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Just as the ocular lens gathers and focuses light, so too has it captured and focused the attention of developmental biologists. Since Spemann's first experiments introduced the concept a century ago, the vertebrate lens has served as a model for the phenomenon of embryonic induction. Figure 2.1 provides a diagrammatic representation of the major steps in vertebrate lens determination to illustrate the physical relationships among developing tissues during stages pertinent to this review. The figure is based on the chick embryo, as its relatively flat topology during the earliest stages of development is particularly convenient for illustrative purposes. The lens differentiates from a region of head ectoderm that early in development lies adjacent to the region of the neural plate from which the retina will form (Fig. A). As development proceeds, the region of presumptive lens ectoderm (PLE) is not in contact with the retinal rudiment, as the neural plate folds up into a closed tube (Fig. C), but it is brought into close proximity to the retinal anlage by virtue of the outgrowth of the optic vesicle (OV) from the forebrain (Fig. D). The first overt signs of lens formation appear only after the OV establishes close contact with the PLE. After contact is made, the PLE thickens to form a placode (Fig. E) that subsequently invaginates simultaneously as the inward collapse of the OV forms the double-layered optic cup (Fig. F).
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