"Who would believe that so small a space could contain the images of all the universe?" Leonardo da Vinci The last years of the 20th century have found the discipline of Developmental Biology returning to its original position at the forefront of biological re search. This progress can be attributed to the burgeoning knowledge base on molecules and gene families, and to the power of the molecular genetic ap proach. Topping the list of organ systems which have provided the most significant advances would have to be the eye. The vertebrate eye was one of the classic embryologic models, used to demonstrate many important prin ciples, including the concepts of inductive tissue interactions first put forth in the early 1900s. Within the last decade of this century, a return to some of the old questions with the new approaches has put eye development back into the limelight. I find this a highly appropriate topic for a book which aims to spark research for the new millennium. We begin with a chapter that discusses the anatomy of eye development, providing the basic reference information for the chapters that follow. A novel aspect of this introduction is the connection made between develop mental strategies and the eye's optical function. What also emerges from this chapter is the number of important eye structures that have barely been touched by the modern developmental biologist. Work on cornea and ante rior chamber development has lagged behind lens and retina.

Vision is our primary sensory modality, and we are naturally curious as to how the visual system assembles. The visual system is in many ways remarkably simple, a repeating assemblage of neurons and support cells that parse the visual field through precision and redundancy. Through this simplicity the eye has often led the way in our exploration of how an organ is assembled. Eye development has therefore long been a favorite for exploring mechanisms of cell fate choice, patterning and cell signaling.This volume, which is part of the Current Topics in Developmental Biology series, highlights the exceptional advances over the past 20 years. Chapters emphasize our knowledge of transcription factors and how these generate networks to direct the eye field and associated structures. Topics such as cell fate specification are also explored, along with the potential of Drosophila as a model for lens formation and the progress made in using the Drosophila eye to examine planar cell polarity. - Contributions from researchers who are active in identifying new paradigms to explore - Review of our current state of knowledge - Chapters written by authors with a new generation approach that takes a more systems approach to identifying factors and better defines cell subtypes

1 Kevin Moses It is now 25 years since the study of the development of the compound eye in Drosophila really began with a classic paper (Ready et al. 1976). In 1864, August Weismann published a monograph on the development of Diptera and included some beautiful drawings of the developing imaginal discs (Weismann 1864). One of these is the first description of the third instar eye disc in which Weismann drew a vertical line separating a posterior domain that included a regular pattern of clustered cells from an anterior domain without such a pattern. Weismann suggested that these clusters were the precursors of the adult ommatidia and that the line marks the anterior edge of the eye. In his first suggestion he was absolutely correct - in his second he was wrong. The vertical line shown was not the anterior edge of the eye, but the anterior edge of a moving wave of patterning and cell type specification that 112 years later (1976) Ready, Hansen and Benzer would name the "morphogenetic furrow". While it is too late to hear from August Weismann, it is a particular pleasure to be able to include a chapter in this Volume from the first author of that 1976 paper: Don Ready! These past 25 years have seen an astonishing explosion in the study of the fly eye (see Fig.

The vertebrate retina has a form that is closely and clearly linked to its func tion. Though its fundamental cellular architecture is conserved across verte brates, the retinas of individual species show variations that are also of clear and direct functional utility. Its accessibility, readily identifiable neuronal types, and specialized neuronal connectivity and morphology have made it a model system for researchers interested in the general questions of the genet ic, molecular, and developmental control of cell type and shape. Thus, the questions asked of the retina span virtually every domain of neuroscientific inquiry-molecular, genetic, developmental, behavioral, and evolutionary. Nowhere have the interactions of these levels of analysis been more apparent and borne more fruit than in the last several years of study of the develop ment of the vertebrate retina. Fields of investigation have a natural evolution, rdoving through periods of initial excitement, of framing of questions and controversy, to periods of synthesis and restatement of questions. The study of the development of the vertebrate retina appeared to us to have reached such a point of synthesis. Descriptive questions of how neurons are generated and deployed, and ques tions of mechanism about the factors that control the retinal neuron's type and distribution and the conformation of its processes have been posed, and in good part answered. Moreover, the integration of cellular accounts of development with genetic, molecular, and whole-eye and behavioral accounts has begun.

The vertebrate eye has been, and continues to be, an object of interest and of inquiry for biologists, physicists, chemists, psychologists, and others. Quite apart from its important role in the development of ophthalmology and related medical disciplines, the vertebrate eye is an exemplar of the ingenuity of living systems in adapting to the diverse and changing environments in which vertebrates have evolved. The wonder is not so much that the visual system, like other body systems, has been able to adapt in this way, but rather that these adaptations have taken such a variety of forms. In a previous volume in this series (VII/I) Eakin expressed admiration for the diversity of invertebrate photoreceptors. A comparable situation exists for the vertebrate eye as a whole and one object of this volume is to present to the reader the nature of this diversity. One result of this diversification of ocular structures and properties is that the experimental biologist has available a number of systems for study that are unique or especially favorable for the investigation of particular questions in visual science or neurobiology. This volume includes some examples of progress made by the use of such specially selected vertebrate systems. It is our hope that this comparative approach will continue to reveal new and useful preparations for the examination of important questions.

This book has been considered by academicians and scholars of great significance and value to literature. This forms a part of the knowledge base for future generations. So that the book is never forgotten we have represented this book in a print format as the same form as it was originally first published. Hence any marks or annotations seen are left intentionally to preserve its true nature.

The eye is a complex sensory organ, which enables visual perception of the world. Thus the eye has several tissues that do different tasks. One of the most basic aspects of eye function is the sensitivity of cells to light and its transduction though the optic nerve to the brain. Different organisms use different ways to achieve these tasks. In this sense, eye function becomes a very important evolutionary aspect as well. This book presents the different animal models that are commonly used for eye research and their uniqueness in evaluating different aspects of eye development, evolution, physiology and disease. - Presents information on the major animal models used in eye research including invertebrates and vertebrates - Provides researchers with information needed to choose between model organisms - Includes an introductory chapter on the different types of eyes, stressing possible common molecular machinery