全文获取类型
收费全文 | 644篇 |
免费 | 71篇 |
专业分类
715篇 |
出版年
2023年 | 5篇 |
2022年 | 6篇 |
2021年 | 16篇 |
2020年 | 9篇 |
2019年 | 10篇 |
2018年 | 15篇 |
2017年 | 7篇 |
2016年 | 25篇 |
2015年 | 30篇 |
2014年 | 26篇 |
2013年 | 38篇 |
2012年 | 47篇 |
2011年 | 32篇 |
2010年 | 24篇 |
2009年 | 23篇 |
2008年 | 35篇 |
2007年 | 28篇 |
2006年 | 38篇 |
2005年 | 17篇 |
2004年 | 19篇 |
2003年 | 21篇 |
2002年 | 25篇 |
2001年 | 15篇 |
2000年 | 14篇 |
1999年 | 13篇 |
1998年 | 4篇 |
1997年 | 6篇 |
1996年 | 9篇 |
1995年 | 8篇 |
1994年 | 3篇 |
1993年 | 4篇 |
1992年 | 14篇 |
1991年 | 16篇 |
1990年 | 18篇 |
1989年 | 5篇 |
1988年 | 8篇 |
1987年 | 6篇 |
1986年 | 5篇 |
1985年 | 6篇 |
1984年 | 4篇 |
1983年 | 10篇 |
1982年 | 7篇 |
1981年 | 6篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1978年 | 5篇 |
1977年 | 10篇 |
1976年 | 4篇 |
1973年 | 3篇 |
1972年 | 2篇 |
排序方式: 共有715条查询结果,搜索用时 0 毫秒
1.
A detailed morphological staging system for cattle embryos at stages following blastocyst hatching and preceding gastrulation is presented here together with spatiotemporal mapping of gene expression for BMP4, BRACHYURY, CERBERUS1 (CER1), CRIPTO, EOMESODERMIN, FURIN and NODAL. Five stages are defined based on distinct developmental events. The first of these is the differentiation of the visceral hypoblast underlying the epiblast, from the parietal hypoblast underlying the mural trophoblast. The second concerns the formation of an asymmetrically positioned, morphologically recognisable region within the visceral hypoblast that is marked by the presence of CER1 and absence of BMP4 expression. We have termed this the anterior visceral hypoblast or AVH. Intra-epiblast cavity formation and the disappearance of the polar trophoblast overlying the epiblast (Rauber’s layer) have been mapped in relation to AVH formation. The third chronological event involves the transition of the epiblast into the embryonic ectoderm with concomitant onset of posterior NODAL, EOMES and BRACHYURY expression. Lastly, gastrulation commences as the posterior medial embryonic ectoderm layer thickens to form the primitive streak and cells ingress between the embryonic ectoderm and hypoblast. At this stage a novel domain of CER1 expression is seen whereas the AVH disappears. Comparison with the mouse reveals that while gene expression patterns at the onset of gastrulation are well conserved, asymmetry establishment, which relies on extraembryonic tissues such as the hypoblast and trophoblast, has diverged in terms of both gene expression and morphology. 相似文献
2.
J. L. van Leeuwen 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》1999,354(1385):839-840
Preface to Mechanisms of neuromuscular control. A Theme organized and edited by J. L. van Leeuwen. 相似文献
3.
W. O. van der Knaap Jacqueline F. N. van Leeuwen Tomasz Goslar Robert Krisai Willy Tinner 《Vegetation History and Archaeobotany》2012,21(1):37-60
Three mires and a small lake in the Swiss and Austrian Alps were studied palynologically at high resolution, covering the
last 1,000, 400, 50 and 1,200 years, respectively. Methodological lessons include: (1) Sub-decadal resolution in upper, little-decomposed
peat layers reveals recurrent marked fluctuations in both percentages and influx of regional tree-pollen types, reflecting
variations in pollen production rather than in plant-population sizes. (2) Intermittent, single-spectrum pollen maxima in
samples of sub-decadal resolution indicate pollen transport in clumps. This type of pollen transport may remain unrecognized
in sections with lower sampling resolution, which may then lead to inappropriate interpretation in terms of plant-population
sizes. (3) The detection of short-lived phases of human impact in decomposed peat requires sampling intervals as close as
0.2 cm. (4) PAR (pollen influx) may reflect vegetation dynamics more faithfully than percentages. Reliable PAR, however, is
difficult to achieve in Alpine mires due to past human impact on peat growth, even when complex depth–age modelling techniques
are used. Critical comparison of PAR with percentages is therefore essential. (5) Careful consideration of spatial scales
in pollen signals (local–regional and subdivisions) is essential for a realistic palaeo-ecological interpretation. Results
in terms of past human impact on vegetation are summarized as follows: (1) Trends in pollen types reflecting regional human
action are in general agreement with earlier findings for the western Swiss Alps, allowing for regional differences. (2) All
mires in the Alps investigated here and in an earlier study experienced human impact during the last millennium. The studied
small lake, lying in sub-alpine pasture, records forest dynamics at a lower elevation since a.d. 800. 相似文献
4.
C. van Leeuwen 《Biotechnology and bioengineering》1979,21(11):2125-2131
The determination of the overall volumetric mass-transfer coefficient with the dynamic measurement technique involves modeling, parameter estimation, and experimental design. The combination and extension of previous efforts lead to some suggestions for improvements. 相似文献
5.
The role of cAMP and calcium in the induction of ornithine decarboxylase (ODC, E.C.4.1.1.17) activity in the osteogenic sarcoma cell line, UMR 106-01, was studied, with particular interest for parathyroid hormone (PTH). PTH and forskolin dose-dependently induced the ODC activity and the cAMP production. Protein synthesis is involved in the effect of PTH and forskolin on ODC activity but not on cAMP production. Using quin2 we showed that 20 nM PTH and 10 microM forskolin increased the intracellular ionized calcium concentration ([Ca2+]i), thereby offering the possibility for calcium to play a role as cellular mediator in the action of PTH and forskolin in bone. Data obtained with A23187 showed that solely an increase of the [Ca2+]i is not sufficient to stimulate basal or potentiate PTH- and forskolin-induced ODC activity. However, the effects of calcium channel blockers and EGTA on basal and PTH- and forskolin-induced ODC activity point to a specific role for calcium. Moreover, the effects of calcium channel blockers and EGTA on basal and PTH- and forskolin-induced cAMP production indicate that the involvement of calcium in the induction of ODC activity is primarily located at another site than the adenylate cyclase. These data indicate that calcium is involved in the control of basal ODC activity. Furthermore, these data suggest that both cAMP and calcium are involved in the induction of ODC activity by PTH and forskolin. More precisely, ODC activity in UMR 106-01 cells can be induced by PTH and forskolin via a calcium-dependent cAMP messenger system. 相似文献
6.
J L van Leeuwen 《Journal of theoretical biology》1991,149(2):229-256
A model of a "general" sarcomere is presented for the calculation of power output as a function of (i) contraction range, (ii) contraction velocity, (iii) muscle fibre stimulation (active state) and (iv) structural parameters of the sarcomere (i.e. lengths of actin, myosin, and bare zone on myosin, and thickness of the Z-disc). The model is applicable to virtually all types of striated muscle fibres. By computer simulation, particular combinations of actin and myosin lengths were found that maximize the specific power output for particular functional demands, specified in terms of contraction range and contraction velocity. The accuracy of the prediction of the optimum sarcomere design by the model depends on the quality of its input, i.e. the available knowledge of the in vivo spectrum of contraction velocities and sarcomere excursions. Predictions of sarcomere design from model simulations were compared with ultrastructural data from the literature. With the present model, the complete variation in the ratio of myosin length over actin length (from about 1.05 down to 0.65, as observed in insect and vertebrate sarcomeres) can be explained as a series of adaptations for optimum power output from a small to a large contraction range, respectively. 相似文献
7.
Rapid and simple isolation of pure photosystem II core and reaction center particles from spinach 总被引:2,自引:2,他引:0
Peter J. van Leeuwen Maaike C. Nieveen Erik Jan van de Meent Jan P. Dekker Hans J. van Gorkom 《Photosynthesis research》1991,28(3):149-153
Pure and active oxygen-evolving PS II core particles containing 35 Chl per reaction center were isolated with 75% yield from spinach PS II membrane fragments by incubation with n-dodecyl--D-maltoside and a rapid one step anion-exchange separation. By Triton X-100 treatment on the column these particles could be converted with 55% yield to pure and active PS II reaction center particles, which contained 6 Chl per reaction center.Abbreviations Bis-Tris
bis[2-hydroxyethyl]imino-tris[hydroxymethyl]methane
- Chl
chlorophyll
- CP29
Chl a/b protein of 29 kDa
- Cyt b
559
cytochrome b
559
- DCBQ
2,5-dichloro-p-benzo-quinone
- LHC II
light-harvesting complex II, predominant Chl a/b protein
- MES
2-[N-Morpholino]ethanesulfonic acid
- Pheo
pheophytin
- PS H
photosystem II
- QA
bound plastoquinone, serving as the secondary electron acceptor in PS II (after Pheo)
- SDS
sodiumdodecylsulfate 相似文献
8.
9.
10.
No major differences have been found in series of Staphylococcus aureus strains which reverted from L-phase, either by pyrolysis mass spectrometry or by phage-typing or sensitivity testing. In L-phase they have been subcultured for a long time or transformed/reverted many times into/from L-phase. Plasmids were lost during transformations/reversions, but there was some difference between the tetracycline-connected plasmids on the one hand and the erythromycin-connected ones on the other.This investigation was supported by the Foundation for Fundamental Research on Matter (F.O.M.), subsidized by the Netherlands Organization for the Advancement of Pure Research (Z.W.O.). 相似文献