Toads were raised in the laboratory from the egg stage through three weeks after foreleg emergence. Three to six toads were preserved from each of ten stages of development between stage XX (foreleg emergence) and three weeks after stage XX. The specimens were cleared and stained by the alizarin technique, examined under a binocular dissecting microscope, and the skull bones drawn freehand.At stage XX ossification had begun in the frontoparietals, parasphenoid, premaxillae, septomaxillae, prootics, and exoccipitals. By one day after stage XX the squamosals, dentaries, and angulosplenials had appeared; by two days, the nasals and maxillae. By three days, two of four specimens possessed two ossification centers of each titerygoid bone. At three weeks the beginnings of the hyoid apparatus were apparent.Comparisons were made to the ossification of skull bones reported in the literature for Pseudacris triseriata and Rana pipiens. / Department of Biology

Mercury (Hg) is an important environmental contaminant due to its global distribution, tendency to bioaccumulate, and toxicity to wildlife. However, Hg has received little attention in amphibians compared to other vertebrates, despite the fact that amphibian population declines have been documented worldwide and environmental contaminants are believed to contribute to some declines. During my dissertation research, I used a pluralistic approach which combined field studies and manipulative laboratory and mesocosm experiments to examine the bioaccumulation and ecological effects of environmentally relevant Hg exposure routes acting at various early life stages in amphibians. By collecting amphibians in the field at the Hg-contaminated South River, VA, I confirmed that amphibians exhibiting different life histories and occupying different ecological niches (Plethodon cinereus, Eurycea bislineata, and Bufo americanus) can bioaccumulate sufficient levels of Hg to warrant concern (Chapter 2) and female Bufo americanus transfer accumulated Hg to their eggs (Chapter 3). Maternal transfer of contaminants is a parental effect which typically has negative consequences for offspring because early development is a critical organizational period in the ontogeny of vertebrates. Through laboratory observations and mesocosm experiments, I examined the short and long-term effects of maternal contaminant exposure on offspring, and found the negative effects of maternal Hg exposure manifested either immediately at the embryonic stage or later during the larval stage, depending on the year in which the study was conducted (Chapters 4 and 5). Lastly, using a factorial laboratory experiment, I examined whether the latent effects of maternal transfer of contaminants manifests differently depending on the environment in which offspring develop, and found both maternal and dietary Hg exposure independently produced negative, but different, sublethal effects on larval development. Most importantly, maternal exposure to Hg combined with high dietary Hg exposure later in ontogeny had a lethal effect in larvae (Chapter 6). This study is one of the first to demonstrate that the latent effects of maternally transferred contaminants may be exacerbated by further exposure later in ontogeny, findings that may have important implications for both wildlife and human health. / Ph. D.

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