Nuclear divisions with reduced numbers of microtubules in Tetrahymena.

Ultrastructural observations on micronuclei of Tetrahymena thermophila, WH-6 dividing in the presence of colchicine suggest that the microtubules present at the time of drug addition persist, but no new microtubules are formed. It is suggested that the great amount of elongation seen in these drug-treated micronuclei may not be due to the persisting microtubules alone. On the basis of ultrastructural and light-microscopic observations, it is proposed that at least part of the motive force responsible for the elongation which occurs in colchicine may be due to an active sliding of the micronuclear membrane on some element of the cell cortex. In addition, the macronucleus of this species of Tetrahymena was found to be capable of dividing in colchicine in the absence of internal nuclear microtubules.