The Role of Microtubules in Macronuclear Division of Blepharisma*

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Abstract
SYNOPSIS. The macronucleus of the heterotrich ciliate Blepharisma undergoes a spectacular change in form in preparation for its division. The elongation phase of this cycle has been examined by light and electron microscopy. Coincident with elongation, microtubules appear closely applied to the outer surface of the macronucleus and parallel to the direction of elongation, and disappear as elongation is completed. It is demonstrated that the antimitotic drugs colchicine and podophyllotoxin reversibly block macronuclear elongation but do not entirely inhibit morphogenetic events including cell division. The failure of colchicine‐treated macronuclei to begin or continue elongation is correlated with the prevention of formation, disruption, and/or disorientation of the extranuclear microtubules.