全文获取类型
收费全文 | 1605422篇 |
免费 | 146520篇 |
国内免费 | 1437篇 |
专业分类
1753379篇 |
出版年
2021年 | 18233篇 |
2019年 | 16200篇 |
2018年 | 19512篇 |
2017年 | 18343篇 |
2016年 | 29237篇 |
2015年 | 42787篇 |
2014年 | 50892篇 |
2013年 | 77009篇 |
2012年 | 46423篇 |
2011年 | 37852篇 |
2010年 | 47451篇 |
2009年 | 47623篇 |
2008年 | 34750篇 |
2007年 | 33876篇 |
2006年 | 36601篇 |
2005年 | 37952篇 |
2004年 | 36777篇 |
2003年 | 34128篇 |
2002年 | 32044篇 |
2001年 | 47869篇 |
2000年 | 45280篇 |
1999年 | 41938篇 |
1998年 | 27138篇 |
1997年 | 26852篇 |
1996年 | 26157篇 |
1995年 | 24296篇 |
1994年 | 23930篇 |
1993年 | 23218篇 |
1992年 | 35532篇 |
1991年 | 34141篇 |
1990年 | 32664篇 |
1989年 | 33091篇 |
1988年 | 30273篇 |
1987年 | 29063篇 |
1986年 | 27359篇 |
1985年 | 28860篇 |
1984年 | 27124篇 |
1983年 | 23957篇 |
1982年 | 22703篇 |
1981年 | 21697篇 |
1980年 | 20295篇 |
1979年 | 23256篇 |
1978年 | 20930篇 |
1977年 | 19661篇 |
1976年 | 18930篇 |
1975年 | 19037篇 |
1974年 | 19876篇 |
1973年 | 20137篇 |
1972年 | 17368篇 |
1971年 | 15774篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
962.
963.
964.
965.
Stephen B. Vander Wall 《Animal behaviour》1982,30(1):84-94
Five hypotheses of cache recovery behaviour in Clark's nutcracker (Nucifraga columbiana) were examined experimentally. Most caches were made in soil within 5 cm of conspicuous large objects. Both seed-caching and non-seed-caching nutcrackers were able to locate caches. Seed-caching nutcrackers relocated caches using large objects as remembered visual cues. Soil microtopography and small (<2 cm diameter) objects may be used as cues to facilitate cache recovery but are not essential. Non-seed-caching nutcrackers located caches by using soil disturbances at cache sites as visual cues and by searching preferentially near objects where caches were concentrated. Success rates of seed-caching nutcrackers ranged from 52 to 78% and those of non-seed-caching nutcrackers ranged from 8 to 12%. Nutcrackers do not use random search or olfactory cues to locate caches. 相似文献
966.
967.
968.
969.
970.
We report a quantification of the maximum mucus accumulation in proliferating rat colonic epithelial cells. The proliferative potential was determined by radioautographic study of one-hour pulse exposures to tritiated thymidine, mucous content was determined by Periodic-acid Schiff (PAS) staining. We examined 55 labeled mucous cells in 0.5- to 1-micrometer serial sections. The maximum thecal and nuclear profiles of these cells were photographed and their surface areas were determined utilizing a coordinate sensor. The data were expressed as a theca-to-nucleus (T/N) ratio. The maximum (T/N) ratio for a labeled mucous cell was 3.0. We performed a similar analysis on 22 unlabeled mucous cells from upper crypt regions and surface epithelium to derive the range of (T/N) ratios for terminally differentiated mature mucous cells. The range of (T/N) ratios from these cells was from 4.8 to 16.4. Our study shows that proliferative potential of mucous cells is determined by the interrelationship between mucus accumulation and nuclear size. 相似文献