The present paper reports the results and consequences of transverse cuts made between vestibule and cytoproct of Paramecium aurelia. The structural developments of the cortex were observed by the silver impregnation method in successive opisthes (posterior products of fission). Restitution of the posterior region in successive opisthes. Restitution was associated with the process of fission, most of it occurring at the first fission which yielded opisthes having anterior regions 90% of normal length and posterior regions 70% (as compared with the 30% remaining after amputation). Return to normal was nearly complete during the second fission; after the third fission there was no significant difference from normal. Incomplete kineties, i.e., kineties that fail to extend to regions where normal kineties do. Incomplete kineties were found in successive opisthes of cut cells. Growth of complete kineties at latitudes posterior to the termini of incomplete kineties shifted these termini more and more anterior at successive fissions. Eventually the posterior terminations of the incomplete kineties fell anterior to the fission plane, removing them from the opisthe formed at this and subsequent fissions. The kinety patterns in the regenerated posterior region, i.e., the spacing and the angular arrangement of the kineties on the two sides of the cytoproct line. In some opisthes the kinety patterns were similar to but not exactly the same as those of normal cells; in others. The kinety pattern on one side or both sides of the cytoproct line was distinctly abnormal. Regeneration of the cytoproct. The cytoproct line reappeared within two hours after the first fission of three‐quarters of the opisthes; by the end of the second fission, it was present in almost all opisthes. The cytoproct regenerated in cells with abnormal kinety patterns as well as in those with normal kinety patterns. Copyright © 1969 Wiley‐Liss, Inc., A Wiley Company
