1. Paramecium caudatum may be easily cut with a scalpel, one, the larger, part, continues to live in about 30 per cent. of cases, and both parts never for more than 24 hours. If the nucleus is injured by the knife neither part lives for more than a few hours. The present paper is a summary of about 150 recorded experiments. 2. Cells cut at either extremity are just as much demoralized as those cut in apparently more vital parts. 3. The power of regeneration varies in different races of "giant" Paramecium. In one race only about 1 per cent. regenerated; in another race about 10 per cent., in a third race about 30 per cent. regenerated, and in a fourth race all, or 100 per cent. regenerated. This last race is not included in the present paper. 4. There is strong evidence of a division zone in Paramecium which lies in the center of the cell. If the cell is cut anterior or posterior to this zone the fragment divides in the original plane into a truncated abnormal form and a normal form. The truncated form may divide again not through its center, but through the center of the cell were it perfect. 5. A fragment, whether anterior or posterior, dies without division in the majority of cases; in cases of fission, it divides asymmetrically as stated above. The division, however, is retarded. In a few cases the division is abortive and a monster results. 6. After such a division into an abnormal and a normal cell, the normal cell continues to divide normally and forms a race of individuals entirely unaffected by the operation. The abnormal cell in some cases, divides again asymmetrically and forms another normal cell and an abnormal cell; in other cases the second division is abortive and monsters are formed; in a few cases it continues to divide with a gradually decreasing abnormality until normal forms are regained; in many cases, finally, it dies without further division. 7. The monsters represent as many individuals as there are mouths. In one monster as many as fourteen mouths were present at one time. There is a well-marked tendency of the protoplasm to assume the form of a normal Paramecium about each of the mouths, and such individuals bud out of the protoplasmic mass, remain for several hours, and may be absorbed again into it. 8. Free cells, complete in all respects, may be given off from the protoplasm of a monster. These may live for days and may even divide, but vitality is weak and they invariably die. 9. Cell division and cell regeneration are entirely independent phenomena. A fragment divides without regenerating and the abnormal product of this division may divide again without regenerating. In other cases the fragment divides asymmetrically and the resulting abnormal cell regenerates before the following division. The cell need not regenerate to divide, and may or may not regenerate before dividing. 10. The Paramecium cell acts as a unit; cytoplasm and nucleus are equally important, a small loss at either extremity is usually enough to throw the physiological mechanism out of gear and lead to death, to asymmetrical division, or to monster formation. 11. The phenomena resulting from cutting Paramecium cannot be explained on the Kernplasmarelation theory of the Munich school. Mass of nucleus in relation to mass of protoplasm is only a morphological index of the physiological (chemical, electrical) activity of the cell—an effect and not a cause of the various vital reactions. 12. Regeneration of a cut cell in races with a limited power of regeneration, is more often observed in cells that have recently conjugated, or in cells that were cut while in conjugation, than in ordinary vegetative cells.