Cytoplasmic phases in the first cell cycle of the activated frog egg.

The first cell cycle of the activated frog egg is longer than subsequent cycles and several developmentally important events such as the determination of bilateral symmetry occur at this time. When eggs of Rana pipiens or Xenopus laevis are dissected at times after activation, differences in the consistency of the animal half cytoplasm can be detected visually, and the first cell cycle has been divided into four cytoplasmic phases on this basis. Phase 1 includes the events of activation and lasts about one-third of the first cycle. In phase 2, the cytoplasm becomes fluid except for the rigid, growing sperm aster, and most of the migration of the pronuclei occurs in phase 2. In phase 3, the cytoplasm becomes firm whether or not a sperm aster had been present, and the grey crescent forms, indicating the plane of bilateral symmetry. The firmness of the cytoplasm is colchicine sensitive but cytochalasin B insensitive as is grey crescent formation. In phase 4, the cortex detaches from the firm cytoplasm, and the firmness is now cytochalasin B sensitive and colchicine insensitive. The changes in cytoplasmic consistency during the first cell cycle probably reflect changes in the cytoskeleton, and the cytoplasmic consistency is functionally correlated with developmental events in the first cell cycle.