共查询到20条相似文献，搜索用时 15 毫秒
1.
The identities of the digits of the avian forelimb are disputed. Whereas paleontological findings support the position that the digits correspond to digits one, two, and three, embryological evidence points to digit two, three, and four identities. By using transplantation and cell-labeling experiments, we found that the posteriormost digit in the wing does not correspond to digit four in the hindlimb; its progenitor segregates early from the zone of polarizing activity, placing it in the domain of digit three specification. We suggest that an avian-specific shift uncouples the digit anlagen from the molecular mechanisms that pattern them, resulting in the imposition of digit one, two, and three identities on the second, third, and fourth anlagens. 相似文献
2.
蔡奕强 《农业图书情报学刊》2009,21(5):130-133
信息数字资源服务与建设发展是当今高校图书馆发展的特色,数字资源的大量涌现,将逐渐成为高校图书馆的主流载体。在数字资源环境下,高校图书馆数字资源服务与建设有了新的构思,将重新思考在数字资源环境下高校图书馆数字资源服务架构与馆员队伍建设的合理化．并解析及应对措施。 相似文献
3.
Newcombe AG 《Science (New York, N.Y.)》1963,140(3573):1312-1313
New fields have been devised, for a double-row edge-punched card, which require only three edge holes per field in place of the customary four. The new fields record every digit and no digit appears more than once. Any letter of the alphabet can be readily punched into a single field. Sorting of the new fields (compared to sorting of existing fields) requires fewer needle passes for both alphabetical and numerical searching, if a single search is made; this does not apply in arranging the cards in sequence. 相似文献
4.
te Welscher P Zuniga A Kuijper S Drenth T Goedemans HJ Meijlink F Zeller R 《Science (New York, N.Y.)》2002,298(5594):827-830
Distal limb development and specification of digit identities in tetrapods are under the control of a mesenchymal organizer called the polarizing region. Sonic Hedgehog (SHH) is the morphogenetic signal produced by the polarizing region in the posterior limb bud. Ectopic anterior SHH signaling induces digit duplications and has been suspected as a major cause underlying congenital malformations that result in digit polydactyly. Here, we report that the polydactyly of Gli3-deficient mice arises independently of SHH signaling. Disruption of one or both Gli3 alleles in mouse embryos lacking Shh progressively restores limb distal development and digit formation. Our genetic analysis indicates that SHH signaling counteracts GLI3-mediated repression of key regulator genes, cell survival, and distal progression of limb bud development. 相似文献
5.
地方高校图书馆数字化存在资金、人才、设备与技术等方面的不足,在新信息环境下,要以建设服务主导型数字图书馆实体作为自己的目标与定位,强调主动接轨上级数字图书馆联盟,适当进行资源数字化建设,整合图书馆的各类资源与服务,并进而整合校内各种资源,同时加强数字图书馆的理论与实务的跟踪与研究。 相似文献
6.
数字图书馆读者服务工作的变革 总被引：4，自引：0，他引：4
文中分析了数字环境下文献形态、传播手段、读者范围及阅览环境的变化导致图书馆读者服务工作的变革 ,提出个性化主动服务的理念 ,探讨了数字图书馆读者服务的新途径。数字图书馆要成为知识的喷泉 ,要主动为读者提供高效率、高层次的个性化服务 ,才能保持旺盛的生命力 相似文献
7.
BUSCHKE H 《Science (New York, N.Y.)》1963,140(3562):56-57
Report of the missing member of a set depends upon retention of the other members presented. Such a missing scan reveals greater retention than does a digit span and, unlike the digit span, better retention of later than earlier presentations. 相似文献
8.
Windows环境下提供了大量的矢量字库,具有字体非常丰富、添加新的字库比较容易等特点．但在实际应用中,难点是如何能把矢量字变成点阵字来显示．文中通过DELPHI编程,实现了这一转换,并在广告牌上的显示得到了应用．利用将显示汉字指令放在工控机中,上位机通过输出的点阵二进制数来控制广告牌上的显示汉字．并给出了实例． 相似文献
9.
本文首次报道了一匹胸肢和盆肢均为双趾的马第Ⅱ趾(指)的骨、关节、肌腱的结构,以及血管和神经的来源、走向和分支分布.同时对该马第Ⅱ趾(指)的形成原因进行了探讨. 相似文献
10.
A primitive enantiornithine bird and the origin of feathers 总被引：2，自引：0，他引：2
A fossil enantiornithine bird, Protopteryx fengningensis gen. et sp. nov., was collected from the Early Cretaceous Yixian Formation of Northern China. It provides fossil evidence of a triosseal canal in early birds. The manus and the alular digit are long, as in Archaeopteryx and Confuciusornis, but are relatively short in other enantiornithines. The alula or bastard wing is attached to an unreduced alular digit. The two central tail feathers are scalelike without branching. This type of feather may suggest that modern feathers evolved through the following stages: (i) elongated scale, (ii) central shaft, (iii) barbs, and finally (iv) barbules and barbicel. 相似文献
11.
Initiation of proximal-distal patterning in the vertebrate limb by signals and growth 总被引：1，自引：0，他引：1
Cooper KL Hu JK ten Berge D Fernandez-Teran M Ros MA Tabin CJ 《Science (New York, N.Y.)》2011,332(6033):1083-1086
Two broad classes of models have been proposed to explain the patterning of the proximal-distal axis of the vertebrate limb (from the shoulder to the digit tips). Differentiating between them, we demonstrate that early limb mesenchyme in the chick is initially maintained in a state capable of generating all limb segments through exposure to a combination of proximal and distal signals. As the limb bud grows, the proximal limb is established through continued exposure to flank-derived signal(s), whereas the developmental program determining the medial and distal segments is initiated in domains that grow beyond proximal influence. In addition, the system we have developed, combining in vitro and in vivo culture, opens the door to a new level of analysis of patterning mechanisms in the limb. 相似文献
12.
13.
14.
15.
雷音 《农业图书情报学刊》2006,18(7):107-109
在知识经济背景下，知识管理成为全球范围内越来越受重视的热门话题。作为具有知识性工作本质的图书馆。其未来的发展应立足于新的管理观念——知识管理的导入，并在知识管理的要求下，变革其内在管理和外在服务。同时结合知识经济的社会环境和数字化、网络化的技术背景深化图书馆服务工作。 相似文献
16.
高校数字参考咨询服务队伍建设的思考 总被引：1，自引：0，他引：1
张丽萍 《农业图书情报学刊》2009,21(6):195-198
在简要分析高校图书馆实施数字参考咨询服务工作必要性的基础上,重点对数字参考咨询服务的内容和核心问题,以及如何加强数字参考咨询服务队伍建设的问题进行探讨。 相似文献
17.
A partial skeleton of a primitive bird, Rahona ostromi, gen. et sp. nov., has been discovered from the Late Cretaceous of Madagascar. This specimen, although exhibiting avian features such as a reversed hallux and ulnar papillae, retains characteristics that indicate a theropod ancestry, including a pubic foot and hyposphene-hypantra vertebral articulations. Rahona has a robust, hyperextendible second digit on the hind foot that terminates in a sicklelike claw, a unique characteristic of the theropod groups Troodontidae and Dromaeosauridae. A phylogenetic analysis places Rahona with Archaeopteryx, making Rahona one of the most primitive birds yet discovered. 相似文献
18.
Cell death during early morphogenesis: parallels between insect limb and vertebrate limb development
J M Whitten 《Science (New York, N.Y.)》1969,163(874):1456-1457
The complex jointed leg of the adult fly is derived, in the pupal stage, from a simple lobed sac of cells. The gross morphological changes that result in adult shape are effected by a combination of differential cell growth and the programmed death of a large number of imaginal disc cells. Events are closely similar to those occurring in digit formation and limb contouring during vertebrate morphogenesis. In both cases phagocytic blood cells are intimately involved. 相似文献
19.
朱跃华 《农业图书情报学刊》2004,16(11):38-40
本文从编目的角度阐述了图书馆数字化建设，联机编目的网络模式，元数据、都柏林核心集在图书馆编目工作中的作用，以及网络资源的收集等方面的内容。 相似文献
20.
The human brain is found to produce a magnetic field near the scalp which varies in synchrony with periodic electrical stimulation applied to a finger. Use of a highly sensitive superconducting quantum interference device as a magnetic field detector reveals that the brain's field is sharply localized over the primary projection area of the sensory cortex contralateral to the digit being stimulated. The phase of the response at the stimulus frequency varies monotonically with the repetition rate and at intermediate frequencies yields a latency of approximately 70 milliseconds for cortical response. 相似文献