The effects of embryonic overgrowth on the avian optic tectum were determined during late developmental stages in 121 chick embryos in which overgrowth was induced surgically. The embryos ranged in incubation age from seven days through 21 days and showed a variety of gross brain defects, including fused optic lobes, fusion of one or both optic lobes with a cerebral hemisphere or with the cerebellum, absence or reduction of one optic lobe, convolutions of the tectal wall, and hydrocephalus. At seven days to 11 days of incubation, histological defects of the tectum included irregularities in the migration patterns of neuroblasts and abnormal stratification among the migrated neuroblasts. The most severe defects in the 12-day through 21-day tectum were in the stratification of the outer region which corresponds to the adult stratum griseum and fibrosum superficiale, whereas deeper areas of the tectum underwent relatively normal histodifferentiation. Epithelial rosettes were common until the sixteenth day, after which they disappeared almost entirely except for a few occasional ones near the ventricular zone. Preliminary behavioral observations of the defective embryos revealed random motility similar to that in the control embryos, but a failure to achieve the proper hatching position. Of 61 operated embryos maintained until hatching, only one was capable of emerging from the shell.