全文获取类型
收费全文 | 1949篇 |
免费 | 29篇 |
出版年
2017年 | 11篇 |
2016年 | 9篇 |
2015年 | 14篇 |
2014年 | 12篇 |
2013年 | 47篇 |
2012年 | 63篇 |
2011年 | 71篇 |
2010年 | 133篇 |
2009年 | 133篇 |
2008年 | 112篇 |
2007年 | 156篇 |
2006年 | 81篇 |
2005年 | 91篇 |
2004年 | 44篇 |
2003年 | 25篇 |
2002年 | 19篇 |
2001年 | 18篇 |
2000年 | 30篇 |
1999年 | 33篇 |
1998年 | 29篇 |
1997年 | 53篇 |
1996年 | 64篇 |
1995年 | 29篇 |
1994年 | 36篇 |
1993年 | 40篇 |
1992年 | 45篇 |
1991年 | 39篇 |
1990年 | 41篇 |
1989年 | 40篇 |
1988年 | 30篇 |
1987年 | 38篇 |
1986年 | 18篇 |
1985年 | 33篇 |
1984年 | 30篇 |
1983年 | 22篇 |
1982年 | 25篇 |
1981年 | 26篇 |
1980年 | 17篇 |
1979年 | 23篇 |
1978年 | 27篇 |
1977年 | 14篇 |
1976年 | 17篇 |
1975年 | 16篇 |
1974年 | 16篇 |
1973年 | 12篇 |
1972年 | 24篇 |
1971年 | 24篇 |
1970年 | 5篇 |
1969年 | 5篇 |
1968年 | 4篇 |
排序方式: 共有1978条查询结果,搜索用时 46 毫秒
991.
CONTROL mechanisms responsible for determining the intensity and duration of the immune response have been studied extensively. Jerne1 and Uhr and Möller2 described the phenomenon of feedback inhibition whereby the production of a 7S class of antibodies terminates the formation of antibodies with the same specificity. An immunosuppressive effect has also been observed in antigenic competition, in which the administration of one antigen results in a reduced response to a subsequently administered second antigen3. Recently, several investigations have focused on the mechanism of antigenic competition4,5. A humoral rather than cellular mechanism is generally believed to be responsible for this phenomenon4–6, but attempts to identify the humoral mediator have been unsuccessful. We have also searched for a humoral substance responsible for the immunosuppressive effect in antigenic competition and found in the sera of adult untreated mice an immunosuppressive agent whose concentration was substantially increased after antigenic stimulation. 相似文献
992.
993.
SANDRA A. ALLAN NICOLAS BARRÉ DANIEL E. SONENSHINE & MICHAEL J. BURRIDGE 《Medical and veterinary entomology》1998,12(2):141-150
The efficacy of tags impregnated with pheromone and acaricide for control of Amblyomma variegatum on cattle in Guadeloupe was determined for a 13-week trial. Comparisons were made between untreated cattle and cattle with tags containing either pheromones alone (o-nitrophenol, methyl salicylate, 2,6-dichlorophenol and phenylacetaldehyde), pheromones plus acaricide (cyfluthrin or deltamethrin), or acaricide alone. Tags were fastened to cattle both on collars on the neck and with adhesive to tail hairs, with over 98 and 90% of tags retained, respectively, during the trial. By the end of the trial, tick infestations increased on untreated cattle (311.7%) and cattle with pheromone tags (154.8%) but decreased on cattle with pheromone/cyfluthrin (–45.0%), cyfluthrin (–42.8%), pheromone/deltamethrin (–68.7%) and deltamethrin tags (–87.6%). Cattle with pheromone tags had greater proportions of ticks on the hind regions (81%) compared to untreated cattle (62.5%) and on the front regions (18.2%) compared to untreated cattle (8.2%) indicating that ticks aggregated in response to pheromones. Analysis of hair samples by gas chromatography indicated that, within 2 days of placement, cyfluthrin spread rapidly over the body from the tags. Both cyfluthrin and deltamethrin were present in detectable levels on all eight body regions examined throughout the trial. Analysis of the tags indicated that they all still contained pheromone components after 13 weeks under field conditions. 相似文献
994.
995.
EDWARD C. McMANUS MICHAEL T. OBERDICK ASHTON C. CUCKLER 《The Journal of eukaryotic microbiology》1967,14(3):379-381
SYNOPSIS. Sulfaquinoxaline and ethopabate are 2 chemically distinct types of antagonist of PABA with anticoccidial action. Reversal experiments with PABA and synergism studies with pyrimethamine indicated that both compounds interfered with the PABA-folic acid metabolic sequence. Six pure strains of Eimeria brunetti responded differently to the 2 compounds. The strain most sensitive to ethopabate was one of 2 strains most resistant to sulfaquinoxaline. Conversely, the strain most sensitive to sulfaquinoxaline was unresponsive to the highest doses of ethopabate employed. Hypotheses to explain the differences in biological effects of the 2 compounds were considered. 相似文献
996.
997.
MICHAEL W. DICK F.L.S. DAVID L. HAWKSWORTH V.-P.L.S. 《Botanical journal of the Linnean Society. Linnean Society of London》1985,91(1-2):175-179
A synopsis of the biology of the Ascomycotina. A wide variety of classifications of the Ascomycotina has been proposed but a consensus is being reached on the main orders that it is appropriate to recognize. Most of these orders are well characterized with respect to their ecology and nutritional requirements although defined primarily on morphology. The 43 orders are displayed diagrammatically to illustrate their host and substratum requirements. This display is intended to stimulate argument and research by broadening the consideration of evolutionary pathways to include ecological and nutritional factors. It will also be of value as a teaching aid; overlays can be constructed to show additional features not treated here. 相似文献
998.
999.
The mole rat, Spalax ehrenbergi Nehring, is a complex group involving four chromosomal types of In = 52, 54, 58 and 60. These are clinally distributed from Mount Hermon in the north to the Negev in the south, covering humid, semihumid, semiarid and arid climates. Forty-seven breeding nests of Sehrenbergi were collected from 23 localities throughout the range and processed for nidicolous arthropods. A total of five species of fleas and 53 species of gamasid mites were collected. Their distribution in the investigated area has been analysed in relation to that of the chromosomal types of S.ehrenbergi. The following conclusions have been reached:(a) Only three of the species (two haematophagous gamasids and one fungivorous acarid) range across the whole investigated area, showing strong host specificity, (b) A correlation is apparent between the distribution of several nidicolous arthropods (fleas and gamasids) and the ‘54’ geographic region. This probably reflects geographical distribution rather than host specificity, (c) Palaearctic species reach their southern limit of distribution on Mount Hermon and the Golan Heights; possibly Ethiopian ones reach their northern limit in the Negev. (d) No direct correlation has been found between nidicolous arthropods and the chromosomal types of S. ehrenbergi. (e) Subjects for future investigation are outlined. 相似文献
1000.
There are presently two hypotheses concerning the modulationof avian vocalizations, one holding that resonance phenomenaplay an important role, the other that modulations are sourcegenerated. A wide variety of data tend to question the formerand support the latter. The Greenewalt model is examined in the light of recent findings,many of which support its major contentions and implications.On the other hand, the model seems to have certain weaknessesin describing the interactions of pressure, tension, and membranemovement, hence the linkage of AM and FM. Mathematical expressionsof these relationships are developed. They show that the amplitudeof oscillation will vary with the magnitude of the driving pressureperturbation and with the interaction of the frequencies ofthe perturbation and the resonant frequency of the membrane.This suggests an alternative mechanism for the AM-FM linkage. Caution is advised in the development of generalized models.Although all birds may exploit certain fundamental principles,the possibility remains that a variety of techniques may beemployed. A major difference between the techniques utilizedby oscines and non-oscines is especially probable. 相似文献