The micronucleus of Paramecium caudatum is distinctive from other closely related Paramecium such as P. multimicronucleatum and P. tetraurela. P. caudatum has only one micronucleus of a type in which the chromatin is spread equally throughout the nucleus as opposed to P. multimicronucleatum which has four very small micronuclei of the vesicular type and P. aurelia which has two micronuclei of the vesicular type where the chromatin forms a central electron opaque mass in the center of the nucleus. The micronucleus in this micrograph contains many microtubules so it may be in early division. The nuclear envelope has many nuclear pores. TEM taken on 3/23/73 by R. Allen with Hitachi HU11A operating at 75kV. Neg. 13,250X.