Studies on cell proliferation in the mesencephalon of chick embryos by short-term labelling method

The proliferative activity of the mesencephalon was studied from stage 8- to stage 30 in chick embryos by the short-term labelling method. Recorded values of DNA synthesis, estimated as cp 10 m/DNA, were plotted against the stage of development. The high proliferative activity in young stages decreased gradually to stage 20. It then rose rapidly, reaching a peak at stage 22, before decreasing again towards stage 30. This result supports data previously obtained by mitotic figures counting by Källén (1961) but differs from the results obtained by Cowan et al. (1968) who found the peak of activity at stage 28 in a study of the optic tectum. Possible reasons for this discrepancy are discussed.