The migration and aggregation of isolated cells occurs naturally as a functional aspect of embryogenesis and continues later as a natural phenomenon in adult organisms. As an example of the former one may cite the complexities of gastrulation and early embryological development. In a blastula the cells of the animal hemisphere may spread and overgrow the vegetal hemisphere. Consequently the external surface may be covered with ectodermal cells in a process known as epiboly. The process of gastrulation is itself a series of cell movements culminating in differential cell aggregation. Later, in early embryological development there are many instances of continual cell movements as is evidenced by migrating neural crest cells, endodermal cells or nesenchymal cells, all forming a variety of special aggregations which further specialize into tissue structure. One might not that the process of healing a regeneration involves cell movement and specific reaggregation of isolated cells.