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   4478篇
  免费   199篇
  国内免费   247篇
  2023年   32篇
  2022年   47篇
  2021年   83篇
  2020年   94篇
  2019年   113篇
  2018年   88篇
  2017年   73篇
  2016年   104篇
  2015年   100篇
  2014年   141篇
  2013年   221篇
  2012年   150篇
  2011年   163篇
  2010年   150篇
  2009年   182篇
  2008年   220篇
  2007年   243篇
  2006年   193篇
  2005年   222篇
  2004年   220篇
  2003年   168篇
  2002年   194篇
  2001年   133篇
  2000年   107篇
  1999年   117篇
  1998年   136篇
  1997年   119篇
  1996年   127篇
  1995年   90篇
  1994年   87篇
  1993年   86篇
  1992年   75篇
  1991年   72篇
  1990年   60篇
  1989年   58篇
  1988年   53篇
  1987年   38篇
  1986年   34篇
  1985年   58篇
  1984年   46篇
  1983年   17篇
  1982年   29篇
  1981年   29篇
  1980年   23篇
  1979年   28篇
  1978年   25篇
  1977年   18篇
  1976年   17篇
  1975年   13篇
  1973年   9篇
排序方式: 共有4924条查询结果,搜索用时 46 毫秒
951.
Abstract

We attempted to determine the growth characteristics of cultured lung fibroblasts of layer type chickens and to investigate presence of matrix metalloproteinases (MMPs) and their tissue inhibitors (TIMPs) in these cells in vitro. Lung fibroblasts were isolated, characterized and subcultured from one-day-old layer type chicken lungs. Two different methods, explant culture and enzymatic techniques, were used for culturing and the results were compared. The presence of MMP-2 and TIMP-1 was shown in cultured fibroblasts by immunocytochemical staining, immune blotting and zymography methods. Immune expressions of neither MMP-9 nor TIMP-2 enzymes could be detected.  相似文献   
952.
Cercariae of Himasthla quissetensis were cultivated on the chick chorioallantois maintained at 38 ± 1°C and a relative humidity of 70–75%. Of 68, 6-day-old eggs, each inoculated with 100 cercariae, 28 (41%) were examined 1–9 days post-inoculation. Of the 28 eggs, 23 (82%) were infected with a total of 224 live worms plus 10 encysted metacercariae. Worms contained blood or hematin-like material in their intestinal caeca and showed considerable development of reproductive structures. A total of 58, 6- or 7-day-old chorioallantoic-worms were serially transferred to new 6-day-old embryos, but only a single live worm was recovered after 13 days on two membranes. Based on fixed and stained specimens, 7-day-old chorioallantoic-worms increased their body area about 4× compared to cercariae and the 13-day-old worm increased its body area about 10×.  相似文献   
953.
Total protein was extracted from zygotic embryos and from somatic embryos of Picea abies (L.) Karst. (Norway spruce) cultured in vitro at different times during their development. An analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and 2-dimensional gel electrophoresis of the protein extracts showed that protein composition and the temporal changes in protein abundance were very similar in the two embryo types. Both zygotic and somatic embryos accumulated storage proteins in abundance during their maturation phase of growth; the somatic embryos when cultured on medium containing 90 m M sucrose and 7.6 μ M ABA. The major storage proteins are composed of polypeptides with molecular masses of about 22, 28, 33 and 42 kDa and they are identical in both embryo types according to their molecular mass and average isoelectric points. These proteins are also the most abundant proteins in the female gametophytic tissue of the mature seed.  相似文献   
954.
Mary Bettey  Alison M. Smith 《Planta》1990,180(3):420-428
The aim of this work was to discover why pea (Pisum sativum L.) embryos recessive at the r locus (rr) have a higher lipid content than embryos dominant at this locus (RR). The r locus is a gene encoding starch-branching enzyme, rr embryos have a much lower activity of this enzyme than RR embryos, and hence a reduced rate of starch synthesis. The higher lipid content of rr embryos must be a consequence of this. We suggest that neither differences in the availability of substrate for lipid synthesis as a consequence of different rates of starch synthesis, nor differences in the capacity of the pathway for malonyl-CoA synthesis, account for the different lipid contents of RR and rr embryos. Lipid contents of the two sorts of embryo first diverge at a much later stage in development than divergence in starch content. Amounts of pyruvate and acetate, and activities of enzymes that convert triose phosphate to malonyl CoA are the same in the two sorts of embryo. Most of the lipid in developing embryos is polar, structural lipid, and polar lipid accounts for a large proportion of the difference in lipid content between the two sorts of embryo. This difference in structural-lipid content reflects considerable structural differences between the two sorts of embryo and is presumably the consequence of differences in rates of lipid turnover.Abbreviations DW dry weight - FW fresh weight - FAME fatty-acid methyl esters This work was supported by a grant-in-aid from the Agricultural and Food Research Council to the John Innes Institute. We are very grateful to Alan Jones for his valuable advice on lipid analysis and to Dr. Kay Denyer (Advanced Technologies, Cambridge, UK) for valuable discussions. We thank Dr. Cliff Hedley for the gift of the seed of the peas used in this work.  相似文献   
955.
Callus cultures were established from immature embryos of Calotropis gigantea (Linn.) R. Br. on a modified basal medium of Murashige & Skoog supplemented with 1 mgl-1 2,4-D. In addition to 0.1 mgl-1 of NAA the optimal BAP concentration for promoting shoot bud formation and growth was 2 mgl-1. Rooting was induced when shoots were transferred to auxin-supplemented Bonner's solution or half-strength MS basal salt solutions.Abbreviations NAA -naphthaleneacetic acid - 2,4-D 2,4-dichlorophenoxyacetic acid - IAA indole-3-acetic acid - IBA indole-butyric acid - BAP 6-benzylaminopurine - Kin kinetin  相似文献   
956.
Hyperoxia and gamma-irradiation were found to be mutagenic in a transformed Syrian hamster cell line in a dose-dependent manner. The frequency of resistance to 6-thioguanine increased from 10 per 10(6) survivors after 48 h of growth in 70% O2 to 32.6 (highly significant) after 75 h. Increasing the oxygen tension to 95% resulted in a significant mutagenic response in only 44 h. At equitoxic doses, gamma-irradiation was 4 times more mutagenic than 70% O2. After growth in hyperoxia, the cells showed an enhancement of catalase activity, glutathione peroxidase activity and glutathione levels but there was little effect on superoxide dismutase activity. Diethyldithiocarbamate (3 mM, 1.5 h) was mutagenic in normoxia and potentiated the mutagenic activity of both gamma-irradiation and hyperoxia. Cells thus treated showed an 855 reduction in superoxide dismutase activity. When diethyldithiocarbamate was used in conjunction with a direct-acting alkylating agent, the mutagenic response was only additive. Depletion of cellular glutathione with buthionine sulfoximine (0.2 mM) or inhibition of catalase activity with aminotriazole (100 mM) was also effective in potentiating the mutagenic response of gamma-irradiation and hyperoxia. The data demonstrates that endogenously produced activated oxygen species are mutagenic to hamster cells in culture and suggest that aerobic organisms are subject to an unavoidable background risk due to living in an oxygen atmosphere.  相似文献   
957.
958.
During late embryogenesis in a cockroach, the epidermal cells secrete two cuticles: the embryonic cuticle and the pharate first larval cuticle. Late embryogenesis begins with the deposition of the cuticulin layer of the embryonic cuticle. The embryonic cuticle is an atypical one. It remains relatively thin and a well lamellated endocuticle is usually lacking. After general apolysis of the embryonic cuticle the epidermis secretes the epicuticle of the first larval cuticle and, subsequently, a typical lamellate procuticle. During the penultimate phase of late embryogenesis (i.e. before general apolysis) the epidermis becomes larvally committed. Some epidermal cells start to differentiate into specialized structures of the dermal glands, whereas the differentiated oenocytes appear to have acquired some stability. Nevertheless, shortly before general apolysis some oenocytes display signs of an increased alteration of the SER. When general apolysis occurs, the oenocytes contain a well-developed SER. The whole of the oenocyte population is programmed to regress after epicuticle deposition of the first larval cuticle. The correlation of oenocyte regression with available data on cuticulogenesis, ecdysteroid titres and cuticular lipid synthesis is discussed.  相似文献   
959.
Following synchronisation of oestrus using FGA and PMSG, ewes were inseminated by either the conventional cervical (CC) method or directly into the uterus by laparoscopy (LI). The CC method was carried out either at 48 and 60 hours following progestagen withdrawal with 480 x 10(6) spermatozoa per inseminate or once only at 56 hours with 600 x 10(6) spermatozoa. The laparoscopic method was performed at 52 hr using 48 x 10(6) spermatozoa per ewe. In the first two trials eggs were recovered at laparotomy. The egg recovery rate was significantly lower (P<0.05) for those ewes which had been inseminated by the LI method (74%) compared with those inseminated by the CC method (85%); fertilization rates were not significantly different (92% and 89% respectively). In the third trial 20 ewes were bled to determine their periovulatory LH concentrations and the timing of peak LH concentrations correlated with the outcome of each insemination. Ewes inseminated using laparoscopy did not conceive when their LH surge occurred >58 hr after progestagen withdrawal. In this and in the final experiment, the combined pregnancy rates and litter sizes (assessed radiographically) were 67% (n = 51) and 2.21 (n = 34) for the CC method and 75% (n = 48) and 1.97 (n = 36) for the LI method (P>0.05).  相似文献   
960.
An examination has been made of embryos from infertile 1-29 translocation carrying heifers. Cytogenetic measurements of C-banded chromosomes from these cattle, together with those from fertile 1-29 cows, have been made and compared with those from normal karyotypes. There are relative differences between normal animals and translocation carriers in the ratios of total to C-banded material throughout the karyotypes: their possible effects on mitosis and meiosis are discussed. Of the embryos recovered from the infertile heifers, one did not survive transplantation and others had various indications of inviability.  相似文献   
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