The Duration of Mitosis in the Crypts of Lieberkühn of the Terminal Ileum of the Albino Rat

The duration of mitosis in the epithelium of the terminal ileum of the adult rat has been determined by counting normal mitotic figures, and colchicine‐metaphases, in Feulgen‐stained squashes of whole, micro‐dissected crypts of Lieberkühn. The duration, which depends on the morphological definition of mitosis adopted, was calculated to be 23 min. This value is similar to those determined by other authors using other methods, except in the case of tritiated‐thymidine autoradiography, where the reported duration of mitosis is considerably longer. The possibility that this is due to an effect of thymidine on mitosis is discussed.