全文获取类型
收费全文 | 4311篇 |
免费 | 350篇 |
国内免费 | 242篇 |
出版年
2024年 | 5篇 |
2023年 | 86篇 |
2022年 | 110篇 |
2021年 | 144篇 |
2020年 | 165篇 |
2019年 | 249篇 |
2018年 | 236篇 |
2017年 | 180篇 |
2016年 | 195篇 |
2015年 | 186篇 |
2014年 | 239篇 |
2013年 | 490篇 |
2012年 | 208篇 |
2011年 | 224篇 |
2010年 | 196篇 |
2009年 | 221篇 |
2008年 | 205篇 |
2007年 | 226篇 |
2006年 | 160篇 |
2005年 | 134篇 |
2004年 | 127篇 |
2003年 | 106篇 |
2002年 | 98篇 |
2001年 | 92篇 |
2000年 | 77篇 |
1999年 | 67篇 |
1998年 | 37篇 |
1997年 | 50篇 |
1996年 | 52篇 |
1995年 | 38篇 |
1994年 | 40篇 |
1993年 | 30篇 |
1992年 | 27篇 |
1991年 | 24篇 |
1990年 | 25篇 |
1989年 | 13篇 |
1988年 | 16篇 |
1987年 | 13篇 |
1986年 | 24篇 |
1985年 | 12篇 |
1984年 | 13篇 |
1983年 | 12篇 |
1982年 | 8篇 |
1981年 | 6篇 |
1980年 | 5篇 |
1979年 | 7篇 |
1978年 | 4篇 |
1977年 | 6篇 |
1975年 | 4篇 |
1974年 | 5篇 |
排序方式: 共有4903条查询结果,搜索用时 343 毫秒
991.
Ming-San Miao Meng-fan Peng Rui-juan Ma Ming Bai Bao-song Liu 《Saudi Journal of Biological Sciences》2018,25(3):457-464
Objective: To study the effects of the different components of the total flavonoids and total saponins from Mao Dongqing's active site on the rats of TIA model, determine the optimal reactive components ratio of Mao Dongqing on the rats of TIA. Methods: TIA rat model was induced by tail vein injection of tert butyl alcohol, the blank group was injected with the same amount of physiological saline, then behavioral score wasevaluated.Determination the level of glutamic acid in serum, the activity of Na+-K+-ATP enzyme, CA++-ATP enzyme and Mg++-ATP enzyme in Brain tissue, observe the changes of hippocampus in brain tissue, the comprehensive weight method was used to evaluate the efficacy of each component finally. Results: The contents of total flavonoids and total saponins in the active part of Mao Dongqing can significantly improve the pathological changes of brain tissue in rats, improve the activity of Na+-K+-ATP enzyme, Ca++-ATP enzyme and Mg++-ATP enzyme in the brain of rats, and reduce the level of glutamic acid in serum. The most significant of the contents was the ratio of 10:6. Conclusion: The different proportions of total flavonoids and total saponins in the active part of Mao Dongqing all has a better effect on the rats with TIA, and the ratio of 10:6 is the best active component for preventing and controlling TIA. 相似文献
992.
Ondřej Kott Pavel Němec Aneta Fremlová Vladimír Mazoch Radim Šumbera 《Ethology : formerly Zeitschrift fur Tierpsychologie》2016,122(8):682-694
Vision has long been considered purposeless in the dark underground ecotope. However, recent anatomical studies revealed an unexpected diversity of ocular and retinal features and various degrees of development of the visual system in mammals with predominantly subterranean activity, and have suggested retention of basic visual capabilities even in some strictly subterranean mammals such as the African mole‐rats. Behavioural tests assessing image‐forming vision have not yet been conducted in subterranean mammals. Here, we investigated the visual capacities in three species of the African mole‐rats, namely the giant mole‐rat Fukomys mechowii, the Mashona mole‐rat Fukomys darlingi and the silvery mole‐rat Heliophobius argenteocinereus, in the fossorial coruro Spalacopus cyanus and the inbred C57L/J mouse. The behavioural assays performed in this study revealed severe visual deficits in all three species of mole‐rats. The absence of the visual placing reflex suggested impairment of either image‐forming vision or visuomotor integration. The random choice between the shallow and the deep side of a visual cliff clearly demonstrated inability of mole‐rats to perceive depth. The nesting assay did not yield conclusive evidence regarding the capacity for visually guided spatial orientation in the only tested species, the giant mole‐rat. In contrast, both the coruro and the mouse exhibited a clear placing reaction and preferred the shallow side of the visual cliff, implying functional image‐forming vision. Thus, the behavioural data gathered in this study show that vision is seriously compromised in the strictly subterranean, congenitally microphthalmic African mole‐rats but efficient (i.e. comparable to that of surface‐dwelling rodents) in a species with regular surface activity, the coruro. 相似文献
993.
Cost-Effectiveness Analysis of Chemical Testing for Decision-Support: How to Include Animal Welfare?
Toxicity testing for regulatory purposes raises the question of test selection for a particular endpoint. Given the public's concern for animal welfare, test selection is a multi-objective decision problem that requires balancing information outcome, animal welfare loss, and monetary testing costs. This paper demonstrates the applicability of cost-effectiveness analysis as a decision-support tool for test selection in a regulatory context such as, for example, the new European chemicals legislation (REACH). We distinguish different decision-making perspectives, in particular the regulator's and chemical industry's perspectives, and we discuss how cost-effectiveness analysis can be applied to test selection from both perspectives. Furthermore, we show how animal welfare goals can be included in cost-effectiveness analyses, and we provide a three-dimensional extension to the standard cost-effectiveness analysis if animal welfare loss cannot be valued in monetary terms. To illustrate how cost-effectiveness analysis works in different settings, we apply our model to a simple case of selecting short-term tests for mutagenicity. We demonstrate that including sufficiently high values for animal welfare induces cost-effective replacements of animal testing. Furthermore, we show that the regulator and chemical companies face different tradeoffs when selecting tests. This may lead to different choices of tests or testing systems. 相似文献
994.
995.
996.
Aim We provide the first in‐depth study of the Branchiopoda of the rain forests of Cameroon and also of the African continent. Location Surface water environments, Cameroon. Methods Qualitative plankton samples were collected in all types of surface water environments present, ranging from big lakes to water collected in rock crevices or fallen fruit cavities. A tow or hand‐held plankton net of mesh size 100 μm was used, and water volumes filtered were at least several m3 in large water bodies, or half to whole water volume in small water bodies. Results We recorded 61 species (53 first records for the country), based on 700+ samples collected between September 1998 and March 2002. Anomopoda (92%) was the dominant order, followed by Ctenopoda (6.5%) and Cyclestherida (1.5%). Chydoridae (67%) was the most speciose family followed by Macrothricidae (6.5%) and Daphniidae (5%). Alona (11%) was the dominant genus followed by Chydorus (10%) and Pleuroxus (8%). Several species of Chydorinae, especially of the genus Pleuroxus, are shared with continental Eurasia–North America, but are absent from the Mediterranean and desert–steppe–savanna zones of Africa (boreal–tropical disjunction). Daphnia was absent, as in most tropical lowlands. No single species was really abundant, and a majority were rare to very rare, and of restricted occurrence within the rain forest patches. Comparing Africa, South America and Southeast Asia, we found a current total of 196 species for the combined rain forest areas, out of a world total of 500+ species. Systematic trends in richness at three taxonomic levels were the same for all continents: Anomopoda–Ctenopoda–Cyclestherida at ordinal level, Chydoridae–Daphniidae–Macrothricidae–Sididae at family level and Alona–Chydorus–Macrothrix–Diaphanosoma at genus level. Southeast Asia was richest (111 species, 14 endemics) with South America a close second (110 species, 27 endemics). Africa was the most species‐poor (95 species, of which only 5 are endemics). Main conclusions We hypothesize that the post‐Miocene cooling and aridization of the world climate hit the freshwater biota of Africa particularly hard, with more extinction here than elsewhere, and little recolonization. Most extinction occurred in the savanna‐desert belt, and eight disjunct boreal species (four Pleuroxus, Picripleuroxus laevis, Kurzia latissima, Alonella exigua, and Monospilus dispar) survive morphologically unchanged since pre‐Pleistocene times in the Cameroon rain forest. Slow evolution thus appears typical of these cyclic parthenogenetic branchiopods in which sexual recombination occurs only at intervals. Illustrative of the same slow evolution is the fact that the two endemic cladocerans of Cameroon (Nicsmirnovius camerounensis and Bryospilus africanus) belong to tropicopolitan genera of Gondwanan age. 相似文献
997.
W. G. Breed G. A. Cox C. M. Leigh P. Hawkins 《Molecular reproduction and development》1988,19(2):191-202
The morphology of spermatozoa from the red veld rat, Aethomys chrysophilus, of Southern Africa is described; two very different types were found, which came from animals from two separate, as-yet-undescribed, species. In individuals from South Africa the sperm head had a somewhat disc-shaped nucleus and a large acrosome with a huge apical segment that, during epididymal transit, changed in form from initially projecting anteriorly to a highly complex structure that was flexed caudad and lay alongside part of the rest of the sperm head. In addition, the chromatin generally appeared to be not fully condensed. Spermatozoa from animals collected in Malawi were very different in morphology and had a head with a typical apical hook, a perforatorium, fully condensed chromatin, and a 4-μm-long ventral spur. Its sperm tail was also significantly longer. The time of divergence of these two groups of animals from a common ancestor is not known, but the present results show that a considerable morphological change in the sperm nucleus, acrosome, and subacrosomal space can evolve even between two, presumably closely related, species. 相似文献
998.
Renal weights of 430 adult black subjects coming to medicolegal autopsy at the Diepkloof State Mortuary, a large urban area southwest of Johannesburg, South Africa, were analyzed. The subjects were from 10 southern African black ethnic groups-Zulu, Sotho, Tswana, Xhosa, Shangaan, Swazi, Venda, Ndebele, Kalanga, and Malawi. The aims of the study were: 1) to ascertain the anatomical "norm" as it pertains to renal weights in this diverse population group; 2) to formulate standard reference tables that might be of use to the practicing pathologist in the southern African arena; 3) to provide a range of values that take into account the variables of age, sex, race, body weight, and body height; and 4) to provide a standard of comparison with anthropological and anatomical studies conducted on North American black, North American Caucasian, Indian subcontinent, Burmese, and Jamaican population groups. In each of the 430 subjects, age, sex, ethnic group, supine body length, body weight, individual left and right renal weights, and causes of death were noted. The latter were divided into 6 categories: 1) penetrating incised wounds; 2) multiple injuries; 3) gunshot wounds; 4) craniocerebral injuries; 5) various miscellaneous nonnatural causes of death; and 6) natural causes of death. The above variables were analyzed by computer and compared with respect to renal weights. No statistically significant differences were observed between the sexes or the age groups.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
999.
Spier RE 《Cytotechnology》1992,8(2):89-92
Pick up any textbook with ‘Microbiology’ in the title and observe the scant to nonexistent treatment of animal cells in culture. Viruses do not suffer from such an exclusion. Chapters abound regaling the molecular niceties of the complex dances their components undergo while infecting bacteria or animal cells in culture. Indeed, it is normally in the latter sense only that any recognition is given to the existence of animal cells in culture. Such views pervade even the minds of those who initiate and direct activities of societies whoseraison d’être is the growth and exploitation of animal cells in culture. The author of this editorial was appointed to a Chair in Microbiology at the University of Surrey in 1983 and has worked on the growth and use of animal cells in culture continuously since 1970; he, therefore, had to reconcile his position in the university with his chosen calling. In this he was helped by undergraduates too numerous to mention who struggled mightily with essays which asked the question; are animal cells in culture microorganisms? What follows is a digest of their efforts and the views of the author. 相似文献
1000.
Guozheng Wang Wenying Zhang Corey Jacklin David Freedman Lee Eppstein Avinoam Kadouri 《Cytotechnology》1992,9(1-3):41-49
This study describes two packed bed bioreactor configurations which were used to culture a mouse-mouse hybridoma cell line
(ATCC HB-57) which produces an IgG1 monoclonal antibody. The first configuration consists of a packed column which is continuously perfused by recirculating
oxygenated media through the column. In the second configuration, the packed bed is contained within a stationary basket which
is suspended in the vessel of a CelliGen™ bioreactor. In this configuration, recirculation of the oxygenated media is provided by the CelliGen Cell Lift impeller.
Both configurations are packed with disk carriers made from a non-woven polyester fabric. During the steady-state phase of
continuous operation, a cell density of 108 cells per cm3 of bed volume was obtained in both bioreactor configurations. The high levels of productivity (0.5 gram MAb per 1 of packed
bed per day) obtained in these systems demonstrates that the culture conditions achieved in these packed bed bioreactors are
excellent for the continuous propagation of hybridomas using media which contains low levels (1 %) of serum as well as serum-free
media.
These packed bed bioreactors allow good control of pH, dissolved oxygen and temperature. The media flows evenly over the cells
and produces very low shear forces. These systems are easy to set up and operate for prolonged periods of time.
The potential for scale-up using Fibra-cel carriers is enhanced due to the low pressure drop and low mass transfer resistance,
which creates high void fraction approaching 90% in the packed bed. 相似文献