Propagation of a tissue defect. The images highlight the morphogenic paradox of maintaining a defect (e.g. in the embryonic PPF) when the defect is surrounded by cells that are growing and dividing. As cells divide and increase in number, they naturally tend to fill in a central defect (logical behavior), rather than maintain or enlarge a central defect (observed behavior). To explain this observed behavior (e.g. a large CDH defect from a smaller PPF defect), one must consider specific cellular actions. Possible actions that would allow the hole to actually enlarge include: (A) radial (centrifugal) cell migration; (B) position-dependent apoptosis; or (C) enlargement of a central boundary or obstacle.