Microtubular Structures in Macronuclei of Synchronously Dividing Tetrahymena pyriformis

SYNOPSIS. When heat‐synchronized cultures of Tetrahymena pyriformis, amicronucleate strain GL, were examined by electron microscopy, intramacronuclear microtubules were observed in dividing cells. These tubules have a diameter of 180–230 A and occur either singly or packed together in bundles. They are predominantly associated with outpocketings and invaginations of the nucleus. Sections as well as negatively stained preparations of isolated macronuclear envelopes indicate that the microtubules are inserted at the inner nuclear membrane. The findings suggest that microtubules of the spindle type participate in the process of macronuclear division.