By thymidine autoradiography it is shown that the entire isthmic complex is arranged according to a single and very finely grained gradient of proliferation that manifests itself in each of the component cell groups individually. Ventral neurons are generated first, dorsal last. The neurons in the various isthmic cell groups complete their final phase of DNA synthesis mainly between the following stages (of Hamburger and Hamilton, '51): N. semilunaris, HH-22 to HH-24; N. lemnisci lateralis pars ventralis, HH-23 to HH-25; N. lemnisci lateralis pars dorsalis, HH-24 to HH-27; N. isthmi partes principales magnocellularis and parvocellularis, HH-24 to HH-29; N. isthmo-opticus, HH-27 to HH-31. By thymidine autoradiography combined with the peroxidase retrograde transport technique, it is shown that the retinopetal neurons ventral to N. isthmo-optic neurons and substantially later than the nonretinopetal neurons amongst which they are scattered. Using similar autoradiographic methods but in embryos fixed at various ages, it is then shown that the isthmic alar plate gives rise to all the cell groups of the isthmic complex except possibly N. isthmi pars principalis magnocellularis. In confirmation of Vaage's ('73) account, it is shown that two main migrations, "dorsal" and dorsolateral", orignate from the dorsal and dorsolateral parts of the germinal epithelium, traveling first laterally and then ventrally, and that some cells from the former subsequently join the latter to create the mixed migration. The dorsal migration gives rise to N. semilunaris, N. isthmi pars principalis parvocellularis and N. isthmo-opticus, whereas the mixed gives rise to N. lemnisci lateralis partes dorsalis and ventralis and to much of the adjacent reticular formation. The migration is extremely rapid (15-30 micrometer per hour for the dorsal migration), but much of this speed may be due to passive bulk displacement, since there seems to be little relative movement between adjacent cells.