255 research outputs found
The development of the albino rat, Mus norvegicus albinus. I. From the pronuclear stage to the stage of mesoderm anlage; end of the first to the end of the ninth day
No Abstract.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/50236/1/1050260205_ftp.pd
A hexamer origin of the echinoderms' five rays
Of the major deuterostome groups, the echinoderms with their multiple forms
and complex development are arguably the most mysterious. Although larval
echinoderms are bilaterally symmetric, the adult body seems to abandon the
larval body plan and to develop independently a new structure with different
symmetries. The prevalent pentamer structure, the asymmetry of Loven's rule and
the variable location of the periproct and madrepore present enormous
difficulties in homologizing structures across the major clades, despite the
excellent fossil record. This irregularity in body forms seems to place
echinoderms outside the other deuterostomes. Here I propose that the
predominant five-ray structure is derived from a hexamer structure that is
grounded directly in the structure of the bilaterally symmetric larva. This
hypothesis implies that the adult echinoderm body can be derived directly from
the larval bilateral symmetry and thus firmly ranks even the adult echinoderms
among the bilaterians. In order to test the hypothesis rigorously, a model is
developed in which one ray is missing between rays IV-V (Loven's schema) or
rays C-D (Carpenter's schema). The model is used to make predictions, which are
tested and verified for the process of metamorphosis and for the morphology of
recent and fossil forms. The theory provides fundamental insight into the
M-plane and the Ubisch', Loven's and Carpenter's planes and generalizes them
for all echinoderms. The theory also makes robust predictions about the
evolution of the pentamer structure and its developmental basis. *** including
corrections (see footnotes) ***Comment: 10 pages, 6 figure
The embryology of crepidula,A contribution to the cell lineage and early development of some marine gasteropods
Declining Orangutan Encounter Rates from Wallace to the Present Suggest the Species Was Once More Abundant
BACKGROUND: Bornean orangutans (Pongo pygmaeus) currently occur at low densities and seeing a wild one is a rare event. Compared to present low encounter rates of orangutans, it is striking how many orangutan each day historic collectors like Alfred Russel Wallace were able to shoot continuously over weeks or even months. Does that indicate that some 150 years ago encounter rates with orangutans, or their densities, were higher than now?
METHODOLOGY/PRINCIPAL FINDINGS: We test this hypothesis by quantifying encounter rates obtained from hunting accounts, museum collections, and recent field studies, and analysing whether there is a declining trend over time. Logistic regression analyses of our data support such a decline on Borneo between the mid-19th century and the present. Even when controlled for variation in the size of survey and hunting teams and the durations of expeditions, mean daily encounter rates appear to have declined about 6-fold in areas with little or no forest disturbance.
CONCLUSIONS/SIGNIFICANCE: This finding has potential consequences for our understanding of orangutans, because it suggests that Bornean orangutans once occurred at higher densities. We explore potential explanations-habitat loss and degradation, hunting, and disease-and conclude that hunting fits the observed patterns best. This suggests that hunting has been underestimated as a key causal factor of orangutan density and distribution, and that species population declines have been more severe than previously estimated based on habitat loss only. Our findings may require us to rethink the biology of orangutans, with much of our ecological understanding possibly being based on field studies of animals living at lower densities than they did historically. Our approach of quantifying species encounter rates from historic data demonstrates that this method can yield valuable information about the ecology and population density of species in the past, providing new insight into species' conservation needs
A distinguishing feature of Pongo upper molars and its implications for the taxonomic identification of isolated hominid teeth from the Pleistocene of Asia
The taxonomic status of isolated hominoid teeth from the Asian Pleistocene has long been controversial due to difficulties distinguishing between pongine and hominin molars given their high degree of morphometrical variation and overlap. Here, we combine nonmetric and geometric morphometric data to document a dental pattern that appears to be taxonomically diagnostic among Pongo. We focus on the protoconule, a cuspule of wellâdocumented evolutionary history, as well as on shape differences of the mesial fovea of the upper molars
Studies in growth changes in the skull and face of anthropoids. I. The eruption of the teeth in anthropoids and Old World Apes
Beitr\ue4ge zur Anatomie und Systematik der Holothurien
Volume: 17Start Page: 291End Page: 37
Orangutan genetics, races, and reproduction. Review ofthe orang utan. Its biology and conservation, edited by Leobert E. M. de Boer. The Hague, Dr. W. Junk Publishers, 1982, 353 pp. $76.00
Theology, Psychology, and Politics: The Holy Trinity in Morrison's Beloved
This essay dissects the previously unstudied allusion to the Catholic Holy Trinity in Toni Morrison's Beloved and explains how her purpose in using this allusion, like the Trinity itself, is threefold. Excerpts from the novel reveal how Morrison uses the Trinity to develop her three main characters and their relationships to each other, deepening Beloved's already dense plot. Textual evidence for Morrison's exploration of the reality, fear, and enormity of the enslaved experience is also presented, and how Morrison unites these specific psychologically damaging experiences into a singular commentary on post-Civil War African American consciousness through her trinity of characters is demonstrated. Further, the political climate of the 1980s is investigated and the assertion that Morrison uses her allusion to the Trinity to comment on the tacit reversion of civil rights at the hands of the neglectful Reagan administration and Rehnquist
Court is made. These three interdependent aspects of Morrison's Trinity allusion are also used to illuminate Beloved's hauntingly obscure closing message
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