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The time of origin and the pattern of survival of neurons in the isthmo‐optic nucleus of the chick

The time of origin of the cells in the isthmo‐optic nucleus (ION—the nucleus of origin of centrifugal fibers to the avian retina) has been determined in the chick by a variant of the cumulative labeling method, using 3H‐thymidine autoradiography. All the neurons of the ION are generated (i.e., pass through their last phase of DNA synthesis) over a 50‐hour period between the latter part of the fifth and the seventh days of incubation (stages 28–31 of the Hamburger and Hamilton [′51] series) but the cells come to be assembled with in the nucleus along a distinct temporo‐spatial gradient. The earliest‐formed cells occupy the ventrolateral part of the nucleus while the last neurons to be generated come to lie along its dorsomedial margin.