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   3736篇
  免费   443篇
  2021年   44篇
  2019年   34篇
  2018年   35篇
  2017年   42篇
  2016年   60篇
  2015年   91篇
  2014年   115篇
  2013年   125篇
  2012年   145篇
  2011年   129篇
  2010年   102篇
  2009年   93篇
  2008年   166篇
  2007年   160篇
  2006年   132篇
  2005年   151篇
  2004年   121篇
  2003年   116篇
  2002年   102篇
  2001年   115篇
  2000年   108篇
  1999年   82篇
  1998年   58篇
  1997年   57篇
  1996年   43篇
  1995年   45篇
  1994年   31篇
  1993年   52篇
  1992年   96篇
  1991年   91篇
  1990年   89篇
  1989年   65篇
  1988年   74篇
  1987年   74篇
  1986年   64篇
  1985年   72篇
  1984年   59篇
  1983年   38篇
  1982年   38篇
  1981年   46篇
  1979年   56篇
  1978年   63篇
  1977年   38篇
  1976年   37篇
  1975年   33篇
  1974年   36篇
  1973年   29篇
  1972年   46篇
  1971年   44篇
  1969年   30篇
排序方式: 共有4179条查询结果,搜索用时 0 毫秒
981.
982.
In two retrospective Liverpool surveys the domestic, family, dietary, and medical environments of the mothers of children affected by anencephaly or spina bifida, or both (ASB) were examined. None seemed to be of more than minor importance and the findings of others were therefore examined to see if they suggested a major factor. The most promising lead came from the hypothesis that ASB is usually due to an interaction between twin fetuses or between a fetus and residual trophoblastic material from the previous normal pregnancy, particularly when this is of the opposite sex to the propositus. The sex finding was not confirmed. It was noted that hydatidiform moles, like children with ASB, were usually female and that moles and choriocarcinomas on the one hand and children with ASB on the other occurred in mothers of similar ages, but were to some extent reciprocal in geographical incidence. This suggested that trophoblastic disease might be a factor common to all three and if so then one would expect the frequency of miscarriages to be higher immediately before than immediately after the birth of an ASB child, since mole and choriocarcinoma seldom follow a normal pregnancy. An analysis of published data confirmed that this was almost certainly so. Hence residual pathological trophoblastic material from either a previous miscarriage or a co-twin may interact unfavourably with another fetus to produce ASB and this may be the primary cause of the condition. The hypothesis is consistent with the observed lack of concordance in twins, the high incidence in females, the maternal age effect, and the greater prevalence in social classes IV and V.  相似文献   
983.
Injections of oxytocin and TRH (11 picomoles), centered on the dorsal motor nucleus of the vagus, substantially increased gastric acid secretion. Additionally, oxytocin, but not TRH, simultaneously produced a consistent reduction in heart rate. Vasopressin injected into the same locus, at doses of 11 and 110 picomoles, had no effect on either function. Both the gastric and cardiac effects of oxytocin were eliminated by the central injections of oxytocin antagonist dEt2Tyr(Et)Orn8Vasotocin (ETOV; 6 picomoles) or peripheral administration of atropine (300 μg/kg, IP). Application of oxytocin or TRH to the area postrema, at double the dosage (22 picomoles) yielded no consistent effects on either gastric secretion or heart rate. These findings indicate that oxytocin in the dorsal motor nucleus of the vagus may act as a regulator of vagally-mediated gastric and cardiovascular functions while TRH effects, in this medullary area, seem limited to the regulation of gastric function.  相似文献   
984.
The reversion of protoplasts of Bacillus licheniformis 6346 His- on a medium containing 2.5% agar has been studied in sectioned material after reaction with a ferritin-conjugated antibody specific to the peptidoglycan isolated from the walls of the bacilli. Freeze etching has also been used. Fibrils of material reacting with the antibody have been detected emerging from isolated areas of the protoplasts after 3 h of incubation. This material gradually covers the cell and can eventually (at 6 h) be seen in freeze-etched preparations as a fringe of up to 400 nm around the cells and covering the surfaces with particles that can be removed by lysozyme. At later stages the wall begins to take on a compact, well-defined appearance that can be seen in sections; however, the cells are still grossly deformed. A transitory emergence, beyond the wall of long fibers of 6 nm in diameter, takes place after about 12 h of incubation. These fibers react with the conjugated antibody and after freeze etching show a regular banded structure. They are probably indentical with the fibers isolated elsewhere (Elliott et al., 1975) and shown to contain all the wall constituents (i.e., peptidoglycan, teichoic acid, and teichuronic acid). These fibers are not detectable in the final stages of reversion.  相似文献   
985.
BOOK REVIEW     
Insect Dormancy: an ecological perspective: H. V. Danks. Biological Survey of Canada (Terrestrial Anthropods) 1987. pp 439. Can$50.  相似文献   
986.
Inhibition of the Indole Test Reaction by Sodium Nitrite   总被引:4,自引:2,他引:2       下载免费PDF全文
Sodium nitrite formed by nitrate reduction produced a false-negative reaction in the detection of indole formation when both tests were conducted in one medium.  相似文献   
987.
Abstract Addition of β-lactams to exponentially growing cultures of an autolytically deficient Bacillus subtilis met c3 lyt -2 strain FJ6 caused increase in optical density to stop after 1 h when it had about doubled, and thereafter to remain constant for at least 6 h. The number of protoplasts to be derived per unit dry weight of bacteria started to fall when the antibiotic was added and after 1 h had reached 50% of the initial value. Also during the first hour but after a lag of 20–30 min an increasing number of membrane bound rod shaped protoplasts were seen among the normal spherical ones. These swelled to spheres and ultimately burst if the concentration of sucrose in the suspending fluid was gradually reduced. These results would be best explained by rapid inhibition of initiation of cell division followed by membrane damage to an increasing proportion of cells.  相似文献   
988.
989.
Summary In view of the interest in high productivity fermentations at increased temperatures, the effect of temperature on the kinetics of ethanol production by Saccharomyces uvarum was investigated in the range 25–43°C. Using a mathematical model and a nonlinear computer simulation package, the kinetic parameters at each temperature were estimated. It was found that the optimal temperature for growth was 34°C, while the specific ethanol production rate was maximal at 37–43°C. Up to 37°C, the inhibitory effects of ethanol on growth and specific ethanol production rate were unaffected by temperature. However, above this temperature, ethanol inhibition increased significantly.  相似文献   
990.
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