全文获取类型
收费全文 | 4429篇 |
免费 | 367篇 |
国内免费 | 5篇 |
出版年
2023年 | 18篇 |
2022年 | 24篇 |
2021年 | 67篇 |
2020年 | 42篇 |
2019年 | 48篇 |
2018年 | 63篇 |
2017年 | 51篇 |
2016年 | 112篇 |
2015年 | 184篇 |
2014年 | 198篇 |
2013年 | 232篇 |
2012年 | 327篇 |
2011年 | 312篇 |
2010年 | 186篇 |
2009年 | 183篇 |
2008年 | 264篇 |
2007年 | 278篇 |
2006年 | 230篇 |
2005年 | 249篇 |
2004年 | 226篇 |
2003年 | 225篇 |
2002年 | 216篇 |
2001年 | 60篇 |
2000年 | 64篇 |
1999年 | 73篇 |
1998年 | 59篇 |
1997年 | 39篇 |
1996年 | 32篇 |
1995年 | 52篇 |
1994年 | 51篇 |
1993年 | 39篇 |
1992年 | 45篇 |
1991年 | 42篇 |
1990年 | 37篇 |
1989年 | 46篇 |
1988年 | 20篇 |
1987年 | 20篇 |
1986年 | 21篇 |
1985年 | 36篇 |
1984年 | 22篇 |
1983年 | 29篇 |
1982年 | 19篇 |
1981年 | 19篇 |
1980年 | 9篇 |
1979年 | 17篇 |
1978年 | 16篇 |
1977年 | 10篇 |
1973年 | 13篇 |
1971年 | 14篇 |
1965年 | 9篇 |
排序方式: 共有4801条查询结果,搜索用时 15 毫秒
1.
2.
3.
A. Korte I. Ribera R. G. Beutel D. Bernhard 《Journal of Zoological Systematics and Evolutionary Research》2004,42(4):281-288
The series Staphyliniformia is one of the mega‐diverse groups of Coleoptera, but the relationships among the main families are still poorly understood. In this paper we address the interrelationships of staphyliniform groups, with special emphasis on Hydrophiloidea and Hydraenidae, based on partial sequences of the ribosomal genes 18S rDNA and 28S rDNA. Sequence data were analysed with parsimony and Bayesian posterior probabilities, in an attempt to overcome the likely effect of some branches longer than the 95% cumulative probability of the estimated normal distribution of the path lengths of the species. The inter‐family relationships in the trees obtained with both methods were in general poorly supported, although most of the results based on the sequence data are in good agreement with morphological studies. In none of our analyses a close relationship between Hydraenidae and Hydrophiloidea was supported, contrary to the traditional view but in agreement with recent morphological investigations. Hydraenidae form a clade with Ptiliidae and Scydmaenidae in the tree obtained with Bayesian probabilities, but are placed as basal group of Staphyliniformia (with Silphidae as subordinate group) in the parsimony tree. Based on the analysed data with a limited set of outgroups Scarabaeoidea are nested within Staphyliniformia. However, this needs further support. Hydrophiloidea s.str., Sphaeridiinae, Histeroidea (Histeridae + Sphaeritidae), and all staphylinoid families included are confirmed as monophyletic, with the exception of Hydraenidae in the parsimony tree. Spercheidae are not a basal group within Hydrophiloidea, as has been previously suggested, but included in a polytomy with other Hydrophilidae in the Bayesian analyses, or its sistergroup (with the inclusion of Epimetopidae) in the parsimony tree. Helophorus is placed at the base of Hydrophiloidea in the parsimony tree. The monophyly of Hydrophiloidea s.l. (including the histeroid families) and Staphylinoidea could not be confirmed by the analysed data. Some results, such as a placement of Silphidae as subordinate group of Hydraenidae (parsimony tree), or a sistergroup relationship between Ptiliidae and Scydmaenidae, appear unlikely from a morphological point of view. 相似文献
4.
5.
An attempt has been made to study the extent and nature of the damage occurring in adult Nippostrongylus brasiliensis undergoing immune expulsion from the rat. It was found that worms are not killed nor irreparably damaged when being rejected. On transfer into naive second recipient rats the rate of re-establishment of worms previously incubated in immune rat recipients for 4-17 hr was high (68-69%) and comparable to that shown by worms from normal recipient rats (48-56%). Similarly, worms taken on days 10, 11, and 12 of a primary infection, already passed to the distal half of the small intestine due to immune expulsion effects, on transfer into naive recipient rats re-established themselves well (rates varying from 62 to 80%) compared to those harvested from their normal habitat in the proximal half of the small intestine (rates varying from 44 to 87%). Worm damage is associated with decreased motility and impaired locomotion capacity. The phenomenon of mucosal trapping occurs during expulsion, but merely to the extent of some 30% of the worm population. It is suggested that in principle, worms subjected to immune expulsion are in a state of acute, transient metabolic crisis. The present results support the enteroallergic indirect mechanism for worm rejection. 相似文献
6.
Donal A. Hickey Bernhard F. Benkel Poppo H. Boer Yves Genest Sumaia Abukashawa Gerard Ben-David 《Journal of molecular evolution》1987,26(3):252-256
Summary We constructed a cDNA library for the beetle,Tribolium castaneum. This library was screened using a cloned amylase gene fromDrosophila melanogaster as a molecular probe. Beetle amylase cDNA clones were isolated from this bank, and the nucleotide sequence was obtained for a cDNA clone with a coding capacity for 228 amino acids. Both the nucleotide sequence and predicted amino acid sequence were compared to our recent results forD. melanogaster alpha-amylases, along with published sequences for other alpha-amylases. The results show that animal alpha-amylases are highly conserved over their entire length. A borader comparison, which includes plant and microbial alpha-amylase sequences, indicates that parts of the gene are conserved between prokaryotes, plants, and animals. We discuss the potential importance of this and other enzyme-coding genes for the construction of molecular phylogenies and for the study of the general question of molecular clocks in evolution. 相似文献
7.
I Welpe W Bernhard 《Anthropologischer Anzeiger; Bericht über die biologisch-anthropologische Literatur》1987,45(3):275-287
In a cross-sectional study of 452 girls between 10 and 16 years of age 36 indices of physical and 50 of mental development were tested for their correlation with age at menarche and chronological age, as well as for their predictive power for estimating menarche by multiple regression analysis. Indices of physical maturity and body weight when adjusted for chronological age showed the highest partial correlation coefficients with age at menarche. Among mental characters which show lower intercorrelations with menarche occurred the highest correlation coefficients for a handmotor factor "Spurennachzeichnen" and a factor "Gruppenabh?ngigkeit" (which indicates a type of social motivation). In general physical and mental factors correlate higher with chronological age than with age at menarche. By multiple regression analysis we determined 14 physical and 25 mental predictors explaining 21% and 17% respectively of the variance of age at menarche. The error of the estimate predicting menarche on body weight without knowledge of onset of menarche is +/- 1 year. Using chronological age in a sample of girls before menarche the error of the estimate only is +/- 6 months. To compare the predictive power of chronological age combined with body weight or with skeletal age the time interval is calculated within which 95% of girls attain menarche. The range of prediction extends from 4.3 to 1 year on chronological age (11-16 years); using mean body weight it can be improved by 1.8 to 6 months, while using mean skeletal age an improvement of 0.2 to 3.9 months is possible compared with body weight. The correlations between age at menarche and physical and mental variables are attributed to a common hormonal influence on rate of development. 相似文献
8.
Summary Chromatin fractions from Friend erythroleukemia cells after induction of differentiation by dimethylsulfoxide (DMSO) were compared in their biochemical characteristics to fractions from uninduced cells. Fractions were prepared by extracting chromatin from nuclei after mild micrococcal nuclease treatment with increasing concentrations of NaCl according to Sanders [1]. This procedure has been found to release chromatin containing hyperacetylated histones preferentially [2]. The fractions obtained by this procedure were analysed in respect to the amount of chromatin released, the amount of histone H1, the degree of acetylation of histone H4, the presence of non-histone proteins and the concentration of transcribed and non-transcribed sequences. It was found that the fractions differ in the amount of histone H1 present, in several non-histone proteins and in the acetylation of histonie H4, regardless whether induced or uninduced cells were analysed. The distribution of transcribed sequences versus non-transcribed sequences among the fractions was the same, demonstrating that this fractionation procedure, although leading to fractions with biochemical differences, is not able to discriminate functional states of chromatin and that the biochemical characteristics of the fractions may be common to both, active as well as inactive states of chromatin. 相似文献
9.
A new strictly anaerobic bacterium was isolated from an enrichment culture with glutarate as sole substrate and freshwater
sediment as inoculum, however, glutarate was not metabolized by the pure culture. The isolate was a mesophilic, spore-forming,
Gram-negative, motile curved rod. It fermented various organic acids, alcohols, fructose, acetoin, and H2/CO2 to acetate, usually as the only product. Other acids were fermented to acetate and propionate or acetate and butyrate. Succinate
and malonate were decarboxylated to propionate or acetate, respectively, and served as sole sources of carbon and energy for
growth. No inorganic electron acceptors except CO2 were reduced. Yeast extract (0.05% w/v) was required for growth. Small amounts of cytochrome b were detected in membrane
fractions. The guanine-plus-cytosine content of the DNA was 44.1±2 mol%. The isolate is described as a new species of the
genus Sporomusa, S. malonica. 相似文献
10.
Effects of serum type on growth and permeability properties of cultured endothelial cells 总被引:2,自引:0,他引:2
Bernhard Hennig Gilbert A. Boissonneault Howard P. Glauert 《Experimental cell research》1989,181(2):589-596
Serum is frequently added to defined basal media as a source of certain nutrients and macromolecular growth factors essential for cell growth. The many different sera commercially available may not be equally suitable for all cell types. The effects of four sera, fetal bovine serum (FBS), calf bovine serum (CS), equine serum (ES-1), and plasma-derived equine serum (ES-2), on growth and permeability properties of cultured porcine endothelial cells were determined. The rate of DNA synthesis, measured as [3H]thymidine incorporation, reached a peak at around 24 h, regardless of serum type, and was most marked with ES-1- or ES-2-treated cells. However, when estimated by total DNA, FBS, CS, or ES-1 treatment resulted in greater cell proliferation than ES-2. Based on protein synthetic rate and total cell protein, both FBS and CS appeared to be most growth supporting. At 72 h after cell plating, albumin passage across cultured endothelial monolayers was elevated in ES-1- and ES-2-treated cells compared with FBS- or CS-treated cells. "Leaky" cell monolayers were most marked with ES-1-treated cells. Cells grown in ES-2- and particularly in ES-1-enriched media were larger and more spindle-shaped compared with the typical cobblestone appearance of cells cultured in media enriched with either FBS or CS. These data suggest that CS, but not ES-1 or ES-2, is an excellent substitute for FBS to support desirable growth properties of macrovascular endothelial cells in culture. 相似文献