全文获取类型
收费全文 | 119284篇 |
免费 | 8167篇 |
国内免费 | 214篇 |
专业分类
127665篇 |
出版年
2021年 | 1252篇 |
2020年 | 1115篇 |
2019年 | 1082篇 |
2018年 | 2649篇 |
2017年 | 2375篇 |
2016年 | 3187篇 |
2015年 | 4087篇 |
2014年 | 4356篇 |
2013年 | 5650篇 |
2012年 | 6571篇 |
2011年 | 5929篇 |
2010年 | 3909篇 |
2009年 | 3107篇 |
2008年 | 4891篇 |
2007年 | 4670篇 |
2006年 | 4650篇 |
2005年 | 3903篇 |
2004年 | 3874篇 |
2003年 | 3498篇 |
2002年 | 3258篇 |
2001年 | 4641篇 |
2000年 | 4215篇 |
1999年 | 3148篇 |
1998年 | 1194篇 |
1997年 | 947篇 |
1996年 | 951篇 |
1995年 | 830篇 |
1992年 | 2130篇 |
1991年 | 1941篇 |
1990年 | 1848篇 |
1989年 | 1865篇 |
1988年 | 1609篇 |
1987年 | 1625篇 |
1986年 | 1431篇 |
1985年 | 1473篇 |
1984年 | 1132篇 |
1983年 | 1011篇 |
1979年 | 1232篇 |
1978年 | 885篇 |
1977年 | 854篇 |
1975年 | 1016篇 |
1974年 | 1139篇 |
1973年 | 1101篇 |
1972年 | 1067篇 |
1971年 | 994篇 |
1970年 | 1012篇 |
1969年 | 1080篇 |
1968年 | 944篇 |
1967年 | 811篇 |
1966年 | 827篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Deletion of 13q in two patients with retinoblastoma,one probably due to 13q- mosaicism in the mother
Summary We examined the peripheral blood chromosomes of eight patients with retinoblastoma. In two of them an interstitial deletion of 13q was found. The breakpoints were determined as follows: case 1, 13q1221; case 2, 13q1231. In both cases, band 13q14 was deleted. In case 2 the lymphocytes of the mother showed the identical interstitial 13q deletion in 3 of 100 mitoses, thus raising the possibility of maternal origin of the 13q deletion in a child. In one patient, retinoblastoma was unilateral; in the other, bilateral. Both patients were mentally retarded. 相似文献
992.
Synopsis The Río Bayano in eastern Panama is one of many tropical rivers in which bull sharks (Carcharhinus leucas) and largetooth sawfish (Pristis perotteti) have been known to occur. Since both species can osmoregulate in fresh water throughout life, theoretically, both could survive in landlocked situations for many years.P. perotteti reproduces in fresh water, butC. leucas ordinarily does not, so only the former would appear to have the potential for establishing a breeding stock in such a landlocked freshwater body.The damming of the Rio Bayano and the creation of a large impoundment in 1976 provides a test of the ability of members of both species trapped there to survive and to establish breeding stocks. In 1980 and 1981 three mature femaleC. leucas were found dead in Lake Bayano and three sub-adult femaleP. perotteti were taken by trammel net. These events confirm the ability of both to survive in fresh water for long periods, but the establishment of breeding stocks appears doubtful and the question may not be answered for many years. 相似文献
993.
Virus removal during groundwater recharge: effects of infiltration rate on adsorption of poliovirus to soil. 总被引:3,自引:3,他引:0 下载免费PDF全文
Studies were conducted to determine the influence of infiltration rate on poliovirus removal during groundwater recharge with tertiary-treated wastewater effluents. Experiments were conducted at a uniquely designed, field-situated test recharge basin facility through which some 62,000 m3 of sewage had been previously applied. Recharge at high infiltration rates (75 to 100 cm/h) resulted in the movement of considerable numbers of seeded poliovirus to the groundwater. Moderately reduced infiltration rates (6 cm/h) affected significantly improved virus removal. Very low infiltration rates (0.5 to 1.0 cm/h), achieved by partial clogging of the test basin, yielded the greatest virus removal efficiencies. 相似文献
994.
Cultivation ofAspergillus niger 97A in baffled flasks on a shaker brings about metabolic changes demonstrated in increased citric acid biosynthesis, morphological
changes and alterations in the mycelial cell wall. 相似文献
995.
Effect of interferon on phospholipid methylation by peripheral blood mononuclear cells 总被引:1,自引:0,他引:1
The effect of human interferon (IFN) preparations on the metabolic pathway leading to the synthesis of phosphatidylcholine (PC) by a stepwise addition of methyl groups to phosphatidylethanolamine (PE) was investigated in human peripheral blood mononuclear (PBMN) cells. An inhibition of the synthesis of PC via this pathway was regularly observed with both alpha- (recombinant or natural) and beta-IFN. This inhibition was apparent within the first 5 min of treatment, reached its maximum between 15 min and 1 hr, and persisted at the same level until 6 hr, the last time point examined. Each of the transmethylated products of PE underwent a similar inhibition, as measured by the turnover rate of individual products. The intracellular pool of the methyl donors, methionine and S-adenosyl-methionine (SAM), was shown to be unaffected. The methyltransferase activity of IFN-pretreated cell extracts was unchanged. These findings support the hypothesis that IFN induces a functional change in phospholipid methylation at the level of organized membrane-bound phospholipid methyltransferase enzymes in intact cells. 相似文献
996.
Development of immunohistochemical characteristics of intrafusal fibres in normal and de-efferented rat muscle spindles 总被引:1,自引:0,他引:1
Summary Intrafusal muscle fibres in adult muscle spindles differ in their myosin composition. After selective motor denervation intrafusal muscle fibres develop mature ultrastructural characteristics. In order to evaluate the role of fusimotor innervation on the maturation of the myosin composition of intrafusal muscle fibres we have examined with immunohistochemical techniques i) the postnatal development of muscle spindles in new-born rats and in 7–21 day old rats; ii) muscle spindles in the EDL of 21-day-old rats de-efferented at birth. For the characterization of myosins in intrafusal fibres we used three myosin antisera: antipectoral myosin, antiheart myosin and antiheart myosin adsorbed with muscle powder from the soleus muscle of guinea pig. We show in this study that during development intrafusal fibres change immunoreactivity and that in the absence of motor innervation bag fibres do not fully develop the myosin characteristics of control spindles. We conclude that the maturation of bag1 and bag2 fibres apparently requires next to the inductive influence of sensory axon terminals the presence and activity of fusimotor axons. 相似文献
997.
998.
999.
1000.
The distribution of small negative ions with mobility kr=0.5+ (cm2/Vs) in an air conditioned room is presented. The air was ionized by an ion generator, which was placed at the outlet of the air duct. The measurements were made at 100 different locations in the room. It was established, that the main factors in the distribution of ions were the coordinates of points in the room space. Regression analysis method was applied to determine optimal estimate of identified static characteristics of ions distribution. The main purpose of this research was to obtain data for the correct placement of ion generators in air conditioned or ventilated rooms. 相似文献