全文获取类型
收费全文 | 7053篇 |
免费 | 1579篇 |
国内免费 | 969篇 |
专业分类
9601篇 |
出版年
2024年 | 100篇 |
2023年 | 355篇 |
2022年 | 367篇 |
2021年 | 513篇 |
2020年 | 458篇 |
2019年 | 423篇 |
2018年 | 339篇 |
2017年 | 366篇 |
2016年 | 309篇 |
2015年 | 313篇 |
2014年 | 399篇 |
2013年 | 502篇 |
2012年 | 325篇 |
2011年 | 352篇 |
2010年 | 294篇 |
2009年 | 357篇 |
2008年 | 362篇 |
2007年 | 377篇 |
2006年 | 319篇 |
2005年 | 305篇 |
2004年 | 255篇 |
2003年 | 263篇 |
2002年 | 205篇 |
2001年 | 168篇 |
2000年 | 142篇 |
1999年 | 145篇 |
1998年 | 115篇 |
1997年 | 98篇 |
1996年 | 103篇 |
1995年 | 92篇 |
1994年 | 88篇 |
1993年 | 69篇 |
1992年 | 68篇 |
1991年 | 69篇 |
1990年 | 46篇 |
1989年 | 50篇 |
1988年 | 41篇 |
1987年 | 45篇 |
1986年 | 33篇 |
1985年 | 54篇 |
1984年 | 42篇 |
1983年 | 32篇 |
1982年 | 48篇 |
1981年 | 34篇 |
1980年 | 26篇 |
1979年 | 23篇 |
1978年 | 30篇 |
1977年 | 25篇 |
1976年 | 16篇 |
1975年 | 12篇 |
排序方式: 共有9601条查询结果,搜索用时 15 毫秒
121.
In vitro autoradiographic localization of [I]-angiotensin II binding sites in the rat and dog kidney
Light microscopic autoradiographic techniques have been utilized to demonstrate specific regions of the rat and dog kidney where angiotensin II receptors exist. Slide mounted tissue sections were labeled with [125I]-angiotensin II using conditions which provided for highly specific binding. These angiotensin II binding sites were localized to several distinct renal structures. In the renal cortex, angiotensin II binding sites were found concentrated in all parts of the glomeruli including the vascular components, the macula densa and the juxtaglomerular apparatus. Angiotensin II binding in the medulla was more diffusely associated with the vasa recta, and to a lesser extent, the thick ascending segment of the loop of Henle. Binding sites specific for angiotensin II were also found in the smooth muscle laminae of the ureter. Scatchard analysis of the binding kinetics allowed the demonstration of two subpopulations of binding sites which differ slightly in their affinities for [125I]-angiotensin II. These subpopulations appear to be associated with distinct components of the renal structure. 相似文献
122.
Margaret R. Bolick 《Plant Systematics and Evolution》1978,130(3-4):209-218
Two basic patterns of exine ultrastructure are found in theCompositae, the caveate Helianthoid pattern and the non-caveate Anthemoid pattern. TheHeliantheae, Astereae, Inuleae, Sececioneae, Calenduleae andEupatorieae all have pollen with caveate exines. TheMutisiseae, Vernonieae andCardueae have predominately Anthemoid pollen. TheAnthemideae, Arctoteae andLactuceae have pollen with exines of both patterns. Recent investigations of pollen in theVernonieae suggest that these exine ultrastructures in the family have evolved in response to mechanical stresses on the wall which are caused by changes in volume of the grain as it loses or gains water from its environment. 相似文献
123.
124.
The asymptotic information in censored survival data 总被引:2,自引:0,他引:2
125.
Immunogenetic investigations on two serum beta-lipoprotein allotypes of rhesus monkeys (Macaca mulatta) are reported. The allotypes, designated Lmb1 and Lmb11, are associated with the main lipoprotein family, LP-B or beta-lipoprotein, expressed on independent beta-molecules, and classify rhesus monkeys into three phenotypes: Lmb1, Lmb11, and Lmb1,11. Genetic and molecular studies indicate that the allotypes are encoded by two codominant autosomal allelic genes, Lmb1 and Lmb11. Anti-Lmb1 cross-reacts with the sera of two other macaque species, whereas anti-Lmb11 with sera of all Old World monkeys. Heteroimmune sera, antihuman apo-B and antirhesus LP-B, showed high but diversified degrees of cross reactivity with other primates. 相似文献
126.
Activities of Phosphorylase, glyceraldehyde-3 -phosphate dehydrogenase, lactate dehydrogenase, malate dehydrogenase and succinate
dehydrogenase in the rat endometrial tissue are significantly inhibited by an intrauterine copper device, while it stimulated
glucose-6-phosphate dehydrogenase activity. The copper device decreased the lactate/pyruvate ratio in the tissue; pyruvate
utilizationin vitro by the rat endometrium is also blocked by copper. These findings suggested that the normal carbohydrate metabolism of the
tissue may be affected in presence of copper, thus resulting in a change of the endometrial function, which may be one of
the factors responsible for the contraceptive and pharmacological action of an intrauterine copper device. 相似文献
127.
128.
Summary The ultrastructure of the tanycyte ependyma in male 160–180 g Wistar albino rats was studied under normal conditions and in experiments involving long-term suppression of ACTH secretion and its long-term stimulation. The former was accomplished by daily (for 8 days) intraperitoneal administration of dexamethasone phosphate at low (5 g/100 g) and high (100 g/100 g) concentrations. The effectiveness of suppression of the hypothalamic-hypophyseal-adrenal system in the experimental animals was judged by their reaction to two-minute ether stress (determination of plasma corticosterone) and by the results of measurement of the adrenal weights. Stimulation of ACTH secretion was achieved by bilateral adrenalectomy; the animals were examined on days 8, 10, 14, and 22 following the operation. The results obtained were in agreement with the previously established fact that there is a negative correlation between tanycyte activity and hypophyseal adrenocorticotrophic function (Akmayev and Fidelina, 1974). They also testified to the predominant involvement of the median eminence tanycyte ependyma (beta-tanycytes according to the authors' nomenclature) in these relationships.It is supposed that these correlations are regulated by a feedback mechanism and attest to the involvement of beta-tanycytes in the inhibiting control of hypophyseal adrenocorticotrophic function. The mechanism of this control may be explained alternatively: either the tanycytes transport ACTH-suppressing substances (catecholamines, corticosteroids, ACTH) from the CSF to the hypophyseal portal system or they themselves secrete substances possessing ACTH-suppressive activity. The authors distinguish several types of vesicles in the beta-tanycytes, the number of which changed with experimentally induced shifts in hypophyseal adrenocorticotrophic function. These vesicles are discussed in connection with the transport and secretory activity of the tanycytes and are considered to be a possible substrate of the hypothalamic inhibiting effect on ACTH secretion. 相似文献
129.
Shoukry N. Messieh 《Environmental Biology of Fishes》1977,2(3):195-210
Synopsis Examination of more than 2,000 alewives (Alosa pseudoharengus) and blueback herring (A. aestivalis) from various areas in the Saint John River, N.B., showed differences in their morphometric and meristic characters. They differed also in length and age, compositions, spawning time, length and age at maturity, back-calculated lengths, and growth parameters. Within each species, between-area comparisons showed significant differences in some of their meristic characteristics.To test a homing hypothesis for alewives, a multivariate analysis was performed on eight meristic characters. Two discriminant function programs, discriminant analysis for two groups and a multiple group stepwise discriminant analyses were used. Values of the generalized Mahalanobis D2 and percent overlap in frequency distributions of meristic characters suggest that considerable straying of fish, especially between adjacent areas occurs during upstream spawning migration.This article is one of several papers presented at the Second European Ichthyological Congress, Paris, 8–15 September 1976, to be published in Environmental Biology of Fishes. 相似文献
130.
Summary Ribosomal RNA (rRNA) sequence homology (as determined by comparisons of T1 oligonucleotide catalogs of32P-labeled 16S rRNAs) has been used to assess phylogenetic relationships within the filamentous and unicellular blue-green bacteria, and to identify regions of evolutionary conservatism within blue-green bacterial 16S rRNAs.Nostoc andFischerella, representatives of two morphologically distinct and highly differentiated orders, are shown to be as closely related (on the basis of RNA sequence homology) as typical members of the non-blue-green bacterial genusBacillus. They are further shown to be (on the same basis) indistinguishable from typical unicellular members of a subgroup of the unicellular blue-green bacterial order Chroococcales. These results have general implications for studies of the origin of differentiated prokaryotes and of evolutionary change in prokaryotic macromolecules. In particular, they provide indirect evidence that the divergences of contemporary major prokaryotic groups are truly ancient ones. 相似文献