全文获取类型
收费全文 | 851篇 |
免费 | 85篇 |
国内免费 | 68篇 |
出版年
2024年 | 4篇 |
2023年 | 17篇 |
2022年 | 29篇 |
2021年 | 31篇 |
2020年 | 24篇 |
2019年 | 38篇 |
2018年 | 32篇 |
2017年 | 37篇 |
2016年 | 29篇 |
2015年 | 39篇 |
2014年 | 58篇 |
2013年 | 85篇 |
2012年 | 36篇 |
2011年 | 55篇 |
2010年 | 33篇 |
2009年 | 43篇 |
2008年 | 44篇 |
2007年 | 54篇 |
2006年 | 61篇 |
2005年 | 41篇 |
2004年 | 25篇 |
2003年 | 34篇 |
2002年 | 20篇 |
2001年 | 15篇 |
2000年 | 14篇 |
1999年 | 9篇 |
1998年 | 18篇 |
1997年 | 7篇 |
1996年 | 5篇 |
1995年 | 8篇 |
1994年 | 3篇 |
1993年 | 7篇 |
1992年 | 5篇 |
1991年 | 4篇 |
1990年 | 6篇 |
1989年 | 4篇 |
1988年 | 3篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1983年 | 3篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1974年 | 3篇 |
1973年 | 4篇 |
1972年 | 2篇 |
1971年 | 1篇 |
排序方式: 共有1004条查询结果,搜索用时 15 毫秒
1.
Sequential medical trials involving paired data 总被引:2,自引:0,他引:2
2.
There is a major risk that many of the remaining semi-natural pastures in Swedish forest dominated regions will lose their grazing in the near future with lost biodiversity as a result. The reason is the high costs of grazing small pastures with cattle from generally small herds. The approaching decoupling of the present EU income support per head of cattle will increase the risk. Calculations based on economies of scale in beef production and opportunity cost of forest and arable land suggest that re-creating extensive pasture-forest mosaics consisting of existing semi-natural pastures and adjacent arable fields and forests can secure economically sustainable grazing. The risk of local extinction of grassland species due to habitat isolation is also lower in large mosaics than in small, scattered pastures. 相似文献
3.
Peter Lewis 《Journal of bioethical inquiry》2007,4(1):51-55
The display of personal photographs in hospital is a common practice that has yet to be rigorously examined. The photographs
displayed are subject to interpretation by the viewer and may lead to misunderstandings or miscommunication if clarification
of meaning is not sought. This paper explores a range of possible meanings that the display of photographs in hospital may
hold, based on a case study of a 15 year old boy hospitalised with a life threatening illness. Further research is needed
into the actual meanings attributed to the display of photographs in hospital by patients and family members. 相似文献
4.
Identification of different protein functions facilitates a mechanistic understanding of Japanese encephalitis virus (JEV) infection and opens novel means for drug development. Support vector machines (SVM), useful
for predicting the functional class of distantly related proteins, is employed to ascribe a possible functional class to Japanese encephalitis virus protein. Our study from SVMProt and available JE virus sequences
suggests that structural and nonstructural proteins of JEV genome possibly belong to diverse protein functions, are expected to occur in the life cycle of JE virus. Protein functions common to both structural and
non-structural proteins are iron-binding, metal-binding, lipid-binding, copper-binding, transmembrane, outer membrane, channels/Pores - Pore-forming toxins (proteins and peptides) group of proteins. Non-structural
proteins perform functions like actin binding, zinc-binding, calcium-binding, hydrolases, Carbon-Oxygen Lyases, P-type ATPase, proteins belonging to major facilitator family (MFS), secreting main terminal branch (MTB)
family, phosphotransfer-driven group translocators and ATP-binding cassette (ABC) family group of proteins. Whereas structural proteins besides belonging to same structural group of proteins (capsid, structural, envelope),
they also perform functions like nuclear receptor, antibiotic resistance, RNA-binding, DNA-binding, magnesium-binding, isomerase (intra-molecular), oxidoreductase and participate in type II (general) secretory pathway (IISP). 相似文献
5.
Maria L. Boccia Martin L. Reite Kristine Kaemingk Polly Held Mark L. Laudenslager 《Primates; journal of primatology》1991,32(2):255-263
The social environment affects both behavioral and physiological responses to separation from the mother. Less information
is available on the impact of the social environment on the response to separation in peer-reared infant monkeys. This study
reports the responses of peer-reared pigtail macaque infants to repeated separations, and the impact of social versus isolation
housing during the separation. The responses of two pairs of monkeys were studied during four three-day separations. One of
each pair was housed in isolation during the separation, and the other was with another pair of peers, with whom they had
been living for one month prior to the separation. The isolation-housed peer responded to the separation with behavioral agitation,
but no depression. The socially-housed peer's behavior did not differ from baseline during the separation. During successive
reunions, all the separated monkeys, regardless of housing condition, exhibited declining levels of behaviors related to maintaining
proximity to their attachment figure. Although the number of subjects is small, the results suggest that the presence of social
support, in the form of a familiar peer, can ameliorate the response to separation, and that with repeated separations the
responses of the monkeys changes significantly. 相似文献
6.
The ultrastructure of the mature spermatozoa and spermatogenesis of the bivalve Scrobicularia plana are described. Support cells extend from the basal lamina to the lumen of the testis and are laterally connected to the germinal epithelium. Germ cells present intercellular bridges and flagella since the spermatogonial stage. While spermatogonia and spermatocytes appear connected to support cells by desmosome-like junctions, elongated spermatids are held at the acrosomal region by support cell finger-like processes. During spermiogenesis, the acrosomal vesicle differentiates from a golgian saccule and then migrates to the nuclear apex. A microtubular manchette arising from centrioles surrounds the acrosomal vesicle, the nucleus, and the mitochondria at the time these three organelles start their elongation, disappearing after that. The mature spermatozoon of S. plana lacks a distinct midpiece because the mitochondria extend from the region of the pericentriolar complex along the nucleus anteriorly for approximately 1.4 μm. The features of this bivalve type of modified spermatozoon are compared with those of other animal groups having similar modifications. 相似文献
7.
Shoji Machida 《Primates; journal of primatology》1990,31(2):205-211
The vocal behavior of threat calls was investigated in a captive group of Japanese monkeys (Macaca fuscata fuscata). The vocalizations were heard most often when they undertook winner-support during triadic agonistic interactions. The likelihood
of call emission in support of the winner was affected by the attributes of the participants, and not by the types of agonistic
behavior. The calls were emitted by intermediate ranking animals frequently in support of high ranking animals and in support
of females. The calling behavior of winner-supporters appears to advertise the partner and distant group members of their
support for reciprocation in the near future. 相似文献
8.
Giving between generations in American families 总被引:4,自引:0,他引:4
This paper documents the types and amounts of aid exchanged between adults and their non-coresidential parents. Data for the
study are drawn from a representative national sample survey of Americans age 19 and older conducted in 1987–1988. Exchanges
of monetary and material resources, childcare, household assistance, and companionship and advice are considered.
Patterns of intergenerational exchange are found to differ by gender, family structure, age, ethnicity, and socioeconomic
situation. Differences in exchange between males and females and between whites and Mexican-Americans are related to other
life-course characteristics, and to the availability and proximity of kin. Blacks and persons living in poverty are shown
to be less involved than other groups in intergenerational exchanges. Finally, patterns of prior assistance and the available
needs and resources of the respondents and their parents are found to influence current patterns of exchange.
Support for this research was provided by NICHD Grant No. 1 R01 HD26070-01, “Intergenerational Exchanges in Families with
Children,” Dennis P. Hogan, Principal Investigator. Funds for the computer analysis were provided by the Pennsylvania State
University Intercollege Research Programs.
David Eggebeen is an Assistant Professor of Human Development in the Department of Human Development and Family Studies and
a research associate at the Population Issues Research Center at Pennsylvania State University. He trained in sociology and
demography at the University of North Carolina. His current research interests, besides those related to intergenerational
relations, are the recent changes in the demographic structure of childhood in America and their implications for children’s
social and economic well-being.
Dennis P. Hogan is a professor of sociology and the director of the Population Issues Research Center at Pennsylvania State
University. His current research interests, besides those related to intergenerational relations, are in the interrelation
of social structures and the demographic life course. He is coauthor with David I. Kertzer ofFamily, Political Economy, and Demographic Change: The Transformation of Life in Casalecchio, Italy, 1861–1921, University of Wisconsin Press, 1989. 相似文献
9.
The temporal and spatial scales employed by foraging bees in sampling their environment and making foraging decisions should depend both on the limits of bumble bee memory and on the spatial and temporal pattern of rewards in the habitat. We analyzed data from previous experiments to determine how recent foraging experience by bumble bees affects their flight distances to subsequent flowers. A single visit to a flower as sufficient to affect the flight distance to the next flower. However, longer sequences of two or three visits had an additional effect on the subsequent flight distance of individual foragers. This suggests that bumble bees can integrate information from at least three flowers for making a subsequent foraging decision. The existence of memory for floral characteristics at least at this scale may have significance for floral selection in natural environments. 相似文献
10.
Acyl Group Migrations in 2-Monoolein 总被引:6,自引:0,他引:6
Anna Millqvist Fureby Carmen Virto Patrick Adlercreutz Bo Mattiasson 《Biocatalysis and Biotransformation》1996,14(2):89-111
Acyl migration in 2-monoolein dissolved in solvents under conditions common in lipid modification reactions has been studied. The effects on acyl migration of solvent, incubation temperature, water activity, polar additives and solid additives have been investigated. Extensive acyl migration occured in aliphatic hydrocarbons and water-miscible alcohols under dry conditions. The acyl migration rate could be decreased in several nonpolar solvents by adding a small amount of water or an alcohol. Increasing water activity had no effect in isooctane, but decreased the acyl migration rate dramatically in methyl tert-butyl ether and methyl isobutyl ketone. Several commonly used enzyme supports catalysed acyl migration, showing that supports with surface charges could catalyse acyl migration. 相似文献