |
1977年 | 59篇 |
1976年 | 62篇 |
1974年 | 50篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
M J Schilstra J W Slot P H van der Meide G Posthuma A F Cremers L Bosch 《FEBS letters》1984,165(2):175-179
The localization of the elongation factor Tu (EF-Tu) in ultrathin cryosections of E. coli cells was determined with the electron microscope using a highly specific immunological labelling technique. EF-Tu is distributed almost homogeneously throughout the cytoplasm. Although it has often been suggested that EF-Tu could be part of a putative prokaryotic cytoskeleton, we did not find any evidence for supramolecular assemblies, such as fibres or filaments, containing a large amount of EF-Tu. EF-Tu was not observed in association with the outer cell membrane and periplasmic space. A topological relationship with the inner membrane is not apparent in our micrographs. In cells in which the EF-Tu level is raised significantly, the protein piles up in discrete cell regions. 相似文献
32.
Barbara D. Abbott Robert M. Pratt 《In vitro cellular & developmental biology. Plant》1988,24(4):343-352
Summary Retinoids and growth factors seem to be important for normal mammalian reproduction and development. High levels of retinoic
acid are teratogenic and induce cleft palate in the mouse. Little is known concerning the mechanisms through which retinoids
induce cleft palate. Palatal epithelia from CD-1 embryonic mice on Day 12 of gestation were isolated from the mesenchyme and
cultured in serum-free media, with all-trans retinoic acid or 13-cis retinoic acid, with or without epidermal growth factor
(EGF). The epithelia attached and grew, and the cells differentiated over a 72-h culture period. Binding of [125I]EGF was observed in all cultures in a pattern that correlated with thymidine (TdR) uptake by the epithelia. EGF enhanced
growth and [3H]TdR incorporation of the oral cells, but nasal cells generally did not proliferate. In this culture system, both retinoids
suppressed [3H]TdR incorporation in a concentration-dependent manner for epithelia cultured with or without EGF. Medial cells are important
to normal palatogenesis as they play a role in fusion of opposing shelves and subsequently many of these cells undergo programmed
cell death. Death of medial cells in vitro is prevented by EGF and by the retinoids, either with or without EGF. This response
occurs in the absence of a mesenchymal interaction, suggesting that the medial cell response to EGF and retinoids is not mediated
by or dependent on the mesenchymal tissues. The survival of medial cells may be responsible for the failure of opposing shelves
to fuse. 相似文献
33.
E. Frankel 《Biometrical journal. Biometrische Zeitschrift》1984,26(7):741-748
In the present paper we present an example proving that the Varimax rotation method commonly used in factor analysis (see H. F. KAISER, 1958) need not give a unique solution. We also show that changing the original algorithm more than one local maximum of the criterion “V” can be achieved. Hence the loadings of factors and practical interpretations of solutions are affected in this way. 相似文献
34.
Growth hormone-releasing factor (GRF) is a hypothalamic peptide named for its ability to induce release of growth hormone from the anterior pituitary. GRF also acts as a neurotransmitter in the suprachiasmatic nucleus/medial preoptic area (SCN/MPOA) to stimulate food intake. The purpose of this series of experiments was to explore the nature of GRF-induced feeding, with a particular emphasis on macronutrient selectivity, and to examine the role of opiate activity in the paraventricular nucleus of the hypothalamus (PVN). Chow intake stimulated by GRF microinjection (1 pmol/0.5 μl) into the SCN/MPOA was blocked by injection of methyl-naltrexone (3 μg/0.5 μl) into the PVN. In animals habituated to macronutrient diets (Teklad, WI), GRF preferentially stimulated intake of protein at 2 and 4 h postinjection, whereas it had no effect on carbohydrate intake. Further, this effect was blocked by injection of naloxone (40 nmol/0.5 μl) into the PVN. Microinjection of morphine (0, 1, 10, and 17 μg/0.5 μl) into the PVN also specifically stimulated protein intake at 2 and 4 h postinjection. These results suggest that feeding derived from GRF actions in the SCN/MPOA is macronutrient selective, and is dependent on PVN opiate activity for expression. 相似文献
35.
36.
37.
G. J. Venter J. T. Paweska A. A. Van Dijk P. S. Mellor W. J. Table Ick 《Medical and veterinary entomology》1998,12(4):378-385
Abstract .The susceptibility of field-collected Culicoides bolitinos to infection by oral ingestion of bluetongue virus serotypes 1, 3 and 4 (BLU 1, 3 and 4) was compared with that of field-collected C. imicola and laboratory reared C. variipennis sonorensis . The concentration of the virus per millilitre of bloodmeal was 105.0 and 106.0 TCID50 for BLU 4 and 107.2 TCID50 for BLU 1 and 3. Of 4927 C. bolitinos and 9585 C. imicola fed, 386 and 287 individual midges survived 10 days extrinsic incubation, respectively. Midges were assayed for the presence of virus using a microtitration assay on BHK-21 cells and/or an antigen capture ELISA. Infection prevalences for the different serotypes as determined by virus isolation ranged from 22.7 to 82.0% in C. bolitinos and from 1.9 to 9.8% in C. imicola; infection prevalences were highest for BLU 1, and lowest for BLU 4 in both species. The mean log10 TCID50 titre of the three BLU viruses per single fly was higher in C. bolitinos than in C. imicola . The results suggested that C. bolitinos populations are capable vectors of the BLU viruses in South Africa. A high correlation was found between virus isolation and ELISA results for the detection of BLU 1, and less for BLU 4; the ELISA failed to detect the presence of BLU 3 in infected flies. The C. v. sonorensis colonies had a significantly lower susceptibility to infection with BLU 1, 3 and 4 than C. bolitinos and C. imicola . However, since infection prevalence of C. v. sonorensis was determined only by ELISA, this finding may merely reflect the insensitivity of this assay at low virus titres, compared to virus isolation. 相似文献
38.
Abstract: Schwannoma-derived growth factor (SDGF) is a potent mitogen and neuronal differentiation factor. Because of its relationship to epidermal growth factor (EGF) and the heregulins, it was asked if SDGF interacts with the EGF receptor or HER2/neu. SDGF binds to and causes the phosphorylation on tyrosine of the EGF receptor but not HER2/neu. 相似文献
39.
Adventitious root formation in vitro in 1-mm stem slices cut from microshoots of apple cv. Jork 9 was studied using light
and electron microscopy. When indole-3-butyric acid (IBA) had been added to the medium, starch grains accumulated during the
first 24 h of culture in cells of the cambial region and in cells in the vicinity of vascular tissue and in the primary rays.
This accumulation occurred only in the basal part of explants. After that, the nuclei in these cells were activated, and the
density of the cytoplasm and the number of cell organelles increased, whereas starch was broken down. Cambium cells started
to divide transversely and at 96 h, after several divisions, a continuous ring of isodiametric cytoplasmic cells had appeared
around the xylem near the basal cutting surface. The cells in this ring were rich in cell structures, and did not contain
large starch grains and a central vacuole. Root meristemoids regenerated from the portions of the ring that were localized
in the primary rays. From the other cells in the ring, callus developed. The meristemoids did not grow into the direction
of the epidermis as in shoots, but along the vascular bundles. After emergence from the cutting surface, the meristemoids
were transformed into small, dome-like primordia. They developed a typical root apex with root cap, root ground meristem and
tracheid connection with shoot vascular tissue.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
40.
Summary Video-densitometric DNA measurements of Feulgenstained tissues of 42 day old eggs of the corn snake,Elaphe g. guttata (Columbridae, Serpentes), revealed a basic DNA content of 2C=2.17 pg, with somatic polyploidy in the allantois, the chorioallontois, the yolk sac, and other extraembryonic membranes. The maximum value determined was 128C (in binucleate cells 2×128C) at the distal pole of the egg. This is the first report of somatic polyploidy in a snake, and one of the first in reptiles in general. 相似文献