With few exceptions, multicellular organisms begin to develop when a motile sperm meets an egg.
Many cell divisions occur, and then cells migrate to their final positions.
During life, individual cells divide frequently, and certain specialized cells move through the body to accomplish various tasks. In addition, every cell must have a mechanism for moving materials within itself. Balancing the need for movement is the cell's need to maintain its shape against the pressure of surrounding cells. Keeping a cell firm while enabling it to move are the twin roles played by the cytoskeleton . For a long time, microscopists believed that the cytoplasm surrounding the cell's organelles was completely unstructured.