全文获取类型
收费全文 | 19309篇 |
免费 | 1691篇 |
专业分类
21000篇 |
出版年
2023年 | 85篇 |
2021年 | 151篇 |
2020年 | 108篇 |
2019年 | 132篇 |
2018年 | 361篇 |
2017年 | 382篇 |
2016年 | 402篇 |
2015年 | 348篇 |
2014年 | 433篇 |
2013年 | 695篇 |
2012年 | 1327篇 |
2011年 | 1460篇 |
2010年 | 764篇 |
2009年 | 509篇 |
2008年 | 1237篇 |
2007年 | 1239篇 |
2006年 | 1175篇 |
2005年 | 1056篇 |
2004年 | 986篇 |
2003年 | 938篇 |
2002年 | 923篇 |
2001年 | 761篇 |
2000年 | 889篇 |
1999年 | 395篇 |
1998年 | 194篇 |
1997年 | 145篇 |
1996年 | 187篇 |
1995年 | 173篇 |
1994年 | 140篇 |
1993年 | 150篇 |
1992年 | 144篇 |
1991年 | 134篇 |
1990年 | 136篇 |
1989年 | 118篇 |
1988年 | 126篇 |
1987年 | 129篇 |
1986年 | 115篇 |
1985年 | 152篇 |
1984年 | 155篇 |
1983年 | 134篇 |
1982年 | 164篇 |
1981年 | 153篇 |
1980年 | 110篇 |
1979年 | 127篇 |
1978年 | 96篇 |
1977年 | 95篇 |
1976年 | 84篇 |
1975年 | 79篇 |
1974年 | 84篇 |
1973年 | 73篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
W. Schäfer 《Helgoland Marine Research》1985,39(3):341-356
Early developmental stages of theAnemonia germ are characterized by asynchronously dividing nuclei and an extreme delay of blastomere differentiation. The nuclei migrate to the periphery, whereas nutritive substances remain in the interior. Following this stage, the appearance of cell boundaries results in the formation of the blastoderm and the simultaneous division of the yolk into many fragments. Most of them are exclusively filled with reserve material; only very few contain zooxanthellae or nuclei. On the embryo's surface, conically shaped bundles of long microvilli are obvious. They appear to be less regularly arranged than the spines of oocytes before insemination. Pigment granules that have originated from fusing Golgi vesicles are crowded peripherally in the blastoderm cells. In the nucleoplasm single annulate lamellae that seem to be cut off from the nuclear envelope can frequently be found. There is no further cellular differentiation until gastrulation is completed. Though yolk-containing cellular fragments occupy nearly the whole blastocoel, entoderm formation occurs by invagination. Ultrastructural observations provide evidence of the existence of interstices between entoderm cells that allow all nutritive substances to pass gradually into the gastric cavity. In the region of the blastoporus there are cellular processes enveloping reserve material. Presumably, these observations indicate a so-called filtration of nutritive yolk (Korschelt & Heider, 1909) that might represent an additional mode for the transfer of yolk-containing cellular fragments from blastocoel into gastrocoel. 相似文献
62.
63.
Summary In this study the fine structure of the cerebral ganglion of Sagitta setosa (Chaetognatha) is investigated. The ganglion is flat and superficially positioned dorsally, below the basal lamina of the cephalic epidermis. It is surrounded by a specifically differentiated sheath. This sheath is made up of cells, which are interpreted as representing glial cells, and can be divided into an outer and an inner zone. The outer zone is composed of flat sheath cells with pale nuclei and few organelles. The inner zone consists of densely packed, extremely thin lamellar cellular processes. These attenuated lamellae, which still contain cytoplasm, resemble the myelin sheath of vertebrate axons. The intercellular space between the lamellae contains electron-dense material. In the sheath specialized intercellular contacts occur. The inner zone of this sheath extends at definite points into the centre of the ganglion and separates a zone of perikarya from the neuropil, as well as the single perikarya from each other. The perikarya are relatively uniform and do not form a cortex, but are concentrated mainly in lateral parts of the cerebral ganglion. Within the neuropil are axonal endings which have synaptic contacts with several postsynaptic elements. These anatomical findings are discussed with respect to their functional significance. 相似文献
64.
O. Häppölä S. Soinila H. Päivärinta P. Panula O. Eränkö 《Histochemistry and cell biology》1985,83(1):1-3
Summary Using the immunoperoxidase technique in conjunction with specific antisera to -atrial natriuretic polypeptide (-ANP), it was shown that immunoreactive cell bodies and varicose fibers are widely distributed throughout the rat brain. The highest concentrations of -ANP-containing neuronal cell bodies and fibers were found in the hypothalamus and septum. This result confirms the radioimmunological determination of -ANP immunoreactivity in the rat brain. 相似文献
65.
Thomas Hansen-Hagge Volker Lehmann Ulrich Seydel Buko Lindner Ulrich Zähringer 《Archives of microbiology》1985,141(4):353-358
The extraction, purification and structural characterization of two lipid A precursors (Ia and Ib) differing only in one hexadecanoic acid are described. Both precursors were synthesized at elevated temperatures by a new mutant of Salmonella typhimurium (mutant Ts5) which is conditionally defective in synthesis of the 3-deoxy-d-manno-octulosonic acid region of lipopolysaccharides.Both precursors were purified by repeated phenol/chloroform/petroleum ether (PCP) extractions followed by thin layer chromatography. Teh precursor preparation was free of lipopolysaccharides and phospholipids and contained less than 0.1% protein. Structural analysis which included chemical degradation procedures as well as positive ion laser desorption (LDMS) mass spectroscopy of dephosphorylated lipid A precursors showed together that precursor Ia represents a diphosphorylated glucosamine disaccharide containing two ester, two amide-linked residues of 3-hydroxytetradecanoic acid and lacks the ester-linked dodecanoic, tetradecanoic and hexadecanoic acid as well as 3-deoxy-d-manno-octulosonic acid. Precursor Ib has the same basic structure as precursor Ia, but contains in addition one mol of hexadecanoic acid per mol disaccharide which is linked to the 3-hydroxy group of the amide-bound 3-hydroxy-tetradecanoic acid of the reducing, terminal glucosamine residue.The structure of precursor Ib supports the conclusion that hexadecanoic acid incorporation occurs at an early stage in lipid A biosynthesis prior to the attachment of 3-deoxy-d-manno-octulosonic acid and/or other polar substituents.Abbreviations LDMS
laser desorption mass spectrometry
- KDO
3-Deoxy-d-manno-octulosonic acid
- Ts5
Salmonella typhimurium mutant Ts5
- PCP
phenol/chloroform/petroleum ether
- H2F2
hydrogen fluoride
This work is dedicated to Prof. Dr. Drews, Freiburg, on the occasion of his 60th birthday 相似文献
66.
T. Finkenstädt N. T. Adler T. O. Allen S. O. E. Ebbesson J. -P. Ewert 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1985,156(4):433-445
Summary The (14C)2DG autoradiographic technique has been employed to quantitatively map glucose utilization in the mesencephalon, the diencephalon and the cerebellum, of toads in response to configurational moving visual stimuli: (i) a 0.4 cm × 2.8 cm worm-like stripe (W) which elicited prey catching responses, (ii) a 8.4 cm × 8.4 cm square (S) that released predator avoidance responses, and (iii) a 2.8 cm × 0.4 cm antiworm-like stripe (A) which elicited no motor activity.For various brain nuclei different relationships were obtained: The optic tectum showed statistical significant higher 2DG uptake during worm-stimulation (¯X
W) than during antiworm stimulation (¯X
A), i.e.¯X
W>¯X
A. The latter visual pattern led to a 2DG utilization that was statistically significant stronger than during stimulation with a square (¯X
S), i.e.¯X
A>¯X
S. Thus, in comparison between right and left hemisphere as well as between brains the following ratios were obtained:Optic tectum:¯X
W>¯X
A>¯X
S; nucleus isthmi:¯X
W>¯X
A-¯X
s; posterodorsal lateral thalamic nucleus:¯X
S>¯X
A>¯X
W; posteroventral lateral thalamic nucleus:¯X
S>¯X
A¯X
W; posterior thalamic nucleus:¯X
W>¯X
A¯X
S; anteripr division of the lateral thalamic nucleus:¯X
W>¯X
A¯X
S; anterior thalamic nucleus:¯X
A>¯X
S>¯X
W; nucleus of Bellonci and dorsal division of the ventrolateral thalamic nucleus:¯X
W¯X
A¯X
S; cerebellum:¯X
S¯X
W>¯X
A.Abbreviations
A
anterior thalamic nucleus
-
Cb
cerebellum
-
Hyp
hypothalamus
-
Ist
nucleus isthmi
-
cl. Ist
contralateral Ist
-
La
lateral thalamic nucleus, anterior division
-
Lpd
lateral thalamic nucleus, posterodorsal division
-
Lpv
lateral thalamic nucleus, posteroventral division
-
MP
medial pallium
-
NB/VLd
nucleus of Bellonci and ventrolateral thalamic nucleus, dorsal division
-
P
posterior thalamic nucleus
-
PO
preoptic area
-
Sna
snapping evoking area=ventrolateral tectum
-
Str
striatum
-
Tec
tectum opticum 相似文献
67.
G. Ruhenstroth-Bauer H. Baumer J. Kugler R. Spatz W. Sönning B. Filipiak 《International journal of biometeorology》1984,28(4):333-340
The possibility of connections between weather and the onset of epileptic seizures has long been suggested (see, for example, the Hammurabi Codex 1600 BC). Work in the 20th Century points to a probability that the onset of both local and generalised epilepsy is significantly influenced by an interaction between genetic and extrinsic factors. In an attempt to clarify the situation a detailed study of the history of 315 attacks from 1 Jan. to 31 July 1981 suffered by a small number of patients in Munich has been undertaken. Although linkages between classical meteorological parameters and the onset of seizures are very weak, links with more generalised indexes (e.g. passage of fronts and disturbances) are more promising. However, the correlation between onsets and atmospherics of 28 KHz (positive) and 10 KHz (negative) impulses, are significant and call for urgent study. 相似文献
68.
Hahn-Hägerdal B 《Biotechnology and bioengineering》1984,26(7):771-774
The efficiency of beta-glucosidase and Saccharomyces cerevisiae in directly converting cellobiose to ethanol was studied for various combinations of the two catalytic species, both free and immobilized, in order to elucidate the advantages of using a coimmobilized system. The coimmobilized preparation was superior to a combination of separately immobilized biocatalysts. However, in this preparation, one-half the enzyme activity was lost within a week when incubated at the operational temperature in the absence of substrate. In continuous experiments, an 80% conversion of cellobiose to ethanol was obtained using the coimmobilized preparation, compared to 40% using separately immobilized biocatalysts when applying a dilution rate of 0.1 h(-1) in a packedbed reactor. The immobilized biocatalysts showed no decline in productivity during two weeks of continuous operation. 相似文献
69.
Summary Clonal stock (grafts of 15-, 50-, and 100-year-old trees) of spruce (Picea abies (L.) Karst.) and fir (Abies alba Mill.) was exposed either to 150 ppb ozone (300 gm-3) during 9 daylight hours or to carbon-filtered air on 35 days between 29 August and 21 October 1983. At the end of the fumigation the trees did not exhibit any visible signs of injury. Transpiration and leaf conductance in light and darkness, measured with a Licor porometer, revealed, however, stomatal sluggishness and — in spruce-increased transpiration. 相似文献
70.
Peptidases, highly specific toward several synthetic chromogenic peptides, were found in the mycelia of four arthropod pathogenic fungi: Aphanomyces astaci, Beauveria bassiana, Metarrhizium anisopliae, and Paecilomyces farinosus. A. astaci peptidases had high hydrolyzing activities toward most of the peptides, especially those with arginine in the P1 position, while those of B. bassiana and P. farinosus readily hydrolyzed peptides with valine and arginine, as well as proline and tyrosine in the P2 and P1 positions, respectively. The hydrolyzing capacities of M. anisopliae peptidases were similar to A. astaci, but showed lower specific activities. Casein or azocoll was only hydrolyzed by A. astaci peptidases. B. bassiana and M. anisopliae had a very low hydrolyzing capacity toward casein and could not degrade azocoll. P. farinosus had no hydrolyzing activity toward casein or azocoll. Only peptidases from the crayfish pathogen A. astaci could degrade the crayfish cuticle. The peptidase preparations of A. astaci and B. bassiana hydrolyzing MeO-Suc-Arg-Pro-Tyr-pNA or Bz-Phe-Val-Arg-pNA were of the serine type. The possible importance of peptidase activity of arthropod pathogenic fungi in the infection process is discussed. 相似文献