The nuclear changes during binary fission in a heterotrichous ciliate, Blepharisma wardsi were studied by electron microscopy.
The macronucleus in the interphase is in the form of 4-6 nodes. At the beginning of binary fission, the macronuclear nodes aggregate and coalesce into a mass (stage 1), then the macronucleus elongates (stage 2) and finally becomes nodulatod (stage 3). The chromatin bodies in the macronucleus gradually uncoil into their component fine fibrils at the end of stage 1, and the macronucleus becomes entirely filled with these fine fibrils in stage 2, then the fine fibrils gradually recoil to form chromatin bodies in stage 3. Nucleoli decrease both in size and in number in stage 2 and increase again in size and number during stage 3. The spherical bodies containing protein, on the other hand, have maximum size and electron-density in stage 2. The macronuclear envelope persists throughout the division cycle. The extra-nuclear microtubules seem to arise from the nuclear pores. These microtubules become most prominent in stage 2, and probably participate in the elongation of the macronucleus.
Micronuclear division is initiated in stage 1, proceeds rapidly and ends in stage 2 of macronuclear division.