Variability of mitotic activity in chicken embryo tissues

On testing antimitotic substances in chicken embryos the authors encountered considerable changes in the mitotic activity which were difficult to explain. Mitotic indexes (MI) of several tissues sampled during a 24 hour period were measured and compared. The statistical significance of the results was verified by the U- and t-tests. Marked changes in mitotic activity dependent on daytime were found in all cases. Maximum MI values were measured around the 12th, minimum between the 2nd and 6th hours. These variations cannot be explained by the facts known so far. The question arises whether this time regulation of mitotic activity is typical only for embryonic or bird tissues, or whether it may be a general biological phenomenon.