Rhythmic incorporation of thymidine by chick pineal glands in vitro

Cultured chick pineal glands showed a cycle in the cumulative incorporation of thymidine into DNA. In undisturbed cultures the rate of thymidine incorporation, amount of thymidine incorporated per 1-day cycle, and persistence of the incorporation process were all markedly affected by the concentration of exogenous precursor. However, more than two full cycles of incorporation were found when culture medium of low thymidine content was renewed daily, or when medium of intermediate concentration was replaced on the 3rd day of culture. At a high thymidine concentration the second cycle of incorporation sometimes appeared to be impaired. At all concentrations tested, less than 2% of the available thymidine was incorporated.