Ultrastructure and 3H-thymidine incorporation by chromosome vesicles in sea urchin embryos

The ultrastructural features of chromosome vesicle formation in early sea urchin embryos and chromosome vesicle uptake of tritiated thymidine is described. Envelopes which resemble typical nuclear envelopes form around the condensed anaphase chromosomes. In late anaphase or early telophase, the chromosomes swell and decondense and it is at this time when tritiated thymidine is incorporated. This study shows that DNA synthesis in the rapidly dividing cells or early sea urchin embryos occurs in chromosome vesicles which form during anaphase.