Serial sections of embryonic rat eyes were stained with hematoxylin and eosin, quantified (by counting pycnotic and viable nuclei), reproduced by camera lucida on wax plates, and moulded into reconstructions in order to study the normal progression of cellular death during morphogenesis. At least nine distinct necrotic loci (A through I) can be distinguished. Immediately following contact between the retina and surface ectoderm (day 11) degenerating cells were observed in (A) the ventral extent of the optic vesicle, beginning i n the mid-retinal primordium and continuing ventrally i n the optic stalk, (B) in the rostra1 optic stalk base, and (C) in the surface ectoderm encircling the early lens placode. No degeneration was observed i n the dorsal half of the presumptive retina, i n the entire pigment epithelium, or i n the lens placode proper. During day 11.5 the lens placode thickens and forms a degenerating locus (D) i n its ventral portion opposite the underlying pycnotic zone in the retina (A). During day 12 the ventral pycnotic zone (A) divides into two subunits (A1 and A2). Invagination of the lens displaces its marginal and ventral components (C and D) so that they come to occupy the lens pore area and presumptive corneal epithelium. Simultaneous invagination of the retinal rudiment juxtaposes the pigment epithelium which concurrently forms a necrotic area (E) adjacent ventrally to that i n the retina (Al). Degeneration appears in the caudal optic stalk (I). The density of viable cells decreases adjacent to pycnotic areas in the retina and pigment epithelium and increases within these death centers. During day 13 the optic fissure forms within the subunits of the ventral pycnotic zone (A1 and A2). Degenerations are seen i n the dorsal optic stalk (F) and in the walls of the optic fissure (G and H). Throughout these stages necrosis appears only in those portions of the eye rudiment where invagination is either retarded or completely absent.
In part, these observations suggest that cell death serves (I) to retard or inhibit invagination within death centers, ( 2 ) to integrate the series of invaginations which mould the dorsal optic cup and optic fissure, ( 3 ) to assist formation of the pigment epithelium monolayer, and (4) to orient the lens vesicle within the eye cup. The spatio-temporal relationship between necrotic loci suggests that pycnotic cells in the retina may influence their production i n the lens and pigment epithelium. Preliminary observations on the mouse, pig, and human substantiate those on the rat.