Final answer:
The cleavage furrow is the indentation formed during cytokinesis in an animal cell, created by the contraction of a ring of actin filaments, leading to the division of the cell into two separate cells.
Step-by-step explanation:
The slight indentation that forms as a cell divides during cytokinesis is called the cleavage furrow. This feature is observed in animal cells which lack cell walls. During cytokinesis, the cleavage furrows form due to a contractile ring composed of actin filaments that is situated just inside the plasma membrane at the former metaphase plate).
The actin filaments contract, pulling the equator of the cell inward, forming a constriction or fissure. As the actin ring contracts, this furrow deepens, eventually cleaving the membrane and thereby separating the cytoplasm into two distinct new cells. One of these cells functions as a stem cell, re-entering the cell cycle to divide again in the future, while the other transforms into a functional cell, taking the place of an older cell within the tissue.