Thesis: Ph. D., Massachusetts Institute of Technology, Department of Brain and Cognitive Sciences, 2017.Cataloged from PDF version of thesis.Includes bibliographical references.How animals establish and maintain the sizes of myriad tissues and organs in tight proportion to one another is a fundamental question of developmental biology. Planarian flatworms regenerate from diverse injuries, in each case precisely restoring body parts to their appropriate proportions. Underlying this ability is a pluripotent population of dividing cells called neoblasts, which are required for homeostatic maintenance and regeneration of all planarian tissues. Whether neoblasts restore proportion by sensing and responding to the presence or absence of specific tissues during regeneration is unknown. We used the planarian eye lineage to address this problem. Following decapitation, neoblasts normally give rise to a large number of eye progenitors, facilitating eye regeneration. Remarkably, we found that eye absence alone was not sufficient to induce this response. Tissue-specific eye regeneration was achieved by homeostatic eye progenitor production accompanied by a decreased rate of cell death specifically in the regenerating eye. Conversely, large head wounds were sufficient to increase eye progenitors, even in the presence of intact eyes. Therefore, eye absence is not sufficient or necessary for neoblasts to increase eye progenitor production. Our findings suggest a "target-blind" model for planarian regeneration in which progenitor production by neoblasts does not depend on feedback from the presence or absence of specific target tissues to be regenerated.by Samuel Alexander LoCascio.Ph. D.