首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   48035篇
  免费   17066篇
  国内免费   11篇
  2023年   153篇
  2022年   427篇
  2021年   1067篇
  2020年   2532篇
  2019年   4130篇
  2018年   4329篇
  2017年   4462篇
  2016年   4850篇
  2015年   5296篇
  2014年   5051篇
  2013年   5727篇
  2012年   3792篇
  2011年   3333篇
  2010年   4148篇
  2009年   2732篇
  2008年   1970篇
  2007年   1526篇
  2006年   1411篇
  2005年   1365篇
  2004年   1304篇
  2003年   1113篇
  2002年   1083篇
  2001年   490篇
  2000年   358篇
  1999年   349篇
  1998年   191篇
  1997年   156篇
  1996年   147篇
  1995年   115篇
  1994年   117篇
  1993年   95篇
  1992年   109篇
  1991年   101篇
  1990年   101篇
  1989年   116篇
  1988年   91篇
  1987年   72篇
  1986年   62篇
  1985年   64篇
  1984年   64篇
  1983年   45篇
  1982年   37篇
  1981年   37篇
  1980年   41篇
  1979年   37篇
  1978年   31篇
  1977年   22篇
  1976年   31篇
  1973年   20篇
  1970年   22篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
Purdy LM 《Bioethics》1989,3(1):18-34
The morality of surrogate mothering is analyzed from a "consequentialist" framework which attempts to separate those consequences that invariably accompany a given act from those that accompany it only in particular circumstances. Critics of surrogacy argue that it transfers the burden and risk of pregnancy onto another woman, separates sex and reproduction, and separates reproduction and childrearing; none of these acts is necessarily wrong, either morally or for women's or society's basic interests. While surrogate mothering can be rendered immoral if women are coerced into the practice or become victims of subordinating or penalizing contracts, it has the potential to empower women and increase their status in society by providing a job that is less risky and more enjoyable than other jobs women are forced to take and by achieving greater social recognition for reproductive labor.  相似文献   
152.
Summary The epidermal Merkel cells and their sensory innervation serve tactile sensation in vertebrates. In this study the fluorescent cationic mitochondrial dye, 4-(4-diethylaminostyryl)-N-methylpyridinium iodide (4-Di-2-ASP), which has recently been used as a vital stain for motor and autonomic nerve terminals, was tested for its ability to stain Merkel cells and sensory fibers in the snout of the rat. Brightly-fluorescent structures resembling Merkel cells as well as nerve fibers and their terminations were evident in whole mounts of the vibrissal follicle. Unilateral denervation of the vibrissal follicles soon after birth resulted in a staining pattern remarkably similar to that obtained after labelling of the Merkel cells selectively with the fluorescent marker quinacrine, but all fiber staining was abolished. Likewise, in the separated epidermis of other skin regions, including the hairy and glabrous skin of the nose, the staining pattern revealed by 4-Di-2-ASP was indistinguishable from that obtained by quinacrine fluorescence. These results indicate that certain styryl pyridinium dyes may be used as vital stains for epidermal Merkel cells as well as cutaneous mechanosensory axons.  相似文献   
153.
Acetylcholinesterase and fluoride-resistant acid phosphatase activities were contrasted in alternative serial sections of rat dorsal root ganglia. The morphometric analysis demonstrated no correlation between cellular size and enzymatic activity. Differences with previous works in this area are discussed.  相似文献   
154.
155.
Size variation in Brachionus plicatilis resting eggs   总被引:1,自引:0,他引:1  
Serrano  Laura  Serra  Manuel  Miracle  Maria R. 《Hydrobiologia》1989,(1):381-386
The effect of temperature and salinity on resting egg size of two Brachionus plicatilis (Rotifers) clones was investigated. Clones were selected according to their different behaviour in laying resting eggs: one clone ejects them, whereas they remain inside the females body in the other clone. The difference in resting eggs size between the two clones is noticeable, although the difference is not as great as that between female body size. An important temperature-salinity interaction on resting egg size has been observed. The general inverse relationship between size and temperature is only true at lower temperatures. At high temperatures size varies around the mean although could be greater than at intermediate temperatures. This is more evident at the intermediate salinity tested which is considered to be the closest to the optimum in our experiments. This pattern of variation suggests that mean size is bigger than expected, in relation to temperature and salinity, when these factors have values close to the extremes of their range, normally found in nature, and to which adaptative mechanisms can evolve. Size is bigger at the salinity — temperature low - low and high - high combinations which are the most commonly found in the temperate environments.  相似文献   
156.
Purdy LM 《Bioethics》1990,4(4):273-291
Purdy explores the argument that women's rights to control their bodies should be subordinated to the welfare of their fetuses. She gives examples ofinstances where women's decisions about pregnancy and childbirth have been overridden by physicians and judges. She examines the nature of the mother fetus relationship, the social context of the current conflict between maternal and fetal rights, and the extent of a woman's legally enforceable duty to her fetus. Purdy acknowledges that women may owe a reasonable duty to their fetuses to try to prevent disease or handicap. However, given the uncertainties of modern medicine, the value-laden nature of many physicians' decisions, and the lack of adequate adequate prenatal care, women should not be held responsible for situations that are due largely to society's shortcomings. Purdy concludes that it is unjust to use the law to coerce women into accepting medical advice.  相似文献   
157.
Summary The thoracic legs of the moth Manduca sexta acquire a new form and develop a new complement of sensory organs and muscles during metamorphosis from larva to adult. Because of our interest in the reorganization of neural circuitry and the acquisition of new behaviors during metamorphosis, we are characterizing sensory elements of larval and adult legs so that we may determine the contribution of new sensory inputs to the changes in behaviors. Here we describe the sensory structures of adult legs using scanning electron microscopy to view the external sensilla and cobalt staining to examine innervation by underlying sensory neurons. We find that, in contrast to larval legs, the adult legs are covered with a diverse array of sensilla. All three pairs of thoracic legs contain scattered, singly innervated scalelike sensilla. Campaniform sensilla occur singly or in clusters near joints. Hair plates, consisting of numerous singly innervated hairs, are also present near joints. Other more specialized sensilla occur on distal leg segments. These include singly innervated spines, two additional classes of singly innervated hairs, and three classes of multiply innervated sensilla. Internal sensory organs include chordotonal organs, subgenual organs, and multipolar joint receptors.  相似文献   
158.
We have analyzed whether lymphokine-activated killer (LAK) cells, generated from C57BL/6J (B6) spleen cells at different times after recombinant interleukin-2 (rIL-2) culture, could be heterogeneous in their ability to lyse a variety of tumor targets. When tested 3 days after exposure to 250 U/ml rIL-2 (day-3 LAK cells) a significant lysis was detected with the natural-killer(NK)-sensitive YAC lymphoma, the NK-resistant P815 mastocytoma, three different syngeneic melanomas and a syngeneic fibrosarcoma (group 1 targets), whereas no lysis was observed with a reticulum cell sarcoma, two different lymphomas or concanavalin A blasts, all of B6 origin (group 2 targets). LAK cells cultured for 5 days, however, lysed group 2 targets and showed a parallel increase of cytotoxic activity against group 1 targets. At day 7, LAK activity declined on all targets examined. In cold-target inhibition studies, the lysis of group 1 tumor targets by day-3 or day-5 LAK cells could be inhibited only by group 1 and not by group 2 unlabelled tumor cells. All group 1 tumors could effectively compete each other. Conversely, the lysis of group 2 tumor targets by day-5 LAK cells was inhibited by both group 1 and group 2 targets. These data indicate the presence of separate LAK effectors that appear to arise with different time kinetics and have different recognition structures. In vitro antibody depletion at the effector level showed that day-3 LAK cells with cytotoxic activity against group 1 tumors were ASGM1+. Day-5 LAK cells included both ASGM1+ and Lyt2+ effectors and both populations, although to a different extent, contributed to the lysis of all targets. Our results indicate that LAK cells are functionally heterogeneous. This heterogeneity is defined by their susceptible target cells and cannot be ascribed to different (Lyt2+ versus ASGM1+) lineages.  相似文献   
159.
Summary The hematological features of cold-adapted, red-blooded Antarctic teleosts has prompted this study on the relationship between hemoglobin molecular structure and oxygen-binding properties. The hemolysates from 21 species of 5 families contained one component (Hb 1), often accompanied by an additional, minor one (Hb 2, 5%–10% of total). On the other hand, 3 species of Zoarcidae, a non-endemic family, had 4–5 components. All purified hemoglobins from the former group, but only 1–2 of the 4–5 hemoglobins of Zoarcidae, showed a strong Root effect (pH regulation of oxygen binding). Globins from each hemoglobin have been purified and characterised with respect to molecular structure in several species. The similarity between the complete amino acid sequence of one -chain and those of non-Antarctic -chains is lower than that among the latter sequences, suggesting independent pathways of evolution.Presented at the 5th SCAR Symposium on Antarctic Biology, Hobart, Australia (August 29th-September 3rd, 1988)  相似文献   
160.
We studied seven informative kindreds segregating for manic depressive illness (MDI), consistent with X-chromosome transmission of the trait (families do not show affective disease in both a father and a son), using markers mapped to the region of Xq27-Xq28. The lod scores were consistently below -2 in the region extending from about 10 cM centromeric from the Factor IX locus (F9) to the colorblindness region. This study does not replicate previous reports of linkage of MDI to Factor IX (Xq27) and colorblindness region (Xq28) chromosomal markers in other kindreds.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号