OBSERVATIONS ON THE BINARY FISSION OF FOUR SPECIES OF COMMON FREE-LIVING CILIATES, WITH SPECIAL REFERENCE TO THE MACRONUCLEAR CHROMATIN

¹ From the College of the City of New York and Dartmouth College

1. Urocentrum turbo, Colpidium campylum, Colpidium colpoda, and Glaucoma scintillans all exhibit the phenomenon of macronuclear chromatin elimination during the process of binary fission.

2. In Urocentrum a ball of chromatin is differentiated from the elongating macronucleus. This ball contracts in the region of the plane of fission. As the daughter macronuclei separate, the ball is left between the two as a residuum. The residual ball is absorbed into the cytoplasm usually before the daughter ciliates separate.

3. Colpidium campylum, C. colpoda, and Glaucoma scintillans exhibit a peculiar type of chromatin elimination, practically identical in all three species.

4. A ball of deeply-staining chromatin forms in the center of the macronucleus during the prophase of the micronucleus. This ball increases in size, and comes to occupy the region of the division plane. Instead of remaining intact, as the macronucleus divides the ball also divides, one-half going to each daughter macronucleus. After the daughter ciliates separate, the chromatin of the central ball is budded off and cast into the cytoplasm, where it degenerates.

5. This type of post-divisional chromatin elimination is the first reported for holotrichous ciliates.

6. It is suggested that the profound reorganization of the macronuclei of Colpidium and Glaucoma may account for their high division rates and also for the infrequency of the appearance of conjugation.