全文获取类型
收费全文 | 133981篇 |
免费 | 8912篇 |
国内免费 | 2218篇 |
专业分类
145111篇 |
出版年
2021年 | 149篇 |
2019年 | 219篇 |
2018年 | 165篇 |
2017年 | 178篇 |
2016年 | 229篇 |
2015年 | 192篇 |
2014年 | 301篇 |
2013年 | 223篇 |
2012年 | 14374篇 |
2011年 | 15843篇 |
2010年 | 2266篇 |
2009年 | 1308篇 |
2008年 | 12322篇 |
2007年 | 12850篇 |
2006年 | 11974篇 |
2005年 | 11222篇 |
2004年 | 10986篇 |
2003年 | 10124篇 |
2002年 | 8686篇 |
2001年 | 6744篇 |
2000年 | 8967篇 |
1999年 | 3452篇 |
1998年 | 450篇 |
1997年 | 381篇 |
1996年 | 248篇 |
1995年 | 249篇 |
1994年 | 254篇 |
1993年 | 228篇 |
1992年 | 269篇 |
1991年 | 254篇 |
1990年 | 281篇 |
1989年 | 286篇 |
1988年 | 285篇 |
1987年 | 255篇 |
1986年 | 206篇 |
1985年 | 215篇 |
1984年 | 167篇 |
1983年 | 225篇 |
1959年 | 403篇 |
1958年 | 758篇 |
1957年 | 710篇 |
1956年 | 656篇 |
1955年 | 630篇 |
1954年 | 579篇 |
1953年 | 523篇 |
1952年 | 472篇 |
1951年 | 434篇 |
1950年 | 317篇 |
1949年 | 182篇 |
1948年 | 162篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
62.
The semen of six different bulls was used to examine the effects of treatment with caffeine or caffeine plus Ca-ionophre on in vitro fertilization, cleavage and development into morulae of in vitro matured bovine oocytes. In vitro fertilization results (formation of both pronuclei, cleavage and development to >/= four-cell stage were significantly (P<0.01) higher using caffeine plus Ca-ionophre than those using only caffeine. The rates of fertilization and first cleavage were only slightly variable among the bulls. However, the present data showed significant variability in formation of both pronuclei (36 to 75%) of fertilized ova and development to the >/=4cell stage (39 to 71%) by different bulls. Development into morulae of ova recovered from the rabbit oviduct did not show any significant differences in relation to sperm treatments or individual bulls. 相似文献
63.
In most cyclic females, prostaglandin F(2alpha) (PGF(2alpha)) triggers a uterine motility response resembling that of oxytocin (OT). To determine if PGF(2alpha) is a uterokinetic substance in the cycling mare, uterine motility was measured by intrauterine balloon technique in 12 conscious, normally cyclic mares. After 60 min of saline infusion, continuous intravenous (i.v.) infusion with OT (1 i.u./min) was followed by PGF(2alpha) (200 mug/min) for 60 min each. The experiment was repeated 3 wk later except with PGF(2alpha) preceeding OT. A second group of mares was administered OT (60 i.u.) either i.v., intramuscularly (i.m.), or intrauterinely (i.u.). Plasma samples were studied for progesterone concentration. Control uterine motility for the first group of mares was (mean +/- SEM) 545.83 +/- 45.10 mm(2). Significant (P<0.05) elevation in uterine motility was recorded for OT (1118.60 +/- 70.56 mm(2)) regardless if PGF(2alpha) preceded OT infusion or vice-versa. No significant difference (P>0.05) was seen in motility after PGF(2alpha) (423.33 +/- 31.12 mm(2)) infusion. The uterokinetic effect of OT was greatest when OT was administered i.v. (1696.50 +/- 195.46 mm(2)) followed by i.m. (819.82 +/- 39.96 mm(2)), and it was least effective when administered i.u. (607.83 +/- 21.56 mm(2)) as compared to control uterine motility (279.78 +/- 22.33 mm(2)). Skin electrical resistance values rose from 0 to 2000 ohms with PGF(2alpha) infusion (but not with OT), indicating that PGF(2alpha) was bioactive. It was concluded that PGF(2alpha) was not a uterokinetic substance in the cyclic mare. 相似文献
64.
Onishi H Kamekura M Yokoi H Kobayashi T 《Applied and environmental microbiology》1988,54(11):2632-2635
A bioreactor with a column of flocculated cells of the moderate halophile Micrococcus varians subsp. halophilus which adsorbed the halophilic nuclease H was designed to be used in the production of 5' nucleotides from RNA. A remarkable characteristic of the flocculated cells was that they preferentially adsorbed much exogenous nuclease, excluding adsorbed 5' nucleotidase. Furthermore, desalting treatment of the flocculated cells in the presence of 2% MgSO(4) . 7H(2)O gave rise to selective inactivation of 5' nucleotidase without the loss of nuclease H activity, and 5'-guanylic acid was produced with the bioreactor. 相似文献
65.
66.
The dorsal ocelli of bibionid flies, details of which have not previously been described, were examined in males of Dilophus febrilis. The three ocelli are combined within an elevated chitin capsule, in a medial position between the enlarged dorsal compound eyes. The biconvex lenses show a multiple layering of up to 150 regularly spaced, clear and dense cuticle zones (100 nm spacing) which probably provide some spectral filtering, suggested by in vivo observations with an epifluorescence microscope. The corneagenous cells and the retina with 100-200 photoreceptor cells are adjoined proximally. A distal retina zone comprises the rhabdoms, which are laterally connected in an hexagonal network. The rhabdoms are between 4 and 15 mum in length; they decrease gradually from the dorsal to the ventral retina region. A middle retina zone comprises the receptor somata, a proximal zone, their axons. Synaptic contacts between axons and interneuron dendrites, feedback synapses to axons, and axo-axonic synapses are found, showing varying pre-synaptic structures. A possible functional role of the ocelli is discussed. 相似文献
67.
本文研究了念珠菌素所修饰的双层脂膜的离子渗透性、离子选择性和pH—膜电流的关系。发现该膜在一定的钠离子浓度范围内,可以作为Na+离子传感器,当膜两侧溶液存在着质子梯度时,念珠菌素起着一个钠离子载体的作用。 相似文献
68.
在苏芸金杆菌浓缩商品制剂中,通常含有3—4%的二甲苯以防止芽孢的萌发。除此以外,二甲苯还能防止各种腐生微生物污染,尤其是肠杆菌和真菌。最近,一种用苏芸金杆菌血清型H3a3b制备的称为Futufa的超浓缩液已经问世,其中不含二甲苯,用于防治枞色卷蛾(属鳞 相似文献
69.
目前保藏细菌的常用方法有:①根据不同菌种低温保藏,隔双周或隔月移种传代,②在很低的温度下。使微生物细胞快速冻结且保持完整,然后在真空下使水分升华达到干燥,即冻干保存。前种操作简 相似文献
70.
猕猴桃新近在世界上的许多地区均有种植。虽然曾采用许多常用的育苗法,以满足对苗木需求的日益增加,但唯有幼苗嫁接法和扦插法较为实用。软枝扦插发根相当容易,而硬枝扦插则偶有发根困难的现象。有关硬枝扦插的报道甚少,且试验结果不尽一致。应用底温22°—24℃或高至27℃,被认为是一项良好的技术,但尚未探明不同温度 相似文献