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The development of the eye has been studied in mouse embryos from the ninth day through the nineteenth day of gestation. The tissue was fixed i n aldehydes, post-osmicated and embedded in Epon. The entire eye was sectioned and studied under the light microscope. The development of the eye in the prenatal period is described as having seven stages. In the first stage, the close contact of the optic vesicle with the surface ectoderm is observed. The retinal disc and lens placode begins to form. The second stage shows the invagination of the optic vesicle and the contraction of the margin of the lens pit. I n the third stage, the optic cup differentiates into a n outer pigment epithelium and a n inner retinal layer. The cells of the posterior lens wall start to elongate. During the fourth stage, the cavity of the lens vesicle is completely obliterated by primary lens fibers and the inner neuroblastic layer of the retina is formed with nerve fibers being traced from its cells to the optic stalk. The f i f t h stage is dominated by the appearance of lids, cornea and optic nerve, the formation of the anterior chamber and the consolidation of the intraocular poition of the embryonic vascular system. In the sixth stage, the eyelids fuse and the anterior margin of the optic cup begins to grow actively. In the last prenatal stage, the third neuron layer of the retina begins to differentiate. The iris and the folds of the ciliary body can be detected.The structural details of the development of the human eye have been fairly well described by Mann ('64), Duke-Elder and Cook ('63), and Barber ('55). Much less is known about the developing mouse eye. Otis and Brent ('54) in their study of equivalent ages between mouse and human embryos, published some pictures of the embryonic mouse eye. However, these are incomplete and without descriptions. A thorough investigation of the development of the mouse eye was described by Rugh ('68). Since only drawings were used by Rugh to illustrate these developments, we felt it would be of some value to repeat this work, using newer techniques.The purpose of this communication is also to describe several distinct stages of the prenatal development of the mouse eye as observed under the light microscope for subsequent electron microscopic study. The techniques used are also reported in some detail.Selection of stages of the development was determined by the occurrence of significant changes in the structures, rather than by arbitrary time periods. The stages presented generally represent a 24-28 hour interval. Demonstration of the finer de- MATERIALS AND METHODSThroughout the study, the Ha/lCR Swiss strain mice from Carworth Farms were used. The mating method is the same as that used in Rughs earlier studies ('68). Sexually mature females and males were mated overnight and examined the next morning. The presence of a vaginal plug (which occurred in more than 95% of the cases) was taken as evidence of fertilization and considered as zero day and nine hours of gestation. The precise tim...