Fill in the Blank in the Following Paragraph Describing Collective

Short Answer

Fill in the blank in the following paragraph describing collective cell rearrangements. Do not use abbreviations.
"Cells form lamellipodia and attempt to crawl over one another, essentially pulling their neighbors inward into a narrow zone. This is accompanied by elongation along the long axis of the narrow zone. This process of … depends on Wnt signaling and is observed multiple times during the development of a vertebrate."