The effects of proliferative disorders on the time and sequence of neuron origin in the optic tectum were studied in chick embryos in which "overgrowth" had been experimentally induced, Sixteen normal and fourteen abnormal embryos ranging in age from four through ten days of incubation were each given a single dose of thymidine[3H]. Comparisons were then made between 16-day normal and abnormal chick embryos with respect to the distribution tectum midway between its rostral and caudal boundaries. In the normal and abnormal tecta layers I-IV were generated on days 3-6. Differences occurred in the generation of the remaining layers. In the normal embryos layers V-VII arose on days 5-8 and layers VIII-XV on days 4-7; in the abnormal embryos layers V-VII originated on days 6-8 and layers VIII-XV on days 5-8. In addition to the delay in onset of neuron origin, the abnormal tectum also showed distorted lamination, particularly in layers VIII-XV which constitute the superficial regions of the stratum griseum and fibrosum superficiale. Despite the defect in time and sequence of neuron origin, the rostroventrolateral to caudodorsomedial gradient of maturity characteristic of the normal tecta was not disturbed in the abnormal tecta.