Protoplasmic Viscosity Changes During Mitosis in the Egg of Chaetopterus

The viscosity of Chaetopterus egg protoplasm was determined at one minute intervals during the period between fertilization and cleavage. 2. By studying fixed, sectioned and stained material, the course of the mitotic processes in the Chaetopterus egg was followed minute by minute. 3. During the cleavage mitosis, marked changes in protoplasmic viscosity occur, and these are similar to the changes already described for the eggs of Arbacia and Cumingia. 4. The appearance of the mitotic spindle is preceded by an increase in protoplasmic viscosity and is followed by a decrease in protoplasmic viscosity. 5. During the metaphase, the protoplasmic viscosity is low. 6. Just before the cell divides, the protoplasmic viscosity increases markedly.