The number of mitotic figures in the optic tectum of the chick increases from just over 20, 000 on the fourth day of incubation (stages 25 and 26) to a peak of approximately 45, 000 on the fifth and sixth days (stages 28 and 29). Thereafter, the number declines sharply to about 10, 000 and 2, 000 on the eighth and ninth days respectively and to as few as 100 and 18 on the eleventh and twelfth days. The dividing cells are not uniformly distributed throughout the tectum but show a clear rostral to caudal gradient such that at every stage the number of mitoses in the caudal half of the tectum exceeds that in its rostral half. In addition the number of mitotic figures in the ventrolateral half of the tectum exceeds that in its dorsomedial half between the fourth and seventh days of incubation, but this pattern is reversed during the last few days of mitotic activity.
The process of cell proliferation in the optic tectum appears to be completely independent of the developing eye since there is no significant change in either the total number of mitotic figures in the tectum, or in their spatial distribution, after complete removal of the optic vesicle towards the end of the second day of incubation.