全文获取类型
收费全文 | 7114篇 |
免费 | 1133篇 |
国内免费 | 3732篇 |
出版年
2024年 | 88篇 |
2023年 | 252篇 |
2022年 | 360篇 |
2021年 | 410篇 |
2020年 | 406篇 |
2019年 | 443篇 |
2018年 | 280篇 |
2017年 | 294篇 |
2016年 | 290篇 |
2015年 | 427篇 |
2014年 | 619篇 |
2013年 | 529篇 |
2012年 | 737篇 |
2011年 | 672篇 |
2010年 | 575篇 |
2009年 | 590篇 |
2008年 | 625篇 |
2007年 | 624篇 |
2006年 | 557篇 |
2005年 | 472篇 |
2004年 | 378篇 |
2003年 | 314篇 |
2002年 | 248篇 |
2001年 | 263篇 |
2000年 | 261篇 |
1999年 | 178篇 |
1998年 | 95篇 |
1997年 | 71篇 |
1996年 | 46篇 |
1995年 | 45篇 |
1994年 | 27篇 |
1993年 | 26篇 |
1992年 | 46篇 |
1991年 | 52篇 |
1990年 | 34篇 |
1989年 | 46篇 |
1988年 | 33篇 |
1987年 | 46篇 |
1986年 | 41篇 |
1985年 | 29篇 |
1984年 | 44篇 |
1983年 | 34篇 |
1982年 | 36篇 |
1981年 | 29篇 |
1979年 | 25篇 |
1978年 | 25篇 |
1977年 | 35篇 |
1975年 | 18篇 |
1974年 | 23篇 |
1973年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Comparison of zona cutting and zona drilling as techniques for assisted fertilization in the mouse 总被引:2,自引:0,他引:2
H T Depypere K J McLaughlin R F Seamark G M Warnes C D Matthews 《Journal of reproduction and fertility》1988,84(1):205-211
Zona cutting and zona drilling of the mouse oocyte significantly increased the fertilization rate (3.8-90%) at low sperm concentrations (less than 200,000/ml) compared with zona-intact controls (0-45%). More oocytes were fertilized after zona drilling. Zona cutting was associated with a low loss of oocytes (less than 1%), no increase in polyspermy and normal development in vitro and in vivo after fertilization. There was a 4% oocyte loss rate after zona drilling, mostly due to extrusion of the oocyte from the zona during the procedure. Hatching of blastocysts occurred about 12 h earlier for zona-drilled than for zona-cut and zona-intact control oocytes. Zona drilling was associated with a higher, but not statistically significant, rate of polyspermy at all sperm concentrations tested. The proportion of zygotes developing to the blastocyst stage was not different between the techniques (zona cut, 77%; zona drilled, 66%; control, 71%). Similarly, no difference was found in the percentage of embryos implanting after blastocyst transfer to the uterine horns of pseudopregnant female mice (zona cut, 67%; zona drilled, 68%; control, 77%). Transmission electron microscopy demonstrated the induced defects in the zona with no damage to the oocyte or oolemma. Parthenogenetic activation was not seen after either of the micromanipulative techniques. Both techniques have promise for application to the human. 相似文献
22.
23.
蚤数量与宿主数量关系 总被引:9,自引:5,他引:4
无论在自然条件下或在人为条件下,蚤指数和染蚤率的高低与宿主密度的高低是一致的.宿主密度的升降,会导致其寄生蚤指数和染蚤率的升降. 本文讨论了宿主数量下降导致其寄生蚤数量下降的原因,仅是分析推测,提出几方面的看法. 相似文献
24.
Seedlings of two cultivars of wheat (Triticum aestivum L.) differing in tolerance to aluminium (Al) were grown using a split-root sand/soil culture technique. Each culture tube
was divided horizontally into a surface (0–150 mm) compartment and a subsurface (150–250 mm) compartment separated by a root-permeable
paraffin wax barrier. Thus phosphorus (P) supplied to surface roots could not percolate or diffuse into the soil in the subsurface
compartment. The soil in the subsurface compartment was divided into ‘rhizosphere’ and ‘non-rhizosphere’ zones using a porous
(5 μm) membrane.
Root growth of both cultivars into the subsurface zone was enhanced by increased P supply to surface roots, but did not conform
to known relationships between root growth and soil pH, extractable-Al, or pH, Al or P concentrations in soil solution. Concentrations
of Al in soil solution in the rhizosphere were greater than those in solution in the bulk soil. Concentrations of Al reactive
with pyrocatechol violet (30s-RRAI) in the rhizosphere soil solution were generally greater than those in non-rhizosphere
soil. With the Al-sensitive cultivar, root dry weight and length increased as concentrations of RRAl in the rhizosphere soil
solution increased. Increased concentrations of Al in rhizosphere soil solutions were not related to the presence of organic
ligands in solution.
The effect of P in promoting root penetration into the acidic subsurface stratum was not related to differential attainment
of maturity by the plant shoots, but appeared to be related to the effect of P in enhancing the rate of root growth. Thus,
suboptimal supply of P to the surface roots of a plant, even at levels sufficient to preclude development of nutritional (P)
stress symptoms, may seriously reduce tolerance to Al, and hence diminish the ability of roots to penetrate into acidic subsoils. 相似文献
25.
本文报导四川省西部鱼类寄生粘孢子虫粘体虫属六新种,即异型粘体虫,新种Myxosoma disparis sp.nov.,四川粘体虫,新种Myxosoma sichuanensis sp.nov.,光唇粘体虫,新种Myxosoma acrossochilusi sp.nov.鳅粘体虫,新种Myxosoma nemachilusi sp.nov.斜囊粘体虫,新种Myxosoma obliqua sp.nov.,雅安粘体虫,新种Myxosoma yaanensis sp.nov.。 相似文献
26.
关于苦马豆属学名的讨论 总被引:2,自引:0,他引:2
本文根据国内外豆科研究文献的考证,讨论了苦马豆属(Sphaerophysa DC.)及其近缘属的区别,重新确认了国产苦马豆属和苦马豆的学名。 相似文献
27.
28.
Abstract The vitellogenic period of gypsy moth ovarian development starts on day 3 of the pupal stage and continues through adulthood. During this period, rapid increases occur in follicle size, protein content, and wet weight of the ovary. Patency is observed on day 3 of the pupal stage.
Pre-vitellogenic follicles are formed in the last larval stadium. Newly formed follicles detach from the germarium on day 4, and increase rapidly to 140 per ovariole at the end of the last larval stadium. The pre-vitellogenic follicles are uniformly around 50 um in diameter. No vitellogenin is incorporated into the oocytes until the pupal stage.
Polyacrylamide gel electrophosesis (PAGE) in the presence of sodium dodecylsulphate (SDS) analysis of male and female haemolymph samples and vitellogenic ovaries demonstrates the presence of two female-specific subunits of vitellogenin of 180 kD and 160 kD. These proteins are detected only in haemolymph and ovarian extracts of vitellogenic females. The molecular weight of the native protein determined by size exclusion chromatography is approximately 400–420 kD.
A highly sensitive double antibody sandwich enzyme-linked immunosorbent assay (ELISA) was developed to monitor the temporal changes in vitellogenin titre in haemolymph. Vitellogenin production starts on day 2 of the last larval stadium, reaching a maximum level by day 6 of the last larval stadium, and decreasing in the late pupal stage as vitellogenin was internalized into the oocytes. This is the first report of vitellogenin production occurring in the larval stage of a holometabolous insect. The fact that vitellogenin production and uptake occur during different stages of development in the gypsy moth, opens up some interesting questions concerning the underlying regulatory mechanisms controlling each process. 相似文献
Pre-vitellogenic follicles are formed in the last larval stadium. Newly formed follicles detach from the germarium on day 4, and increase rapidly to 140 per ovariole at the end of the last larval stadium. The pre-vitellogenic follicles are uniformly around 50 um in diameter. No vitellogenin is incorporated into the oocytes until the pupal stage.
Polyacrylamide gel electrophosesis (PAGE) in the presence of sodium dodecylsulphate (SDS) analysis of male and female haemolymph samples and vitellogenic ovaries demonstrates the presence of two female-specific subunits of vitellogenin of 180 kD and 160 kD. These proteins are detected only in haemolymph and ovarian extracts of vitellogenic females. The molecular weight of the native protein determined by size exclusion chromatography is approximately 400–420 kD.
A highly sensitive double antibody sandwich enzyme-linked immunosorbent assay (ELISA) was developed to monitor the temporal changes in vitellogenin titre in haemolymph. Vitellogenin production starts on day 2 of the last larval stadium, reaching a maximum level by day 6 of the last larval stadium, and decreasing in the late pupal stage as vitellogenin was internalized into the oocytes. This is the first report of vitellogenin production occurring in the larval stage of a holometabolous insect. The fact that vitellogenin production and uptake occur during different stages of development in the gypsy moth, opens up some interesting questions concerning the underlying regulatory mechanisms controlling each process. 相似文献
29.
Synopsis It is commonly accepted that more active fishes have a greater proportion of red muscle in their trunk musculature than do less active fishes. Further, the proportion of red muscle has been used to classify fish species into functional groups reflecting different activity patterns. Nevertheless, existing measures of both red muscle and mobility have several limitations, and the relationship between these parameters has never been evaluated quantitatively. Using data from the literature, we demonstrate a positive, statistical association between the proportion of red muscle in the caudal peduncle of marine fishes and a qualitative measure of mobility (categorization as sedentary vs. mobile based on natural-history accounts). Analyses of the frequency distribution of the proportion of red muscle also provide evidence for two subdistributions. However, this bimodality does not correspond with sedentary vs. mobile or sit-and-wait vs. active search dichotomies. 相似文献
30.