全文获取类型
收费全文 | 2065篇 |
免费 | 164篇 |
国内免费 | 343篇 |
出版年
2024年 | 2篇 |
2023年 | 26篇 |
2022年 | 31篇 |
2021年 | 33篇 |
2020年 | 61篇 |
2019年 | 79篇 |
2018年 | 85篇 |
2017年 | 61篇 |
2016年 | 59篇 |
2015年 | 78篇 |
2014年 | 83篇 |
2013年 | 118篇 |
2012年 | 73篇 |
2011年 | 83篇 |
2010年 | 77篇 |
2009年 | 105篇 |
2008年 | 103篇 |
2007年 | 111篇 |
2006年 | 92篇 |
2005年 | 76篇 |
2004年 | 90篇 |
2003年 | 94篇 |
2002年 | 88篇 |
2001年 | 82篇 |
2000年 | 51篇 |
1999年 | 55篇 |
1998年 | 68篇 |
1997年 | 68篇 |
1996年 | 54篇 |
1995年 | 56篇 |
1994年 | 31篇 |
1993年 | 37篇 |
1992年 | 33篇 |
1991年 | 30篇 |
1990年 | 21篇 |
1989年 | 31篇 |
1988年 | 47篇 |
1987年 | 20篇 |
1986年 | 26篇 |
1985年 | 28篇 |
1984年 | 24篇 |
1983年 | 15篇 |
1982年 | 25篇 |
1981年 | 20篇 |
1980年 | 11篇 |
1979年 | 15篇 |
1978年 | 7篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1974年 | 2篇 |
排序方式: 共有2572条查询结果,搜索用时 31 毫秒
991.
猪卵母细胞体外成熟和体外受精卵表面元素的研究 总被引:3,自引:0,他引:3
本文应用X-射线能量色散技术对猪卵泡内卵母细胞,培养不同时间卵母细胞和体外受精卵进行了表面元素的定性和定量分析,结果表明,(1)在卵母细胞和受精卵的表面均含有Na、Mg、Al、Si、S、Cl、K、Ca和Fe元素;(2)A级卵母细胞随着体外培养,Ca的含量逐渐升高,而K的含量逐渐降低;(3)B、C级卵母细胞随着其培养Al、Ca~(2+)降低;(4)受精后卵表面的Ca显著升高。 相似文献
992.
The tendency of mammalian sperm-egg fusion to occur at a site away from the first polar body was investigated in a homologous (mouse oocytes and mouse spermatozoa) and in a heterologous model (hamster oocytes and mouse spermatozoa). Following micromanipulation of the zona pellucida either in proximity to or opposite the first polar body, in vitro fertilization was performed and subsequent differences in sperm-egg interaction were evaluated. Since spermatozoa from random-bred mice do not readily penetrate intact zonae pellucidae in vitro, it is likely that zona penetration occurred through the artificial holes in both models. The creation of a gap in the zona pellucida opposite the first polar body was associated with levels of sperm fusion that were significantly higher than those resulting from manipulation near the first polar body. Spermatozoa were rarely found to penetrate the hole completely, and in general few spermatozoa were observed in the pervitelline space. The proximity between pronuclei following sperm penetration was correlated with the position of the incision with respect to the polar body. The findings suggest that breaching the zona pellucida for microsurgical fertilization should be performed away from the microvillus-free area of the oocyte. 相似文献
993.
W. E. Friedman 《Protoplasma》1991,165(1-3):106-120
Summary Fertilization inEphedra trifurca was examined with a combination of light and fluorescence microscopy. Developmental analysis clearly indicates that double fertilization events, similar to those described inE. nevadensis, regularly occur during the process of sexual reproduction inE. trifurca. In addition to the typical fusion of a sperm nucleus and egg nucleus, a second fertilization event occurs between the second sperm nucleus from an individual pollen tube and the ventral canal nucleus. Both of the fertilization events take place within the confines of an individual egg cell of the female gametophyte. Microspectrofluorometric data demonstrate that each nucleus involved in a sexual fusion event proceeds through the synthesis phase of the cell cycle and increases its DNA content from 1C to 2C before the process of nuclear fusion is completed. Photometric data also confirm that the product of the second fertilization event is equal in DNA content (4C) to the zygotic nucleus derived from the first fertilization event, and is prepared to enter into mitosis as a fully functional diploid nucleus.Abbreviations DAPI
4;,6-diamidino-2-phenylindole
- RFU
relative fluorescence units 相似文献
994.
Ca−K exchange isotherms of samples from carribean (Martinique island) volcanic soils differing in the weathering stage and
in the nature of cation-exchange material were studied. Allophanic soils high in organic matter (Troporthents, Hydrandepts)
exhibit a low selectivity for K-ions. Potassium is adsorbed specifically in the halloysitic soils (Humitropepts). A positive
relationship exists between the affinity for K+ and the halloysite content. This relation is most likely due to the close association of 1∶1 hydrated phyllosilicates with
2∶1 smectitic clay minerals. A lower selectivity for K was observed in the kaolinitic soil materials (Dystropepts). Because
parent rocks are very low in K, these exchange properties strongly influence their K status, through differences in susceptibility
to K leaching losses and K availability to banana plants in intensive cropping systems. 相似文献
995.
The effect of varying the sperm concentration between 2 × 105 sperm/ml and 8 × 106 sperm/ml on fertilization of cumulus-free, zona-intact F1 (CBA × C57BL) mouse ova by QS and F1 (CBA × C57BL) mouse spermatozoa was studied. The spermatozoa from both strains of mice exhibited optimal fertilization rates at 2 × 106 sperm/ml. However, at sperm concentrations greater than 4 × 106 sperm/ml and less than 1 × 106 sperm/ml, fertilization rates were significantly reduced. F1 spermatozoa were more susceptible to dilution than QS spermatozoa. A significant interaction between strain and sperm concentration indicated that the two strains produced different fertilization rates at different sperm densities. Extracts of epididymal fluid, medium from capacitated spermatozoa, or ampulla fluid did not improve the fertilization rate at 2 × 105 sperm/ml, but retaining the cumulus oophorus did. The decrease in fertilization rate at 8 × 106 sperm/ml can in part be attributed to a nondialysable inhibitor from the neat sperm preparation that appeared to be of epididymal origin. 相似文献
996.
B. G. Brackett M. A. Cofone M. L. Boice D. Bousquet 《Molecular reproduction and development》1982,5(3):217-227
Zona-free hamster ova interacted with bull and stallion spermatozoa after treatment of ejaculated semen to capacitate the sperm cells. Sperm conditioning by prolonged incubation in BWW medium (18–26 hr) prior to insemination was effective for capacitation of bull and stallion sperm. Preincubation of bull sperm for 70 or 105 min in defined medium (DM) with NaCl content elevated to result in 350 m0sM/kg also led to penetration of hamster vitelli. More rapid sperm conditioning was possible and higher proportions of interacting vitelli followed insemination with bull or stallion sperm exposed to high ionic strength DM (380–390 m0sM/kg) for 10 min before incubation in isotonic DM prior to insemination, the treatment adopted in subsequent work. Initial efforts to assess relative fertilizing ability of freshly ejaculated semen from two fertile bulls (A and B) in A1 usage resulted in uniformly high ( > 90%) levels of sperm-vitelli interaction (for both) when the hamster ova employed resulted from superovulation with PMSG and HCG. Following use of ova from untreated hamsters sperm samples of bull A and bull B interacted with 53.8% and 84.9% (P < 0.05) of zona-free hamster ova, respectively. Conception data (60–90 day nonreturn rates) resulting from A1 with semen collected during the same interval but processed and stored in liquid nitrogen prior to use revealed an inverse relationship to proportions of vitelli interacting with fresh sperm; nonreturn rates were 69.3% and 66.3% for bull A and bull B, respectively. A similar treatment effected capacitation of frozen-stored bull semen to enable sperm-vitelli interaction. These findings encourage additional efforts to correlate testing of processed semen with fertility. 相似文献
997.
Yona Barak Ruth Kaplan P. F. Kraicer Ruth Shalgi 《Molecular reproduction and development》1982,5(3):257-262
The effects of luteinizing hormone (NIH-bovine LH) and progesterone on maturation in vitro of oocyte-cumulus complexes from adult proestrous rats were studied by comparing proportions of oocytes showing germinal vesicle breakdown, mucification of the cumulus oophorus, and fertilizability. Addition of either or both of the hormones to the medium in concentrations between 1.25 and 10 μg/ml during maturation had no discernible effect on germinal vesicle breakdown or on fertilization. Mucification was stimulated by LH and even more by LH plus progesterone. It was concluded that maturation in vivo is the result of concerted action of the two hormones. However, addition of LH + progesterone had no effect on the fertilizability of these oocytes. We attribute this to a relative insensitivity of the system for fertilization in vitro to subtle changes in the oocyte. 相似文献
998.
R. Yanagimachi 《Molecular reproduction and development》1982,5(4):323-344
Using a semi-chemically defined medium, the requirement of extracellular Ca2+ for survival, capacitation, and acrosome reaction of spermatozoa as well as various stages of fertilization in the hamster was studied. A Ca2+-deficient environment is unfavorable for long-term survival of spermatozoa. Sperm capacitation may occur in Ca2+-deficient media, but not as efficiently as in normal media. The acrosome reaction definitely requires extracellular Ca2+. Other processes or phenomena that require extracellular Ca2+ are initiation and maintenance of hyperactivated motility of spermatozoa, penetration of acrosome-reacted spermatozoa into the zona pellucida, fusion of the spermatozoa with eggs, and the development of pronuclear eggs into two-cell embryos. Extracellular Ca2+ is apparently unnecessary for the attachment of spermatozoa to the zona and egg surfaces, decondensation of the sperm nucleus, and the development of sperm and egg pronuclei within the egg. These results were compared with data obtained in other species such as the sea urchin, mouse, rat and guinea pig. 相似文献
999.
"Stochastic survival models which adjust for covariate information have been developed by Beck (1979). These models can include one or two living states and several competing death states. The transitions between stages are assumed irreversible and the transition intensity functions are assumed to be independent of time but dependent upon the covariates." Explicit solutions of the maximum likelihood equations for such models when there are one or two dichotomous covariates are presented. Applications of these models to the case of heart transplants and lung cancer are discussed, and survival in two or four groups is compared. (summary in FRE) 相似文献
1000.
Passive immunization with antiserum prepared against isolated bovine zonae pellucidae inhibited fertilization in the cow. The minimum dosage of antiserum (titer 27 by immunofluorescence) required for complete inhibition was 2 ml/kg of body weight. 相似文献