全文获取类型
收费全文 | 544篇 |
免费 | 8篇 |
国内免费 | 1篇 |
专业分类
553篇 |
出版年
2014年 | 4篇 |
2013年 | 8篇 |
2012年 | 8篇 |
2011年 | 11篇 |
2010年 | 20篇 |
2009年 | 14篇 |
2008年 | 20篇 |
2007年 | 22篇 |
2006年 | 22篇 |
2005年 | 17篇 |
2004年 | 8篇 |
2000年 | 4篇 |
1999年 | 7篇 |
1998年 | 14篇 |
1997年 | 21篇 |
1996年 | 14篇 |
1995年 | 15篇 |
1994年 | 15篇 |
1993年 | 17篇 |
1992年 | 7篇 |
1991年 | 7篇 |
1990年 | 19篇 |
1989年 | 14篇 |
1988年 | 10篇 |
1987年 | 9篇 |
1986年 | 9篇 |
1985年 | 7篇 |
1984年 | 7篇 |
1983年 | 8篇 |
1982年 | 12篇 |
1981年 | 11篇 |
1980年 | 8篇 |
1979年 | 13篇 |
1978年 | 11篇 |
1977年 | 6篇 |
1976年 | 8篇 |
1975年 | 15篇 |
1974年 | 6篇 |
1973年 | 9篇 |
1972年 | 17篇 |
1971年 | 7篇 |
1969年 | 5篇 |
1968年 | 4篇 |
1959年 | 6篇 |
1955年 | 3篇 |
1954年 | 6篇 |
1953年 | 4篇 |
1952年 | 8篇 |
1951年 | 6篇 |
1950年 | 3篇 |
排序方式: 共有553条查询结果,搜索用时 15 毫秒
61.
SUSAN G. LANGRETH JAMES B. JENSEN ROBERT T. REESE WILLIAM TRAGER 《The Journal of eukaryotic microbiology》1978,25(4):443-452
SYNOPSIS. The erythrocytic cycle of the human malaria parasite, Plasmodium falciparum, was examined by electron microscopy. Three strains of parasites maintained in continuous culture in human erythrocytes were compared with in vivo infections in Aotus monkeys. The ultrastructure of P. falciparum is not altered by continuous cultivation in vitro. mitochondria contain DNA-like filaments and some cristae at all stages of the erythrocytic life cycle. The Golgi apparatus is prominent at the schizont stage and may be involved in the formation of rhoptries. In culture, knob-like protrusions first appear on the surface of trophozoite-infected erythrocytes. The time of appearance of knobs on cells in vitro correlates with the life cycle stage of parasites which are sequestered from the peripheral circulation in vivo. Knob material of older parasites coalesces and forms extensions from the erythrocyte surface. Some of this material is sloughed from the host cell surface. The parasitophorous vacuole membrane breaks down in erythrocytes containing mature merozoites both in vitro and in vivo. Merozoite structure is similar to that of P. knowlesi. The immature gametocytes in culture have no knobs. 相似文献
62.
63.
64.
Pollen Ontogeny in Catananche caerulea L. (Compositae: Lactuceae) II. Free Microspore Stage to the Formation of the Male Germ Unit 总被引:1,自引:0,他引:1
The development of Catananche caerulea pollen grains from theearly free microspore stage to the formation of the male germunit has been studied using the scanning electron microscope.The differentiation of the elaborate ectexine from primexineand the accompanying cytoplasmic changes in the microsporesare described. Changes in the tapetum, reflecting first theformation of sporopollenin precursors and then of pollenkitt,are also described. The origin of taxonomically important featuresis discussed. Catananche caerulea, Compositae: Lactuceae, microspore mitosis, male germ unit, ontogeny, primexine differentiation, scanning electron microscopy 相似文献
65.
66.
SYNOPSIS. While chewing is not unique to mammals, it is oneof their most distinctive characteristics. Historically, studiesof food processing in mammals were intended to provide evolutionaryinsights, but more progress has been made in understanding mechanisticaspects. Mastication is considered under five headings. (1)Interaction of teeth with food.Knowledge of comparative dentalanatomy and function is advanced in comparison to understandingof foods and how they are broken down. (2) Chewing force andits resistance by the skull. The traditional assumption thatocclusal force is maximized is not always justified, and experimentalresults suggest that skull loading is far more dynamic and variablethan had been envisioned from theoretical analyses. (3) Howthe jaw moves. The most important masticatory movement is thatof the power stroke, and in most but not all species this isinfluenced more by the inclined planes of the teeth and jawjoints than by the musculature. (4) The role of muscles in producingboth force and movement. The most fundamental distinction amongjaw muscles is whether they have a rostral or caudal directionof pull, as this determines their role in transverse jaw movements.Reliance on anatomical names tends to obscure functional similaritiesand differences among species. (5) Intraoral structures. Becausethey are difficult to study, the actions of the tongue and pharynxare still debated. Even the fundamental question of whethermammals can breathe and swallow at the same time has not beendefinitively answered. 相似文献
67.
Fluorescently-labeled Oligonucleotide Probes Can Be Used To Identify Protistan Food Vacuole Contents 总被引:1,自引:0,他引:1
ABSTRACT. In situ hybridization using fluorescent oligonucleotide probes complementary to unique regions of 16S rRNA molecules provides a way of identifying the food vacuole contents of bactivorous protists. Laboratory experiments with Tetrahymena showed rRNAs in food vacuoles are degraded slowly enough to permit their use as hybridization targets for such probes. A probe specific for a hypervariable region of the small subunit rRNA of an unnamed proteobacterium abundant in a local lake was then synthesized. It was used to probe the food vacuoles of the ciliates present in fixed water samples collected from the same lake. The vacuoles of several filter-feeding ciliates bound the probe, indicating that such probes can be used to identify the food vacuole contents of ciliates collected from natural samples. 相似文献
68.
SYNOPSIS. In the last quarter of the nineteenth century severalinvestigators including Richard and Oskar Hertwig, Theodor Boveri,Hans Driesch, Curt Herbst, T. H. Morgan and others turned theirattention to sea urchin eggs and early embryos. This favorablecombination of outstanding investigators and the sea urchinembryo as an experimental organism contributed to a fundamentalunderstanding of the cell, fertilization and heredity. The advantagesof the sea urchin continued to be recognized as experimentalembryologists used these embryos to develop the concepts ofgradients, regulative development and inductive interactions.Then, as developmental biology arose from chemical embryology,the sea urchin embryo once again emerged as an ideal experimentalanimal, pivotal in the understanding of the molecular and developmentalbiology of eukaryotic organisms. 相似文献
69.
ELIZABETH M. HAUSRATH LAURA J. LIERMANN CHRISTOPHER H. HOUSE JAMES G. FERRY SUSAN L. BRANTLEY 《Geobiology》2007,5(1):49-61
Methanogens, thought to be present on early Earth, have a high requirement for Ni, suggesting that Ni utilization could be a potential biosignature for methanogens if enhanced Ni extraction from surrounding minerals accompanies methanogenic growth. To test the potential for such Ni extraction from minerals by methanogens, Ni release from Ni‐containing silicate glass was measured in Ni‐free growth medium in the presence of the methanogen Methanothermobacter thermoautotrophicus (average pH ∼7.0) and observed to be higher than an abiotic control (average pH ∼6.8). However, batch dissolution experiments and a siderophore assay indicate that cell exudates such as siderophores, low molecular weight organic acids, or lysates accompanying cell death are not responsible for the observed increase in Ni release rate. In addition, scanning electron microscopy (SEM) shows little to no evidence of direct microbe–mineral interactions such as biofilms or pitting. Instead, comparison with abiotic experiments suggests that changes in pH due to CO2 uptake may be responsible for enhanced dissolution in the presence of metabolizing cells. These results document that methanogens may not preferentially extract Ni from surrounding minerals although they may indirectly affect mineral reaction rates that are pH sensitive. Thus identifiable Ni biosignatures may not exist in the rock record to document the presence of methanogens on early Earth or Mars. 相似文献
70.
ASTRID MICHELS KATHLEEN R. LAIRD SUSAN E. WILSON DAVID THOMSON† PETER R. LEAVITT‡ ROBERT J. OGLESBY§ BRIAN F. CUMMING 《Global Change Biology》2007,13(7):1295-1307
Three high-resolution climatic reconstructions, based on diatom analyses from lake sediment cores from the Canadian prairies, show that shifts in drought conditions have prevailed on centennial to millennial time scales for at least the past six millennia. These shifts in mean aridity exhibit broad regional synchrony, with particularly pronounced shifts at all sites between ∼1700–2000 cal. yr bp and ∼3600–3900 cal. yr bp , as well as at ∼5400–5500 cal. yr bp for the two sites which extend back to at least 6000 cal. yr bp . The two Saskatchewan lakes exhibited significant coherence in both the timing and direction of these shifts, whereas inferred changes at the westernmost site in Alberta were significantly correlated to the Saskatchewan sites, but opposite in sign, and exhibited more high-frequency variation on the scale of centuries. The mechanisms behind these abrupt shifts in aridity are poorly understood, but may be linked to changes in oceanic–atmospheric interactions that influence the mean position of the jetstream and the associated storm tracks. Natural shifts in mean climatic conditions may accelerate with increasing carbon dioxide levels intensifying the likelihood of extreme droughts in North American prairies. 相似文献